System:
You are a senior application security auditor reviewing code for potential vulnerabilities.

You will be given a candidate function, repository and file metadata, a reduced call path, and resolved cross-file context such as helper functions, structs, macros, constants, and related definitions. Use the supplied context actively, but stay strictly grounded in what is actually shown.

Work from the supplied code and surrounding context as the primary evidence. Do not assume a bug exists just because the sample was retrieved.

Analyze both control flow and data flow. Trace the relevant execution paths and track security-relevant data through inputs, transformations, conditions, helper calls, outputs, and side effects.

Reason about attacker influence, identify relevant guards or missing guards, and explain exploitability or non-exploitability before reaching a verdict.

When the code is vulnerable, explain the vulnerable behavior, root cause, realistic attack preconditions, likely impact, and the most precise vulnerability classification justified by the supplied code.

When the code is not vulnerable, explain the concrete invariants and guards that prevent the suspected issue, and note any residual uncertainty that remains from the supplied context.

Keep the analysis concrete and code-grounded. Separate observed evidence from inferred conclusion.

Audit the following candidate code to determine whether it presents a real vulnerability.
This sample may come from a noisy retrieval or transformation stage, so reason from the supplied code and context rather than assuming the label.

Repository: github.com/modelcontextprotocol/go-sdk
Language: Go
File: mcp/streamable.go
Function: ServeHTTP

Call path: ServeHTTP (mcp/streamable.go) → http.LocalAddrContextKey (net/http) → util.IsLoopback (internal/util/net.go)

### Primary Function

```go
func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	// DNS rebinding protection: auto-enabled for localhost servers.
	// See: https://modelcontextprotocol.io/specification/2025-11-25/basic/security_best_practices#local-mcp-server-compromise
	if !h.opts.DisableLocalhostProtection && disablelocalhostprotection != "1" {
		if localAddr, ok := req.Context().Value(http.LocalAddrContextKey).(net.Addr); ok && localAddr != nil {
			if util.IsLoopback(localAddr.String()) && !util.IsLoopback(req.Host) {
				http.Error(w, fmt.Sprintf("Forbidden: invalid Host header %q", req.Host), http.StatusForbidden)
				return
			}
		}
	}

	// Allow multiple 'Accept' headers.
	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Accept#syntax
	accept := strings.Split(strings.Join(req.Header.Values("Accept"), ","), ",")
	var jsonOK, streamOK bool
	for _, c := range accept {
		switch strings.TrimSpace(c) {
		case "application/json", "application/*":
			jsonOK = true
		case "text/event-stream", "text/*":
			streamOK = true
		case "*/*":
			jsonOK = true
			streamOK = true
		}
	}

	if req.Method == http.MethodGet {
		if !streamOK {
			http.Error(w, "Accept must contain 'text/event-stream' for GET requests", http.StatusBadRequest)
			return
		}
	} else if (!jsonOK || !streamOK) && req.Method != http.MethodDelete { // TODO: consolidate with handling of http method below.
		http.Error(w, "Accept must contain both 'application/json' and 'text/event-stream'", http.StatusBadRequest)
		return
	}

	sessionID := req.Header.Get(sessionIDHeader)
	var sessInfo *sessionInfo
	if sessionID != "" {
		h.mu.Lock()
		sessInfo = h.sessions[sessionID]
		h.mu.Unlock()
		if sessInfo == nil && !h.opts.Stateless {
			// Unless we're in 'stateless' mode, which doesn't perform any Session-ID
			// validation, we require that the session ID matches a known session.
			//
			// In stateless mode, a temporary transport is be created below.
			http.Error(w, "session not found", http.StatusNotFound)
			return
		}
		// Prevent session hijacking: if the session was created with a user ID,
		// verify that subsequent requests come from the same user.
		if sessInfo != nil && sessInfo.userID != "" {
			tokenInfo := auth.TokenInfoFromContext(req.Context())
			if tokenInfo == nil || tokenInfo.UserID != sessInfo.userID {
				http.Error(w, "session user mismatch", http.StatusForbidden)
				return
			}
		}
	}

	if req.Method == http.MethodDelete {
		if sessionID == "" {
			http.Error(w, "Bad Request: DELETE requires an Mcp-Session-Id header", http.StatusBadRequest)
			return
		}
		if sessInfo != nil { // sessInfo may be nil in stateless mode
			// Closing the session also removes it from h.sessions, due to the
			// onClose callback.
			sessInfo.session.Close()
		}
		w.WriteHeader(http.StatusNoContent)
		return
	}

	switch req.Method {
	case http.MethodPost, http.MethodGet:
		if req.Method == http.MethodGet && (h.opts.Stateless || sessionID == "") {
			if h.opts.Stateless {
				// Per MCP spec: server MUST return 405 if it doesn't offer SSE stream.
				// In stateless mode, GET (SSE streaming) is not supported.
				// RFC 9110 §15.5.6: 405 responses MUST include Allow header.
				w.Header().Set("Allow", "POST")
				http.Error(w, "Method Not Allowed", http.StatusMethodNotAllowed)
			} else {
				// In stateful mode, GET is supported but requires a session ID.
				// This is a precondition error, similar to DELETE without session.
				http.Error(w, "Bad Request: GET requires an Mcp-Session-Id header", http.StatusBadRequest)
			}
			return
		}
	default:
		// RFC 9110 §15.5.6: 405 responses MUST include Allow header.
		if h.opts.Stateless {
			w.Header().Set("Allow", "POST")
		} else {
			w.Header().Set("Allow", "GET, POST, DELETE")
		}
		http.Error(w, "Method Not Allowed", http.StatusMethodNotAllowed)
		return
	}

	// [§2.7] of the spec (2025-06-18) states:
	//
	// "If using HTTP, the client MUST include the MCP-Protocol-Version:
	// <protocol-version> HTTP header on all subsequent requests to the MCP
	// server, allowing the MCP server to respond based on the MCP protocol
	// version.
	//
	// For example: MCP-Protocol-Version: 2025-06-18
	// The protocol version sent by the client SHOULD be the one negotiated during
	// initialization.
	//
	// For backwards compatibility, if the server does not receive an
	// MCP-Protocol-Version header, and has no other way to identify the version -
	// for example, by relying on the protocol version negotiated during
	// initialization - the server SHOULD assume protocol version 2025-03-26.
	//
	// If the server receives a request with an invalid or unsupported
	// MCP-Protocol-Version, it MUST respond with 400 Bad Request."
	//
	// Since this wasn't present in the 2025-03-26 version of the spec, this
	// effectively means:
	//  1. IF the client provides a version header, it must be a supported
	//     version.
	//  2. In stateless mode, where we've lost the state of the initialize
	//     request, we assume that whatever the client tells us is the truth (or
	//     assume 2025-03-26 if the client doesn't say anything).
	//
	// This logic matches the typescript SDK.
	//
	// [§2.7]: https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#protocol-version-header
	protocolVersion := req.Header.Get(protocolVersionHeader)
	if protocolVersion == "" {
		protocolVersion = protocolVersion20250326
	}
	if !slices.Contains(supportedProtocolVersions, protocolVersion) {
		http.Error(w, fmt.Sprintf("Bad Request: Unsupported protocol version (supported versions: %s)", strings.Join(supportedProtocolVersions, ",")), http.StatusBadRequest)
		return
	}

	if sessInfo == nil {
		server := h.getServer(req)
		if server == nil {
			// The getServer argument to NewStreamableHTTPHandler returned nil.
			http.Error(w, "no server available", http.StatusBadRequest)
			return
		}
		if sessionID == "" {
			// In stateless mode, sessionID may be nonempty even if there's no
			// existing transport.
			sessionID = server.opts.GetSessionID()
		}
		transport := &StreamableServerTransport{
			SessionID:    sessionID,
			Stateless:    h.opts.Stateless,
			EventStore:   h.opts.EventStore,
			jsonResponse: h.opts.JSONResponse,
			logger:       h.opts.Logger,
		}

		// Sessions without a session ID are also stateless: there's no way to
		// address them.
		stateless := h.opts.Stateless || sessionID == ""
		// To support stateless mode, we initialize the session with a default
		// state, so that it doesn't reject subsequent requests.
		var connectOpts *ServerSessionOptions
		if stateless {
			// Peek at the body to see if it is initialize or initialized.
			// We want those to be handled as usual.
			var hasInitialize, hasInitialized bool
			{
				// TODO: verify that this allows protocol version negotiation for
				// stateless servers.
				body, err := io.ReadAll(req.Body)
				if err != nil {
					http.Error(w, "failed to read body", http.StatusInternalServerError)
					return
				}
				req.Body.Close()

				// Reset the body so that it can be read later.
				req.Body = io.NopCloser(bytes.NewBuffer(body))

				msgs, _, err := readBatch(body)
				if err == nil {
					for _, msg := range msgs {
						if req, ok := msg.(*jsonrpc.Request); ok {
							switch req.Method {
							case methodInitialize:
								hasInitialize = true
							case notificationInitialized:
								hasInitialized = true
							}
						}
					}
				}
			}

			// If we don't have InitializeParams or InitializedParams in the request,
			// set the initial state to a default value.
			state := new(ServerSessionState)
			if !hasInitialize {
				state.InitializeParams = &InitializeParams{
					ProtocolVersion: protocolVersion,
				}
			}
			if !hasInitialized {
				state.InitializedParams = new(InitializedParams)
			}
			state.LogLevel = "info"
			connectOpts = &ServerSessionOptions{
				State: state,
			}
		} else {
			// Cleanup is only required in stateful mode, as transportation is
			// not stored in the map otherwise.
			connectOpts = &ServerSessionOptions{
				onClose: func() {
					h.mu.Lock()
					defer h.mu.Unlock()
					if info, ok := h.sessions[transport.SessionID]; ok {
						info.stopTimer()
						delete(h.sessions, transport.SessionID)
						if h.onTransportDeletion != nil {
							h.onTransportDeletion(transport.SessionID)
						}
					}
				},
			}
		}

		// Pass req.Context() here, to allow middleware to add context values.
		// The context is detached in the jsonrpc2 library when handling the
		// long-running stream.
		session, err := server.Connect(req.Context(), transport, connectOpts)
		if err != nil {
			http.Error(w, "failed connection", http.StatusInternalServerError)
			return
		}
		// Capture the user ID from the token info to enable session hijacking
		// prevention on subsequent requests.
		var userID string
		if tokenInfo := auth.TokenInfoFromContext(req.Context()); tokenInfo != nil {
			userID = tokenInfo.UserID
		}
		sessInfo = &sessionInfo{
			session:   session,
			transport: transport,
			userID:    userID,
		}

		if stateless {
			// Stateless mode: close the session when the request exits.
			defer session.Close() // close the fake session after handling the request
		} else {
			// Otherwise, save the transport so that it can be reused

			// Clean up the session when it times out.
			//
			// Note that the timer here may fire multiple times, but
			// sessInfo.session.Close is idempotent.
			if h.opts.SessionTimeout > 0 {
				sessInfo.timeout = h.opts.SessionTimeout
				sessInfo.timer = time.AfterFunc(sessInfo.timeout, func() {
					sessInfo.session.Close()
				})
			}
			h.mu.Lock()
			h.sessions[transport.SessionID] = sessInfo
			h.mu.Unlock()
			defer func() {
				// If initialization failed, clean up the session (#578).
				if session.InitializeParams() == nil {
					// Initialization failed.
					session.Close()
				}
			}()
		}
	}

	if req.Method == http.MethodPost {
		sessInfo.startPOST()
		defer sessInfo.endPOST()
	}

	sessInfo.transport.ServeHTTP(w, req)
}
```

### Cross-File Context

[IsLoopback — function — internal/util/net.go:1-26]
```go
func IsLoopback(addr string) bool {
	host, _, err := net.SplitHostPort(addr)
	if err != nil {
		// If SplitHostPort fails, it might be just a host without a port.
		host = strings.Trim(addr, "[]")
	}
	if host == "localhost" {
		return true
	}
	ip, err := netip.ParseAddr(host)
	if err != nil {
		return false
	}
	return ip.IsLoopback()
}
```

[disablelocalhostprotection — var — mcp/streamable.go:228]
var disablelocalhostprotection = mcpgodebug.Value("disablelocalhostprotection")

[DisableLocalhostProtection — field — mcp/streamable.go:176]
DisableLocalhostProtection bool

Provide the final verdict as strict JSON.

Return only one JSON object and no surrounding prose.

Return one JSON object with these keys:
- vulnerable
- cwe
- severity
- reviewed_region
- vulnerable_region
- root_cause
- exploitability
- attack_preconditions
- impact
- non_vulnerable_reason
- security_invariants
- invariant_verification
- residual_risk_notes
- call_path

Control flow: The ServeHTTP function processes incoming HTTP requests through a multi-stage pipeline. First, DNS rebinding protection checks if the request arrives on a loopback interface and validates that the Host header also resolves to a loopback address, returning 403 if mismatched. Next, Accept header parsing determines which content types the client supports. Method-specific validation then enforces Accept header requirements: GET requires text/event-stream, and non-GET/non-DELETE methods require both application/json and text/event-stream. Session ID handling looks up existing sessions under mutex protection and validates user ID consistency for session hijacking prevention. DELETE requests clean up sessions. POST and GET proceed to protocol version validation, followed by session creation if needed (with special stateless mode handling that peeks at the request body). Finally, the transport's ServeHTTP is called to handle the request. Early returns occur on 403 Forbidden, 400 Bad Request, 404 Not Found, and 405 Method Not Allowed responses throughout the pipeline.

Data flow: The local address is extracted from the request context via req.Context().Value(http.LocalAddrContextKey) as a net.Addr, then converted to string via localAddr.String() and passed to util.IsLoopback(). The Host header value from req.Host is passed directly to util.IsLoopback(). Both results feed into the DNS rebinding guard condition. The Accept header values are parsed by splitting on commas and trimming whitespace, setting jsonOK and streamOK boolean flags. Session ID flows from req.Header.Get(sessionIDHeader) through mutex-protected map lookup in h.sessions. The token info flows from auth.TokenInfoFromContext(req.Context()) and is compared against sessInfo.userID for session hijacking prevention. Protocol version flows from req.Header.Get(protocolVersionHeader) with a default fallback, validated against supportedProtocolVersions via slices.Contains.

Verdict: Not Vulnerable

Reviewed region:
[mcp/streamable.go]
```go
func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	// DNS rebinding protection: auto-enabled for localhost servers.
	// See: https://modelcontextprotocol.io/specification/2025-11-25/basic/security_best_practices#local-mcp-server-compromise
	if !h.opts.DisableLocalhostProtection && disablelocalhostprotection != "1" {
		if localAddr, ok := req.Context().Value(http.LocalAddrContextKey).(net.Addr); ok && localAddr != nil {
			if util.IsLoopback(localAddr.String()) && !util.IsLoopback(req.Host) {
				http.Error(w, fmt.Sprintf("Forbidden: invalid Host header %q", req.Host), http.StatusForbidden)
				return
			}
		}
	}

	// Allow multiple 'Accept' headers.
	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Accept#syntax
	accept := strings.Split(strings.Join(req.Header.Values("Accept"), ","), ",")
	var jsonOK, streamOK bool
	for _, c := range accept {
		switch strings.TrimSpace(c) {
		case "application/json", "application/*":
			jsonOK = true
		case "text/event-stream", "text/*":
			streamOK = true
		case "*/*":
			jsonOK = true
			streamOK = true
		}
	}

	if req.Method == http.MethodGet {
		if !streamOK {
			http.Error(w, "Accept must contain 'text/event-stream' for GET requests", http.StatusBadRequest)
			return
		}
	} else if (!jsonOK || !streamOK) && req.Method != http.MethodDelete { // TODO: consolidate with handling of http method below.
		http.Error(w, "Accept must contain both 'application/json' and 'text/event-stream'", http.StatusBadRequest)
		return
	}

	sessionID := req.Header.Get(sessionIDHeader)
	var sessInfo *sessionInfo
	if sessionID != "" {
		h.mu.Lock()
		sessInfo = h.sessions[sessionID]
		h.mu.Unlock()
		if sessInfo == nil && !h.opts.Stateless {
			// Unless we're in 'stateless' mode, which doesn't perform any Session-ID
			// validation, we require that the session ID matches a known session.
			//
			// In stateless mode, a temporary transport is be created below.
			http.Error(w, "session not found", http.StatusNotFound)
			return
		}
		// Prevent session hijacking: if the session was created with a user ID,
		// verify that subsequent requests come from the same user.
		if sessInfo != nil && sessInfo.userID != "" {
			tokenInfo := auth.TokenInfoFromContext(req.Context())
			if tokenInfo == nil || tokenInfo.UserID != sessInfo.userID {
				http.Error(w, "session user mismatch", http.StatusForbidden)
				return
			}
		}
	}

	if req.Method == http.MethodDelete {
		if sessionID == "" {
			http.Error(w, "Bad Request: DELETE requires an Mcp-Session-Id header", http.StatusBadRequest)
			return
		}
		if sessInfo != nil { // sessInfo may be nil in stateless mode
			// Closing the session also removes it from h.sessions, due to the
			// onClose callback.
			sessInfo.session.Close()
		}
		w.WriteHeader(http.StatusNoContent)
		return
	}

	switch req.Method {
	case http.MethodPost, http.MethodGet:
		if req.Method == http.MethodGet && (h.opts.Stateless || sessionID == "") {
			if h.opts.Stateless {
				// Per MCP spec: server MUST return 405 if it doesn't offer SSE stream.
				// In stateless mode, GET (SSE streaming) is not supported.
				// RFC 9110 §15.5.6: 405 responses MUST include Allow header.
				w.Header().Set("Allow", "POST")
				http.Error(w, "Method Not Allowed", http.StatusMethodNotAllowed)
			} else {
				// In stateful mode, GET is supported but requires a session ID.
				// This is a precondition error, similar to DELETE without session.
				http.Error(w, "Bad Request: GET requires an Mcp-Session-Id header", http.StatusBadRequest)
			}
			return
		}
	default:
		// RFC 9110 §15.5.6: 405 responses MUST include Allow header.
		if h.opts.Stateless {
			w.Header().Set("Allow", "POST")
		} else {
			w.Header().Set("Allow", "GET, POST, DELETE")
		}
		http.Error(w, "Method Not Allowed", http.StatusMethodNotAllowed)
		return
	}

	// [§2.7] of the spec (2025-06-18) states:
	//
	// "If using HTTP, the client MUST include the MCP-Protocol-Version:
	// <protocol-version> HTTP header on all subsequent requests to the MCP
	// server, allowing the MCP server to respond based on the MCP protocol
	// version.
	//
	// For example: MCP-Protocol-Version: 2025-06-18
	// The protocol version sent by the client SHOULD be the one negotiated during
	// initialization.
	//
	// For backwards compatibility, if the server does not receive an
	// MCP-Protocol-Version header, and has no other way to identify the version -
	// for example, by relying on the protocol version negotiated during
	// initialization - the server SHOULD assume protocol version 2025-03-26.
	//
	// If the server receives a request with an invalid or unsupported
	// MCP-Protocol-Version, it MUST respond with 400 Bad Request."
	//
	// Since this wasn't present in the 2025-03-26 version of the spec, this
	// effectively means:
	//  1. IF the client provides a version header, it must be a supported
	//     version.
	//  2. In stateless mode, where we've lost the state of the initialize
	//     request, we assume that whatever the client tells us is the truth (or
	//     assume 2025-03-26 if the client doesn't say anything).
	//
	// This logic matches the typescript SDK.
	//
	// [§2.7]: https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#protocol-version-header
	protocolVersion := req.Header.Get(protocolVersionHeader)
	if protocolVersion == "" {
		protocolVersion = protocolVersion20250326
	}
	if !slices.Contains(supportedProtocolVersions, protocolVersion) {
		http.Error(w, fmt.Sprintf("Bad Request: Unsupported protocol version (supported versions: %s)", strings.Join(supportedProtocolVersions, ",")), http.StatusBadRequest)
		return
	}

	if sessInfo == nil {
		server := h.getServer(req)
		if server == nil {
			// The getServer argument to NewStreamableHTTPHandler returned nil.
			http.Error(w, "no server available", http.StatusBadRequest)
			return
		}
		if sessionID == "" {
			// In stateless mode, sessionID may be nonempty even if there's no
			// existing transport.
			sessionID = server.opts.GetSessionID()
		}
		transport := &StreamableServerTransport{
			SessionID:    sessionID,
			Stateless:    h.opts.Stateless,
			EventStore:   h.opts.EventStore,
			jsonResponse: h.opts.JSONResponse,
			logger:       h.opts.Logger,
		}

		// Sessions without a session ID are also stateless: there's no way to
		// address them.
		stateless := h.opts.Stateless || sessionID == ""
		// To support stateless mode, we initialize the session with a default
		// state, so that it doesn't reject subsequent requests.
		var connectOpts *ServerSessionOptions
		if stateless {
			// Peek at the body to see if it is initialize or initialized.
			// We want those to be handled as usual.
			var hasInitialize, hasInitialized bool
			{
				// TODO: verify that this allows protocol version negotiation for
				// stateless servers.
				body, err := io.ReadAll(req.Body)
				if err != nil {
					http.Error(w, "failed to read body", http.StatusInternalServerError)
					return
				}
				req.Body.Close()

				// Reset the body so that it can be read later.
				req.Body = io.NopCloser(bytes.NewBuffer(body))

				msgs, _, err := readBatch(body)
				if err == nil {
					for _, msg := range msgs {
						if req, ok := msg.(*jsonrpc.Request); ok {
							switch req.Method {
							case methodInitialize:
								hasInitialize = true
							case notificationInitialized:
								hasInitialized = true
							}
						}
					}
				}
			}

			// If we don't have InitializeParams or InitializedParams in the request,
			// set the initial state to a default value.
			state := new(ServerSessionState)
			if !hasInitialize {
				state.InitializeParams = &InitializeParams{
					ProtocolVersion: protocolVersion,
				}
			}
			if !hasInitialized {
				state.InitializedParams = new(InitializedParams)
			}
			state.LogLevel = "info"
			connectOpts = &ServerSessionOptions{
				State: state,
			}
		} else {
			// Cleanup is only required in stateful mode, as transportation is
			// not stored in the map otherwise.
			connectOpts = &ServerSessionOptions{
				onClose: func() {
					h.mu.Lock()
					defer h.mu.Unlock()
					if info, ok := h.sessions[transport.SessionID]; ok {
						info.stopTimer()
						delete(h.sessions, transport.SessionID)
						if h.onTransportDeletion != nil {
							h.onTransportDeletion(transport.SessionID)
						}
					}
				},
			}
		}

		// Pass req.Context() here, to allow middleware to add context values.
		// The context is detached in the jsonrpc2 library when handling the
		// long-running stream.
		session, err := server.Connect(req.Context(), transport, connectOpts)
		if err != nil {
			http.Error(w, "failed connection", http.StatusInternalServerError)
			return
		}
		// Capture the user ID from the token info to enable session hijacking
		// prevention on subsequent requests.
		var userID string
		if tokenInfo := auth.TokenInfoFromContext(req.Context()); tokenInfo != nil {
			userID = tokenInfo.UserID
		}
		sessInfo = &sessionInfo{
			session:   session,
			transport: transport,
			userID:    userID,
		}

		if stateless {
			// Stateless mode: close the session when the request exits.
			defer session.Close() // close the fake session after handling the request
		} else {
			// Otherwise, save the transport so that it can be reused

			// Clean up the session when it times out.
			//
			// Note that the timer here may fire multiple times, but
			// sessInfo.session.Close is idempotent.
			if h.opts.SessionTimeout > 0 {
				sessInfo.timeout = h.opts.SessionTimeout
				sessInfo.timer = time.AfterFunc(sessInfo.timeout, func() {
					sessInfo.session.Close()
				})
			}
			h.mu.Lock()
			h.sessions[transport.SessionID] = sessInfo
			h.mu.Unlock()
			defer func() {
				// If initialization failed, clean up the session (#578).
				if session.InitializeParams() == nil {
					// Initialization failed.
					session.Close()
				}
			}()
		}
	}

	if req.Method == http.MethodPost {
		sessInfo.startPOST()
		defer sessInfo.endPOST()
	}

	sessInfo.transport.ServeHTTP(w, req)
}
```

Why it is not vulnerable: The DNS rebinding protection is correctly implemented. When the local address resolves to a loopback interface (util.IsLoopback(localAddr.String()) returns true), the code enforces that the Host header must also resolve to a loopback address (util.IsLoopback(req.Host) must return true). If the Host header does not resolve to loopback, the request is rejected with a 403 Forbidden response. The util.IsLoopback function properly handles multiple address formats: it uses net.SplitHostPort to extract the host component (handling addresses with ports), trims IPv6 brackets, checks for the 'localhost' literal, and uses netip.ParseAddr with ip.IsLoopback() for IP address validation. The protection is properly gated behind DisableLocalhostProtection and disablelocalhostprotection configuration flags, allowing administrators to explicitly disable it when needed (e.g., behind a reverse proxy). Invalid or unparseable addresses in util.IsLoopback return false, which is a safe default.

Security invariants:
- When localhost protection is not explicitly disabled (via h.opts.DisableLocalhostProtection or disablelocalhostprotection), the DNS rebinding guard must execute. Enforced by: if !h.opts.DisableLocalhostProtection && disablelocalhostprotection != "1"
- The local address must be successfully extracted from the request context and must be non-nil before applying the loopback check. Enforced by: if localAddr, ok := req.Context().Value(http.LocalAddrContextKey).(net.Addr); ok && localAddr != nil
- When the server is bound to a loopback interface, the Host header must also resolve to a loopback address. Enforced by: if util.IsLoopback(localAddr.String()) && !util.IsLoopback(req.Host) { return 403 }
- util.IsLoopback must correctly parse addresses with and without port numbers. Enforced by: net.SplitHostPort(addr) to extract host, with fallback handling when it fails
- util.IsLoopback must correctly identify loopback addresses in multiple formats: 'localhost' literal, IPv4 loopback (127.0.0.0/8), and IPv6 loopback (::1). Enforced by: host == "localhost" check and ip.IsLoopback() after netip.ParseAddr
- Invalid or unparseable addresses must not be treated as loopback (safe default). Enforced by: if err != nil { return false } in util.IsLoopback
- Session access is properly synchronized to prevent race conditions. Enforced by: h.mu.Lock()/h.mu.Unlock() around h.sessions map access
- Session hijacking is prevented by verifying user ID consistency. Enforced by: if tokenInfo == nil || tokenInfo.UserID != sessInfo.userID { return 403 }

Invariant verification:
- DNS rebinding protection is enabled by default and can only be explicitly disabled: holds=true. Evidence: The condition !h.opts.DisableLocalhostProtection && disablelocalhostprotection != "1" requires both the config field and the debug variable to explicitly disable the protection
- Local address extraction is safe and guards against missing context value: holds=true. Evidence: The type assertion with ok check: req.Context().Value(http.LocalAddrContextKey).(net.Addr); ok && localAddr != nil ensures the code only proceeds when a valid address is present
- Loopback detection correctly handles host:port format addresses: holds=true. Evidence: util.IsLoopback calls net.SplitHostPort(addr) which correctly separates host and port components, then operates on the host portion only
- Loopback detection correctly identifies localhost, IPv4 loopback, and IPv6 loopback: holds=true. Evidence: util.IsLoopback checks host == "localhost" for the literal, and uses netip.ParseAddr(host) followed by ip.IsLoopback() which covers 127.0.0.0/8 and ::1
- Unparseable addresses default to non-loopback (fail-safe): holds=true. Evidence: Both error paths in util.IsLoopback (net.SplitHostPort error and netip.ParseAddr error) lead to return false or fall through to return false
- Mismatched Host header on loopback interface results in request rejection: holds=true. Evidence: if util.IsLoopback(localAddr.String()) && !util.IsLoopback(req.Host) { http.Error(w, ..., http.StatusForbidden); return } rejects requests where local address is loopback but Host is not
- Session map access is protected by mutex: holds=true. Evidence: h.mu.Lock() before sessInfo = h.sessions[sessionID] and h.mu.Unlock() after; same pattern for write operations with h.sessions[transport.SessionID] = sessInfo

Residual risk notes: The DNS rebinding protection is skipped if the local address cannot be extracted from the request context (e.g., if the HTTP server does not populate http.LocalAddrContextKey). This is acceptable because the protection specifically applies to servers bound to loopback interfaces; if no local address information is available, the condition cannot be evaluated. The util.IsLoopback function does not perform DNS resolution on hostnames in the Host header, but this is correct behavior since the check is designed to detect when the Host header contains a non-loopback address literal or hostname that was not validated as loopback during the connection setup. The req.Host field is controlled by the client, but the IsLoopback check on it is the appropriate defense mechanism.

No concrete vulnerability impact is demonstrated in the supplied code and context.

## Analysis of `ServeHTTP` DNS rebinding protection

The candidate function implements a DNS rebinding check at the very beginning of request handling. The logic is:

- If localhost protection is enabled (not opted out via `DisableLocalhostProtection` or the `disablelocalhostprotection` runtime flag), retrieve the local address of the server connection from the request context via `http.LocalAddrContextKey`.
- If that address is available and non‑nil, check whether it is a loopback address (`util.IsLoopback(localAddr.String())`).
- If the server endpoint *is* loopback but the `Host` header (`req.Host`) does **not** represent a loopback address, reject the request with `403 Forbidden`.

The goal is to prevent DNS rebinding attacks where a malicious website tricks a browser into sending a request to a local service with a `Host` header pointing to an attacker‑controlled domain.

### Examination of the loopback check

The helper `util.IsLoopback` handles:
- The literal string `"localhost"`.
- IPv4 and IPv6 addresses via `netip.ParseAddr`, relying on Go’s `netip.Addr.IsLoopback()`, which correctly covers `127.0.0.0/8` and the IPv6 `::1`.
- Ports and IPv6 bracket notation (`[::1]:8080`) through `net.SplitHostPort`, with a fallback that strips brackets for bare IPv6 addresses.

The way the check is used:
```go
if util.IsLoopback(localAddr.String()) && !util.IsLoopback(req.Host) {
    // block
}
```

### Attempted bypass evaluation

1. **Manipulating the `Host` header to appear loopback while not being loopback**
   `IsLoopback` only yields `true` for the string `"localhost"` or a valid loopback IP. An attacker would have to send a `Host` header that matches these exactly, which necessarily points to the local machine. No ambiguity exists that would let a non‑loopback host impersonate a loopback one.

2. **Exploiting missing context**
   The `http.LocalAddrContextKey` value is set by Go’s `net/http` server for all HTTP/1.x and HTTP/2 connections. If a custom server configuration fails to set it, the type assertion `ok` will be `false` and the protection is skipped. That is a deployment consideration, not a flaw in the handler. In a standard `net/http` deployment the value is always present.

3. **IPv6 quirks**
   Both the local address and the `Host` header can contain IPv6 loopback addresses (with or without brackets, with or without ports). `IsLoopback` parses all valid representations correctly. There is no case where a loopback IPv6 address would be mis‑identified as non‑loopback, or vice versa.

4. **False positives (e.g., `LOCALHOST`)**
   The check is case‑sensitive for the string `"localhost"`. A request with `Host: LOCALHOST` will be treated as non‑loopback and rejected. This is a functional edge case, not a security bypass – it does not allow an attacker to circumvent the protection.

5. **Empty or missing `Host` header**
   An empty `Host` yields `IsLoopback` returning `false`, so a request with no `Host` header on a loopback server is blocked. Again, no bypass.

6. **Reverse proxy scenarios**
   If a reverse proxy forwards requests to a server bound on loopback, the `Host` header may be the original external domain. The check will correctly block such requests if the proxy does not rewrite the `Host` header to a local one. This is the intended DNS rebinding protection; it cannot be bypassed while preserving the external host.

7. **Race conditions or state manipulation**
   The check occurs before any body read or session creation. The context value is per‑request and immutable from the attacker’s perspective. No concurrency or state manipulation can influence it.

### Conclusion

The DNS rebinding protection logic is correctly implemented and grounded in Go’s reliable context values and the well‑tested `IsLoopback` helper. I found **no exploitable bypass** that would allow an attacker to evade the check while actually targeting a local server from a remote origin.

The protection may be overly strict in certain legitimate deployments (e.g., missing `Host` header, case‑insensitive `localhost`, reverse proxies that preserve the external `Host`), but these are functional/resilience choices, not security vulnerabilities in the code itself.

**Verdict: Not vulnerable.**

We need to analyze the candidate code for vulnerabilities. The call path points to ServeHTTP → http.LocalAddrContextKey → util.IsLoopback. The audit should focus on the DNS rebinding protection logic and any potential bypass or issue.

First, let's understand the code:

- ServeHTTP is for a StreamableHTTPHandler.
- It checks if localhost protection is enabled (not disabled via option or env var). If enabled, it retrieves the local address from request context using http.LocalAddrContextKey. This is a context value that Go's net/http server sets for TLS connections? Actually, http.LocalAddrContextKey is used by net/http to store the local address on which the request was received. According to Go's documentation, it is always set on the request context for HTTP/1.x and HTTP/2. So, on a server, req.Context().Value(http.LocalAddrContextKey) returns the local address (e.g., "127.0.0.1:8080" or "[::1]:8080"). Good.

- Then it checks if localAddr is loopback and the request's Host header is not loopback. If so, it returns 403 Forbidden with an error.

Potential vulnerability: DNS rebinding protection bypass? The check uses req.Host (the Host header) directly. But req.Host may contain a port. Go's net/http splits the host from port? Actually, req.Host is the host part of the request URL, which is taken from the Host header, including port if present. In Go's http.Request, the Host field is the value of the Host request header without any trimming of port. But note: req.Host might be something like "example.com:443". The IsLoopback function splits host and port using net.SplitHostPort, which handles IPv6 addresses with brackets. So IsLoopback(req.Host) will correctly parse the host part and check if it's a loopback address.

But there is a subtlety: The localAddr is the server's local address (IP:port). IsLoopback(localAddr.String()) will also split host and port, and then check if the host is loopback. That's fine.

The check: if localAddr is loopback and req.Host is not loopback, then block. That's intended to prevent DNS rebinding attacks where a browser tricks a local server into thinking it's being accessed from localhost but the Host header is a remote host.

Potential issues:

1. What if the localAddr is a non-loopback interface? The protection doesn't apply, but that's fine because the server is not bound to localhost only; thus DNS rebinding is less of a concern? But the spec says "auto-enabled for localhost servers". The protection is only if the server is on loopback.

2. Could an attacker bypass by using an IPv6 loopback address? IsLoopback handles IPv6 (netip.ParseAddr understands "::1" etc.). So that's fine.

3. What about using "localhost" as host but with a different case? IsLoopback checks host == "localhost" (case-sensitive). But DNS is case-insensitive, so "LOCALHOST" would be considered not loopback by IsLoopback? Wait, IsLoopback: if host == "localhost" returns true. Else it tries to parse as IP. So "LOCALHOST" would fail IP parse and return false. That would mean the protection would block request if req.Host is "LOCALHOST" and localAddr is loopback, because it would treat "LOCALHOST" as not loopback? That would cause a false positive 403 on legitimate requests with capitalized host header. But that's a functionality bug, not a vulnerability. However, from a security perspective, could an attacker use "LOCALHOST" to bypass the protection? No, the protection blocks if req.Host is not loopback; if it is "LOCALHOST", it would consider it not loopback, thus block the request. That's overly restrictive, but not a bypass. For bypass, attacker would want to make the check pass (i.e., IsLoopback(req.Host) true while it's not actually localhost) to evade blocking. That would require fooling IsLoopback into returning true for a non-localhost host. That seems unlikely because IsLoopback checks for "localhost" string or valid loopback IP.

Potential bypass: If attacker could make req.Host be something that net.SplitHostPort returns host as "localhost"? For example, "localhost:80", "localhost:443" etc. That's safe because it is localhost. If req.Host is a string like "127.0.0.1:8080", IsLoopback will parse and return true, so it would be considered loopback, no block. That's fine.

But what if req.Host contains a username or something? According to HTTP spec, Host header must be host:port only. Go's http.Request.Host is just that. So no.

Another potential bypass: The code uses localAddr.String() which includes the port. So localAddr.String() for "127.0.0.1:8080" will be "127.0.0.1:8080". IsLoopback splits that, gets host="127.0.0.1", and returns true. So okay.

Now, what if localAddr is a net.Addr that is not a TCP address? Could be Unix socket or something. But in typical HTTP servers, it's TCP. If it's something else, localAddr.String() might not be in host:port format. IsLoopback attempts to split, if fails, trims brackets and sets host. That might yield something that isn't an IP; it would return false, so the check would be if localAddr is loopback? false, so protection wouldn't apply. So that's fine.

The call path also references util.IsLoopback from internal/util/net.go. That function seems correct.

But wait, there's a subtle issue: The check is done before reading the body, etc. It's at the very beginning. But the protection is only for DNS rebinding. However, there is a potential edge case: The code uses req.Host, which is the Host header. Could an attacker send a request with a Host header that is not in the usual form, causing IsLoopback to parse incorrectly and return true for a non-localhost host? For example, if the host part contains a Unicode character, could net.SplitHostPort handle it? I think IsLoopback calls net.SplitHostPort, which expects the format host:port or [host]:port. If it fails, it trims brackets and assumes the whole string is host. Then it calls netip.ParseAddr, which only handles IP addresses, not hostnames. So any hostname that is not "localhost" and not an IP address would return false. So no bypass.

But there is a potential DNS rebinding attack where the attacker controls the DNS for a domain that resolves to 127.0.0.1 initially, then changes to attacker's IP. The Host header would be "attacker.com". IsLoopback("attacker.com") would return false, and localAddr would be loopback, so the request is blocked. That's the intended protection.

What about the reverse scenario: localAddr not loopback but req.Host is loopback? The check doesn't block because condition requires localAddr loopback AND req.Host not loopback. So if the server is on a public interface, requests with Host: localhost are allowed. That could be a security issue? The spec says localhost protection is for servers bound to localhost, not for all servers. That seems intentional.

Now, could there be a bypass if the local address context value is missing? In some server implementations, http.LocalAddrContextKey may not be set. Then the condition ok would be false, and the protection is skipped. That could allow DNS rebinding attacks if the server is actually on localhost but the server doesn't populate that context key. Is that a security vulnerability? The code relies on the net/http server populating that context value. According to Go's documentation, it is set for HTTP/1.x and HTTP/2 servers in the standard library. So in a typical deployment using net/http server, it will be set. But if the user uses a custom server or a different framework, it might not be set. That might be a misconfiguration, but not a bug in this code per se. However, the protection is "auto-enabled for localhost servers" – but if the server binding is not done through net/http? The handler could be used with any http.Handler, but the context value is provided by the server that calls the handler. So if the server fails to provide it, protection is skipped. That could be a risk, but the code cannot guarantee it. However, the code could do a fallback using req.Context().Value might be nil, but the code explicitly checks ok, so it's graceful. That is not a vulnerability introduced by this code.

Another potential vulnerability: The disablelocalhostprotection variable can be set via mcpgodebug.Value("disablelocalhostprotection") which probably reads an environment variable or something. If an attacker can set that env var, they could disable the protection. But that's a runtime configuration issue, not a code vulnerability.

Now, look at the overall flow for potential other vulnerabilities? The primary function is ServeHTTP, the call path points to the IsLoopback check. The audit is to see if there is a real vulnerability. The prompt might be hinting at a specific vulnerability related to DNS rebinding protection bypass. Let's examine the code more carefully:

- It checks `if localAddr, ok := req.Context().Value(http.LocalAddrContextKey).(net.Addr); ok && localAddr != nil {` then `if util.IsLoopback(localAddr.String()) && !util.IsLoopback(req.Host) {`.

What is the type of `localAddr`? It's `net.Addr`. Its `String()` method returns a string representation. For net.TCPAddr, String() returns "IP:port" or "[IP]:port" for IPv6. For net.UnixAddr, it returns something else. So IsLoopback will try to split host:port. That's fine.

But what if the local address is an IPv6 loopback "::1"? For example, localAddr.String() might be "[::1]:8080". net.SplitHostPort handles that and returns host "::1". netip.ParseAddr("::1") returns a valid IP, IsLoopback returns true. Good.

What about req.Host? If Host header is "::1" or "[::1]:8080"? net.SplitHostPort will handle bracket notation. But what if req.Host is just "::1"? net.SplitHostPort will try to split, and since there's no colon, it will error? Actually, for IPv6, the Host header should use brackets if a port is present. Without port, it might be bare "::1". net.SplitHostPort on "::1" will fail because it contains multiple colons but no port, so it returns an error. Then IsLoopback will trim brackets (nothing) and set host = "::1". Then netip.ParseAddr("::1") succeeds and returns true. So it works.

Now, the vulnerability: Could the IsLoopback function be fooled by using an IPv6 address that is loopback but encoded differently? For example, "::1" vs "0:0:0:0:0:0:0:1" vs "0:0:0:0:0:0:0:1" etc. netip.ParseAddr is strict about IPv6 representation? It can parse various forms, but should recognize any valid loopback address. So that's fine.

Now, could a DNS rebinding attack still succeed via WebSockets or other protocols? This handler is for HTTP, so fine.

Alternatively, is there a vulnerability in the way the check is ordered? The protection is applied before any other logic. But what about the session hijacking prevention? That's separate and seems fine.

The call path includes http.LocalAddrContextKey. That's interesting because the HTTP server sets that. But is there any way an attacker could manipulate req.Context() to inject a fake LocalAddrContextKey? No, because context values are per-request and set by the server, not by the attacker. The attacker can control request headers, but not the context values that the server sets internally. So that's safe.

Now, consider the possibility that the server is behind a reverse proxy or load balancer that sets the X-Forwarded-For and Host header, but doesn't modify the local address context. The local address context would still be the reverse proxy's connection's local address (presumably the proxy's IP), which is not loopback, so protection not applied. But if the server is actually bound to localhost and the reverse proxy is also on localhost, the local address might be loopback. That's okay.

But what if the server is dual-stack, listening on both IPv4 and IPv6 loopback, and the DNS rebinding attack targets the IPv6 interface? The check handles both.

Now, a potential issue: The IsLoopback function doesn't handle the case where the address string might have a scope ID (e.g., "::1%lo" on some platforms). Go's net.TCPAddr.String() includes the zone? Actually, net.TCPAddr.String() returns the IP string without zone for IPv6? I think Go's net.TCPAddr's IP field is a net.IP, which does not include zone. So localAddr.String() will not include zone. But if the underlying address has a zone, it might be omitted. This is likely not relevant.

Another potential issue: The check uses localAddr.String(), but if the local address is a net.TCPAddr with a zero port or something, it's still fine.

Now, think about the surrounding code for other vulnerabilities: The Accept header parsing, session handling, protocol version checking. But the audit is focused on the call path. However, we should also look at the overall security.

Is there an instance of a missing Host header? If req.Host is empty, IsLoopback("") will return false (net.SplitHostPort fails, host="", netip.ParseAddr("") fails, so false). So if Host header is missing, localAddr loopback && !IsLoopback("") -> true && !false -> true, so block. That's a bit odd: if a client omits Host header on a loopback server, they get 403. That might be too strict, but it's not a vulnerability. In HTTP/1.1, Host is required, but in HTTP/1.0 it's optional. So legacy clients might be blocked. That's a design choice.

Now, is there a way for an attacker to send a request where localAddr is loopback but the Host header is also loopback but they manage to subvert the connection? That's not a bypass.

Perhaps the vulnerability is that the protection can be circumvented by using a websocket upgrade or other protocol switching that might bypass the initial HTTP request handling? But this handler only deals with HTTP.

Considering the context "repository: github.com/modelcontextprotocol/go-sdk". This is the Model Context Protocol Go SDK. The security best practices section is referenced. The DNS rebinding protection seems correct. However, I recall a known vulnerability in similar implementations: using req.Host directly instead of req.URL.Hostname()? But in Go, req.Host is the value from the Host header, which may differ from req.URL.Host? For HTTP requests, req.URL.Host is usually empty for server requests, as per Go's docs: "For server requests, the URL is parsed from the URI supplied on the Request-Line... For most requests, fields other than Path and RawQuery will be empty." So req.URL.Host is not the Host header. So req.Host is the correct field. So that's fine.

The potential bug: The IsLoopback function checks host == "localhost". However, DNS rebinding attacks could use a subdomain like "localhost.example.com" that initially resolves to 127.0.0.1 and then to attacker IP. The Host header would be "localhost.example.com", which IsLoopback would treat as not loopback, so the request would be blocked. That's correct.

What about using IP address "127.0.0.1" in the Host header? That passes, and the request is allowed. That's expected, because the client is genuinely accessing localhost.

So, no obvious bypass.

But let's look at the call path more carefully: The audit might be about a different vulnerability: the http.LocalAddrContextKey is used, but is there a race or something? No.

Wait, there is a subtlety: The code uses `localAddr, ok := req.Context().Value(http.LocalAddrContextKey).(net.Addr)` directly. However, net/http sets the context value as a *http.Server or something? Actually, the Go documentation for http.LocalAddrContextKey says: "LocalAddrContextKey is a context key. It can be used to retrieve the net.Addr of the local end of the connection from a request context. The value associated with this key is of type net.Addr." So it's guaranteed to be net.Addr. But the type assertion uses net.Addr (interface). So that's fine.

Potential vulnerability: The IsLoopback function may be called with a string that is not a host:port because localAddr.String() could return something like "/tmp/foo.sock" for Unix sockets. That would cause IsLoopback to try SplitHostPort, fail, trim brackets, then ParseAddr, fail, return false. So the condition would be false, which means the protection wouldn't block, but also wouldn't block normal requests because localAddr is not loopback anyway. But what if localAddr is a net.Addr that is a Unix socket but its String() returns something that net.SplitHostPort succeeds? e.g., "localhost:80"? Unlikely, but could it be a security issue? If an attacker can make the server listen on a Unix socket and somehow trick the client to send a Host header that is not loopback, and the localAddr string happens to be a loopback address string? The Unix socket address is a path, not a loopback. So IsLoopback will return false. So no.

Now, perhaps the vulnerability is that the protection can be bypassed by sending an HTTP/2 request with a :authority pseudo-header that is different from the Host header? Go's http.Request.Host takes the :authority pseudo-header as the host for HTTP/2? According to Go docs: "For HTTP/2, the :authority pseudo-header field is used, and Host field is populated with its value." So req.Host in HTTP/2 is the :authority, which is the virtual host. So same protection applies. Good.

Potential vulnerability: The code uses req.Host without checking if it has a port. If the Host header contains a port (e.g., "localhost:8080"), IsLoopback splits and checks host = "localhost", returns true. That's fine. If the Host header contains a non-loopback IP with a port, IsLoopback will parse IP and return true if it's loopback? For "127.0.0.1:8080", host = "127.0.0.1", true. Good.

Now, the audit might be concerned with the fact that the check is done before any authentication, but that's intentional.

After analyzing, I don't see a security vulnerability in the DNS rebinding check. However, the prompt says "reason from the supplied code and context rather than assuming the label." So maybe the intended vulnerability is something else in the function, but the call path points to IsLoopback.

Let's search for potential security issues beyond that specific check:

- Session hijacking prevention: uses auth.TokenInfoFromContext(req.Context()) and compares userID. That seems fine.

- The handling of stateless mode: creates fake sessions and initializes them with default state, peek at body. Potential issue: reading the body and then resetting with io.NopCloser. Could that cause problems if body is large? It reads all body into memory. But that's not a vulnerability per se, more of DoS risk. But not remote code execution.

- There's a TODO about verifying protocol version negotiation for stateless servers. Might be incomplete, but not a vulnerability.

- The method allowed handling: seems fine.

- The session timeout cleanup: uses a timer, with a race? There's a comment "Note that the timer here may fire multiple times, but sessInfo.session.Close is idempotent." So okay.

Now, maybe the vulnerability is that the IsLoopback check uses localAddr.String(), which might be a hostname if the server was started with a hostname instead of an IP, and that hostname could resolve to loopback but IsLoopback would fail to recognize it as loopback because it tries to parse as IP. For example, the server listens on "localhost:8080". net.Listen("tcp", "localhost:8080") returns a net.Listener whose address is "127.0.0.1:8080"? Actually, when you listen on "localhost", Go resolves it to an IP address. So the net.Addr returned by listener.Addr() will be a TCPAddr with IP 127.0.0.1 or ::1. So localAddr.String() will be "127.0.0.1:8080" or "[::1]:8080". That's fine. If the server binds to a hostname that isn't resolved, it fails. So it's always an IP. So no problem.

Potential bypass: if localAddr is loopback but attacker can control the Host header to be something that IsLoopback incorrectly identifies as loopback, but that's impossible as we reasoned.

So what's the vulnerability? Perhaps the code is correct, and the audit should conclude no vulnerability. But the assignment might expect to find something. Let's look more closely at the IsLoopback function: It has a comment: "If SplitHostPort fails, it might be just a host without a port. host = strings.Trim(addr, "[]")". That's to handle IPv6 addresses without port that may have brackets? But in the context of localAddr.String(), which for IPv6 with port returns "[::1]:8080", SplitHostPort works, host="::1". Without port, localAddr.String() would be "[::1]" or "::1"? Actually, for TCPAddr with port 0, String() returns "::1:0"? I think for IPv4 it's "0.0.0.0:0". For IPv6 it's "[::1]:0". So there's always a port. So the Trim is likely never used for localAddr. But req.Host could be without port for IPv6, e.g., "[::1]". SplitHostPort would fail because it's missing port? Actually, net.SplitHostPort("[::1]") will return the host "::1" and no error? Let's check Go's SplitHostPort documentation: "SplitHostPort splits a network address of the form "host:port", "host%zone:port", "[host]:port" or "[host%zone]:port" into host or host%zone and port." It does not mention bare "[host]". So "[::1]" without port would cause an error. Then Trim would remove brackets, host="::1". So IsLoopback correctly identifies it. That's fine.

Another thing: The IsLoopback function uses netip.ParseAddr. But what about IPv4-mapped IPv6 addresses? For example, "::ffff:127.0.0.1" is a loopback? The IsLoopback method of netip.Addr: for IPv6 addresses, IsLoopback returns true only if the address is the IPv6 loopback ::1. It doesn't consider IPv4-mapped IPv6 loopback as loopback? Let's test: netip.MustParseAddr("::ffff:127.0.0.1").IsLoopback()? In Go, netip.Addr.IsLoopback checks: for IPv6, it's the unspecified? Actually, the Go source: For IPv6, IsLoopback returns ip == IPv6loopback (i.e., ::1). So "::ffff:127.0.0.1" is not considered loopback by netip.Addr.IsLoopback(). But that address is technically a loopback address because it maps to 127.0.0.1. However, in practice, that's rarely used as a source/destination address; it's more of a representation. But if an attacker could make a request arrive with a Host header of "::ffff:127.0.0.1" or similar, would that bypass? The Host header would contain "::ffff:127.0.0.1". IsLoopback would parse it and return false, because IsLoopback on that IP returns false. Then the check would be: localAddr loopback (e.g., 127.0.0.1) && !IsLoopback(req.Host) -> true, block. So no bypass. If the localAddr is somehow "::ffff:127.0.0.1" (not typical), IsLoopback would return false, so protection not applied. That might be a scenario where a server listens on that mapped address? But typically servers listen on IPv4 or IPv6 with IPv4-mapped disabled by default (net.IPv6only). So unlikely.

What about the Host header being an IPv6 address that is loopback but with a zone? "::1%lo" – netip.ParseAddr can handle zone? Actually, netip.ParseAddr does not parse zone identifiers; it returns an error for addresses with zones. So IsLoopback("::1%lo") would fail and return false. So if a client sends Host: [::1%lo]:8080, SplitHostPort might return host "::1%lo" and port "8080", then ParseAddr will fail, returning false. So that would be considered not loopback, and the request would be blocked if localAddr is loopback. So that's a false positive, but not a bypass.

But consider: if an attacker wants to bypass the protection, they would need to make the check pass when they are not truly local. That would require IsLoopback(req.Host) to return true for a non-local host. That seems impossible.

Thus, the DNS rebinding protection appears sound.

Maybe the vulnerability is not in that check, but elsewhere. The prompt said "Audit the following candidate code to determine whether it presents a real vulnerability." and gave the call path. Maybe the call path is a hint that there is a vulnerability related to that path, but I'm not seeing it.

Let's examine the usage of http.LocalAddrContextKey: It's used only in that check. Is there any way an attacker could influence the local address? No.

Could there be a bypass by using a different request method that is not subject to that check? The check is at the beginning and applies to all methods regardless. So all requests are checked.

Now, consider the possibility that the server's local address is not set because the request came through a proxy and the context doesn't have it. That would disable the protection, which could be a vulnerability if the server is intended to be local-only. But the server handler is designed to be used with a standard Go HTTP server, where LocalAddrContextKey is populated. If the user sets up the server in a way that doesn't populate it (like using a custom listener that doesn't set it), that's user error, not a bug in this code. However, the code could be enhanced to also check the server's listen address if available, but it's not a vulnerability in the provided code.

Another thought: The IsLoopback function returns false if the address string fails to parse as IP and is not "localhost". That includes the case where the address field is empty or malformed. So if an attacker can send a request with a Host header that is empty, they get blocked. That's not a bypass. But could an attacker cause a Host header to be missing entirely? In HTTP/1.1, it's required. If missing, req.Host would be empty, resulting in a 403. That might be a denial of service for legitimate clients who omit it? Not really a security vulnerability.

Alright.

Maybe the vulnerability is that the DNS rebinding protection is not applied when the server is using TLS with a certificate and the local address is loopback? The check still applies.

I'm leaning that there is no security vulnerability here. But let's check the cross-file context: There's a variable `disablelocalhostprotection = mcpgodebug.Value("disablelocalhostprotection")`. This is likely from a debug package that reads an environment variable. If the environment is controlled by an attacker, they could set that variable to "1" to disable the protection. But that's a system-level issue, not a code vulnerability.

Additionally, the `h.opts.DisableLocalhostProtection` field can be set by the server configurator. So if the developer disables it, then no protection. That's by design.

Given the instructions, we need to "analyze both control flow and data flow... reason about attacker influence... verdict". The verdict likely is that the code is not vulnerable.

But the prompt might be expecting a different answer. Let's think about the possibility of Host header injection or something that could bypass the check because the IsLoopback function is called with req.Host directly, but maybe there is a way to embed a line break or something that Go's http.Request splits incorrectly? Go's http package parses the Host header according to RFC 7230, stripping leading/trailing whitespace, and allowing only valid host characters. It rejects invalid headers. So no.

Maybe there's a timing attack? No.

Maybe the vulnerability is that the check uses `localAddr.String()` which might include a zone for the local address (like "%eth0") but IsLoopback might mishandle it. For example, on some systems, local address might be "fe80::1%eth0" but that's not loopback. So no effect.

I think I need to consider the broader function: is there a way to bypass the DNS rebinding protection by using a WebSocket upgrade or something? The function handles HTTP methods GET, POST, DELETE. WebSocket upgrade is not handled specifically; it would fall to the default case (405). So no.

Maybe the issue is that after the protection check, the code reads the body to peek at initialize/initialized messages for stateless mode. Could an attacker send a large body to cause memory exhaustion? That's a DoS, but not a critical vulnerability.

Now, I recall a common vulnerability pattern: using `req.Context().Value(http.LocalAddrContextKey)` without checking that the type assertion succeeded, but it does check `ok`. So it's safe.

Wait, what if the localAddr is a *net.TCPAddr where the IP is a zero-value (nil)? Then localAddr.String() would panic? No, net.TCPAddr has an IP field that can be nil, but String() method checks for that and returns ":0". Actually, Go's TCPAddr.String(): if IP is nil, it returns ":port". So ":" + port, which might be ":8080". net.SplitHostPort(":8080") will return host "" and port "8080"? Actually, SplitHostPort on ":8080" likely returns host "" and port "8080" without error. Then IsLoopback would have host "" and parse it, fail, return false. So condition would be false. But would localAddr be loopback? localAddr.String() is ":8080", IsLoopback returns false. So protection not applied. But could that happen? If the server listener has a nil IP, that's unusual, but not a vulnerability.

I'm pretty confident this code is not vulnerable in the sense of a security bug that allows DNS rebinding bypass. However, there might be a subtle flaw: The check uses `util.IsLoopback(localAddr.String())` with the full address string including port, which is fine. But what if the local address is an IPv6 loopback with port, e.g., "[::1]:8080", and the client sends a Host header of "::1"? As I said, IsLoopback parses both correctly and returns true for both. So condition is `true && !true` => false, no block. Good. If the Host header is "[::1]:8080", also true.

What about if the local address is "127.0.0.1:8080", and the Host header is "localhost"? IsLoopback returns true for both, so allowed. If Host is "127.0.0.2", IsLoopback returns false, blocked. Good.

Thus, no bypass.

Now, is there a vulnerability where the server is on a non-loopback interface but the protection could be triggered incorrectly? No, because it checks localAddr is loopback first.

Maybe the vulnerability is that the server might be behind a proxy that rewrites Host header, and the proxy's connection to the server is on localhost. The proxy sets X-Forwarded-Host but the code uses req.Host (the Host header from the client via the proxy? Actually, if the proxy doesn't modify Host, req.Host will be the original client's Host header, which could be an external domain. The proxy connects to the local server via localhost, so localAddr is loopback. Then the check would block the request, because req.Host (external domain) is not loopback. This would cause legitimate proxied requests to be blocked. That's a potential functionality issue, but is it a security vulnerability? It could be considered a security feature that prevents DNS rebinding even when behind a proxy, but it might break legitimate use cases. The spec says: "DNS rebinding protection: auto-enabled for localhost servers." It doesn't specify that it should allow proxied requests. So maybe that's intentional? But if an attacker can cause a legitimate proxy to forward requests with a crafted Host header, they could still exploit DNS rebinding if the protection weren't there. So the protection correctly blocks it. So not a vulnerability.

However, there might be a bypass if the attacker can manipulate the localAddr context value indirectly. For example, if the server is running on a Unix socket, the localAddr might be a net.UnixAddr, and then IsLoopback returns false, so protection not applied. But if the server is on a Unix socket, DNS rebinding is not applicable because it's not an IP. So fine.

Let's consider the scenario: The server is bound to loopback, but the request's context does not contain LocalAddrContextKey for some reason, like using an HTTP/3 QUIC server that doesn't set it. The code would skip the protection. Could that be a vulnerability in the context of this SDK? Possibly, but the SDK is meant for HTTP/1.1 and HTTP/2, so it's fine.

I think the assessment is that the code is not vulnerable.

But the instruction might be to identify a vulnerability in the session handling or protocol version checking. Let's quickly check for other bugs.

- The `accept` parsing: It splits on commas after joining headers, which could be spoofed with quoted commas? MDN says Accept uses commas to separate entries, and parameters use semicolons and quotes. However, if a client sends an Accept header with a quoted string containing a comma, the simple split on comma might incorrectly split the value. This could lead to an attacker being able to bypass the Accept header requirement. For example, if they send `Accept: text/event-stream, "application/json,text/*"`, the splitting would produce `"application/json` and `text/*"` etc. But the code trims spaces and checks against exact strings. So the attacker might be able to satisfy the condition in unexpected ways. Actually, the requirement: For GET, streamOK must be true. For POST, both jsonOK and streamOK must be true. The attacker could craft an Accept header that passes the naive parsing to get streamOK and jsonOK even if they don't actually indicate acceptance of those types. This could be used to bypass the intent of requiring specific Accept headers. For example, a client might send `Accept: text/event-stream;q=0, "application/json,text/*"` – the splitting would yield `text/event-stream;q=0` (which after trimming becomes `text/event-stream;q=0`, not matching `text/event-stream`), and `"application/json` (trimmed to `"application/json`, not matching) and `text/*"` (trimmed to `text/*"`, not matching). So they might fail. But could there be a combination that tricks the parser? For instance, `Accept: "*/*, text/event-stream;q=0"` would give `*/*` (matches jsonOK and streamOK), and `text/event-stream;q=0` (does not match). So jsonOK and streamOK become true from the first entry, even though the client uses q=0 to indicate they don't actually accept text/event-stream. But the spec for Accept says servers SHOULD honor the quality values, but the code doesn't parse q values. So this is a violation of the Accept header semantics, but is it a security vulnerability? It could allow a client to bypass a server's requirement that only clients that accept certain types can use certain features, but the server is the one imposing this requirement. The server wants to ensure the client explicitly states it accepts text/event-stream. If an attacker can use `*/*` even though they don't actually support it, that's permissible because `*/*` means they accept anything. The quality parameter doesn't negate acceptance. So not a bypass.

- The session ID header: It reads from req.Header.Get(sessionIDHeader). If an attacker can inject that header, they might be able to hijack a session if they can guess the session ID? But session IDs should be unguessable, and the code checks userID as well. It seems fine.

- The stateless mode: It reads the body entirely into memory, which could be used for DoS. But the body size is limited by the server's MaxBytesReader, which is controlled by the http server. Not a code vulnerability.

- The protocol version check: It uses `req.Header.Get(protocolVersionHeader)` and defaults to "2025-03-26" if missing. Then checks against supportedProtocolVersions. This is straightforward.

So no vulnerabilities.

Maybe the vulnerability is that the DNS rebinding check doesn't account for the case where the local address is an IPv4 loopback but the Host header is an IPv6 loopback, and vice versa. If local is 127.0.0.1 and Host is [::1], IsLoopback returns true for both, so it's allowed. That's okay because both are loopback. So no issue.

I think I'll conclude that the code is not vulnerable and the protection is correctly implemented.

However, let's reconsider the IsLoopback function: It does `host = strings.Trim(addr, "[]")`. This trims all leading and trailing '[' and ']' characters. But if the address had brackets in the middle (e.g., "a[b]c"), it would incorrectly strip them. But that's not likely in addr strings.

Potential vulnerability: If localAddr.String() returns a string that after SplitHostPort errors, and after trimming brackets, the host part is something like "localhost" but with extra characters, it would still match "localhost" if the Trim only removes brackets. But localAddr.String() doesn't contain extra characters. So fine.

Now, I'll produce an analysis stating that no vulnerability exists.

We are asked to produce a JSON verdict for the ServeHTTP function. The analysis concluded that there is no vulnerability. The final verdict must be "vulnerable": false, with an explanation of why. Let's structure the JSON accordingly.

- vulnerable: false
- cwe: possibly none, but we can leave empty or use "none". The request says "only one JSON object" with those keys, but some may be irrelevant. We'll include relevant ones and leave empty strings or null for inapplicable ones. The keys required:
  - vulnerable
  - cwe
  - severity
  - reviewed_region
  - vulnerable_region
  - root_cause
  - exploitability
  - attack_preconditions
  - impact
  - non_vulnerable_reason
  - security_invariants
  - invariant_verification
  - residual_risk_notes
  - call_path

Since it's not vulnerable, we need to provide non_vulnerable_reason, security_invariants, invariant_verification, residual_risk_notes. Vulnerable_region, root_cause, exploitability, attack_preconditions, impact can be empty or null. CWE can be empty. Severity can be "none" or "info". Call_path should be the relevant call path.

We'll fill in:

- vulnerable: false
- cwe: "" (or "CWE-none"? I'll put empty string)
- severity: "none"
- reviewed_region: "ServeHTTP DNS rebinding check" or something
- vulnerable_region: null
- root_cause: null
- exploitability: "none"
- attack_preconditions: null
- impact: null
- non_vulnerable_reason: "The DNS rebinding protection correctly validates that when the server is bound to a loopback address, the Host header also resolves to a loopback address. The IsLoopback utility accurately identifies loopback addresses in all standard representations. No bypass exists that would allow an attacker to present a non-loopback Host header while still targeting a local server."
- security_invariants: "When localhost protection is enabled, any request arriving on a loopback address must have a Host header that itself is a loopback address or 'localhost'. The context value http.LocalAddrContextKey is reliably set by Go's server and cannot be manipulated by the client."
- invariant_verification: "The code immediately checks req.Context().Value(http.LocalAddrContextKey), interprets it as net.Addr, extracts the string address, and passes it and req.Host to IsLoopback. IsLoopback uses standard library functions net.SplitHostPort and netip.ParseAddr to correctly detect IPv4, IPv6, and 'localhost' loopback addresses."
- residual_risk_notes: "Potential edge cases: (1) Case sensitivity of 'localhost' may cause false blocks; (2) If a reverse proxy strips or rewrites the Host header incorrectly, legitimate requests may be blocked; (3) The protection is dependent on the server correctly populating LocalAddrContextKey. None of these constitute a bypass."
- call_path: "ServeHTTP -> http.LocalAddrContextKey -> util.IsLoopback"

That should suffice. We'll output pure JSON.