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/yt-dlp/yt-dlp
Language: Python
File: yt_dlp/utils/_utils.py
Function: prepend_extension

Call path: YoutubeDL.process_info (yt_dlp/YoutubeDL.py) → YoutubeDL._prepare_filename (yt_dlp/YoutubeDL.py) → prepend_extension (yt_dlp/utils/_utils.py) → replace_extension (yt_dlp/utils/_utils.py)

### Primary Function

```python
def _change_extension(prepend, filename, ext, expected_real_ext=None):
    name, real_ext = os.path.splitext(filename)

    if not expected_real_ext or real_ext[1:] == expected_real_ext:
        filename = name
        if prepend and real_ext:
            _UnsafeExtensionError.sanitize_extension(ext, prepend=True)
            return f'{filename}.{ext}{real_ext}'

    return f'{filename}.{_UnsafeExtensionError.sanitize_extension(ext)}'

prepend_extension = functools.partial(_change_extension, True)
replace_extension = functools.partial(_change_extension, False)
```

### Cross-File Context

[_change_extension — function — yt_dlp/utils/_utils.py:2088]
```python
def _change_extension(prepend, filename, ext, expected_real_ext=None):
    name, real_ext = os.path.splitext(filename)

    if not expected_real_ext or real_ext[1:] == expected_real_ext:
        filename = name
        if prepend and real_ext:
            _UnsafeExtensionError.sanitize_extension(ext, prepend=True)
            return f'{filename}.{ext}{real_ext}'

    return f'{filename}.{_UnsafeExtensionError.sanitize_extension(ext)}'
```

[replace_extension — callee — yt_dlp/utils/_utils.py:2101]
replace_extension = functools.partial(_change_extension, False)

[_UnsafeExtensionError — class — yt_dlp/utils/_utils.py:5041]
class _UnsafeExtensionError(Exception): """ Mitigation exception for uncommon/malicious file extensions This should be caught in YoutubeDL.py alongside a warning Ref: https://github.com/yt-dlp/yt-dlp/security/advisories/GHSA-79w7-vh3h-8g4j """ ALLOWED_EXTENSIONS = frozenset([ # internal 'description', 'json', 'meta', 'orig', 'part', 'temp', 'uncut', 'unknown_video', 'ytdl', # video *MEDIA_EXTENSIONS.video, 'avif', 'ismv', 'm2ts', 'm4s', 'mng', 'mpeg', 'qt', 'swf', 'ts', 'vp9', 'wvm', # audio *MEDIA_EXTENSIONS.audio, 'isma', 'mid', 'mpga', 'ra', # image *MEDIA_EXTENSIONS.thumbnails, 'bmp', 'gif', 'heic', 'ico', 'jng', 'jpeg', 'jxl', 'svg', 'tif', 'wbmp', # subtitle *MEDIA_EXTENSIONS.subtitles, 'dfxp', 'fs', 'ismt', 'sami', 'scc', 'ssa', 'tt', 'ttml', # others *MEDIA_EXTENSIONS.manifests, *MEDIA_EXTENSIONS.storyboards, 'desktop', 'ism', 'm3u', 'sbv', 'url', 'webloc', 'xml', ]) def __init__(self, extension, /): super().__init__(f'unsafe file extension: {extension!r}') self.extension = extension @classmethod def sanitize_extension(cls, extension, /, *, prepend=False): if '/' in extension or '\\' in extension: raise cls(extension) if not prepend: _, _, last = extension.rpartition('.') if last == 'bin': extension = last = 'unknown_video' if last.lower() not in cls.ALLOWED_EXTENSIONS: raise cls(extension) return extension

[_catch_unsafe_extension_error — decorator — yt_dlp/YoutubeDL.py:175]
def _catch_unsafe_extension_error(func): @functools.wraps(func) def wrapper(self, *args, **kwargs): try: return func(self, *args, **kwargs) except _UnsafeExtensionError as error: self.report_error( f'The extracted extension ({error.extension!r}) is unusual ' 'and will be skipped for safety reasons. ' f'If you believe this is an error{bug_reports_message(",")}') return wrapper

Control flow: The function _change_extension splits the filename using os.path.splitext to extract the base name and real extension. It checks if expected_real_ext is None or matches the actual real extension. If the condition passes, it sets filename to just the base name. When prepend=True and real_ext exists, it sanitizes the extension with prepend=True and returns the filename with the new extension prepended before the original extension. In all other cases, it sanitizes the extension without prepending and returns the filename with just the new extension. Any _UnsafeExtensionError raised during sanitization propagates up the call stack to be caught by the _catch_unsafe_extension_error decorator in YoutubeDL, which reports the error and returns early.

Data flow: The ext parameter flows through _UnsafeExtensionError.sanitize_extension() which validates for path traversal characters (/ and \), checks against ALLOWED_EXTENSIONS when prepend=False, and handles the .bin special case. The sanitized ext is then concatenated with the filename and optionally the real_ext to form the output. The filename parameter is split via os.path.splitext and only the base name portion is used in construction. The real_ext from the original filename is preserved when prepending but stripped when replacing.

Verdict: Not Vulnerable

Reviewed region:
[yt_dlp/utils/_utils.py]
```python
def _change_extension(prepend, filename, ext, expected_real_ext=None):
    name, real_ext = os.path.splitext(filename)

    if not expected_real_ext or real_ext[1:] == expected_real_ext:
        filename = name
        if prepend and real_ext:
            _UnsafeExtensionError.sanitize_extension(ext, prepend=True)
            return f'{filename}.{ext}{real_ext}'

    return f'{filename}.{_UnsafeExtensionError.sanitize_extension(ext)}'

prepend_extension = functools.partial(_change_extension, True)
replace_extension = functools.partial(_change_extension, False)
```

Why it is not vulnerable: The code enforces multiple safeguards on the ext parameter. First, sanitize_extension checks for / and \ characters and raises _UnsafeExtensionError if found, preventing path traversal sequences. Second, when prepend=False, sanitize_extension validates that the extension is in ALLOWED_EXTENSIONS, an explicit allowlist of safe file types, raising _UnsafeExtensionError otherwise. When prepend=True, while the allowlist check is skipped, path traversal prevention still applies, and the resulting filename preserves the original valid real_ext as the final extension, meaning the OS file type determination is based on the original safe extension. Any sanitization failure propagates _UnsafeExtensionError which is caught by the _catch_unsafe_extension_error decorator, causing the operation to be skipped with an error report rather than producing an unsafe filename.

Security invariants:
- Extension strings containing / or \ must trigger _UnsafeExtensionError: enforced by sanitize_extension checking 'if "/" in extension or "\\" in extension: raise cls(extension)'
- When not prepending, extensions must be in ALLOWED_EXTENSIONS: enforced by sanitize_extension checking 'if last.lower() not in cls.ALLOWED_EXTENSIONS: raise cls(extension)'
- The .bin extension must be renamed to unknown_video: enforced by sanitize_extension checking 'if last == "bin": extension = last = "unknown_video"'
- Failed sanitization must prevent filename construction: enforced by _UnsafeExtensionError propagation, caught by _catch_unsafe_extension_error decorator which returns early
- When prepending, the final extension (real_ext) must remain the original valid extension: enforced by the code structure returning f'{filename}.{ext}{real_ext}' where real_ext comes from os.path.splitext(filename)

Invariant verification:
- Path traversal character detection in extension input: holds=true. Evidence: sanitize_extension contains 'if "/" in extension or "\\" in extension: raise cls(extension)' which is called for every ext parameter usage
- Extension allowlist enforcement for replacement mode: holds=true. Evidence: sanitize_extension contains 'if not prepend: ... if last.lower() not in cls.ALLOWED_EXTENSIONS: raise cls(extension)' which validates against the ALLOWED_EXTENSIONS frozenset when prepend=False
- Safe handling of .bin extension: holds=true. Evidence: sanitize_extension contains 'if last == "bin": extension = last = "unknown_video"' which renames the dangerous extension before the allowlist check
- Error propagation prevents unsafe filename construction: holds=true. Evidence: _UnsafeExtensionError is a subclass of Exception raised during sanitization, and the _catch_unsafe_extension_error decorator catches it with 'except _UnsafeExtensionError as error: self.report_error(...) return', preventing further processing
- Original extension preserved as final extension when prepending: holds=true. Evidence: In the prepend path, the code returns f'{filename}.{ext}{real_ext}' where real_ext is extracted from 'name, real_ext = os.path.splitext(filename)' and is the original file extension

Residual risk notes: When prepend=True, the prepended extension is not validated against ALLOWED_EXTENSIONS, allowing any non-path-traversal extension to be inserted between the filename and original extension (e.g., producing 'file.webm.mp4'). This does not affect file type determination since the OS uses the final extension (real_ext from splitext). The only remaining concern is if the prepended extension could cause issues with downstream filename processing, but the original safe extension is preserved. The filename parameter itself is not validated for path components, relying instead on upstream call site validation.

Based on the provided code and context, the `prepend_extension` function does **not** present a real vulnerability.

**Reasoning:**

1. **Path Traversal Prevention**
   In the prepend path (when a real extension exists and `prepend=True`), `_change_extension` calls `_UnsafeExtensionError.sanitize_extension(ext, prepend=True)`.
   This method explicitly raises an exception if `ext` contains `/` or `\`, blocking directory traversal attempts.
   The resulting filename is constructed as `{name}.{sanitized_ext}{real_ext}`, which stays within the original directory.

2. **Final Extension Remains Unchanged**
   The prepend operation inserts an additional component *before* the original extension, but the original extension (`real_ext`) remains at the end.
   Operating systems and file associations use the **last** extension to determine file type. For example, `video.exploit.mp4` will be treated as an `.mp4` file, not as an executable or otherwise dangerous type.

3. **No Bypass of the Extension Allowlist**
   The `_UnsafeExtensionError` allowlist is only applied when `prepend=False` (i.e., in `replace_extension`). In `prepend_extension`, the prepended text does not become the final extension, so there is no need to restrict it to the allowlist—the final, type‑defining extension is inherited from the safe, already‑vetted input filename.
   An attacker who controls the `ext` parameter cannot change the final extension to an unsafe one; they can only add a prefix, which does not alter the file’s perceived type.

4. **Edge Cases Are Handled Safely**
   - When the original filename has no extension, the prepend branch is skipped and the function falls back to the full allowlist check (raising an error for disallowed extensions).
   - When `expected_real_ext` is provided and mismatches, the function also uses the allowlist‑protected path, preventing injection of dangerous extensions.
   - Sequences like `..` without slashes do not cause directory traversal; they merely add dots to the filename and are harmless.

5. **No Evidence of a Real‑World Attack Scenario**
   The only plausible risk would be if the prepended extension could trick a downstream component into treating the file as something other than its real extension, but the code shown does not include any such pipeline, and standard OS behavior prevents this. The referenced security advisory (GHSA‑79w7‑vh3h‑8g4j) is fully mitigated by the existing allowlist for the final extension; the prepend path does not circumvent that defense.

**Conclusion:**
The `prepend_extension` function correctly blocks path separators and preserves the safe final extension. No vulnerability exists in the supplied code.

We are asked to audit the candidate code to determine whether it presents a real vulnerability. The candidate function is `prepend_extension`, which is a partial of `_change_extension` with prepend=True. The primary function shown is `_change_extension`, and `prepend_extension` uses it with `prepend=True`. The cross-file context includes `_change_extension`, `replace_extension`, `_UnsafeExtensionError` class, and a decorator `_catch_unsafe_extension_error` in YoutubeDL.py. However, the call path shows: `YoutubeDL.process_info` -> `YoutubeDL._prepare_filename` -> `prepend_extension` -> `replace_extension`. Actually `prepend_extension` is used in `_prepare_filename` (maybe for adding extra extensions before the real extension?), and it internally calls `replace_extension`? That seems odd: the call path shows `prepend_extension` -> `replace_extension`. But `prepend_extension` is a partial of `_change_extension` and doesn't call `replace_extension`. Possibly the call path is just showing that both are called separately or nested? The description says "-> prepend_extension (yt_dlp/utils/_utils.py) -> replace_extension (yt_dlp/utils/_utils.py)". Actually in the provided function definition for `prepend_extension` there's no call to replace_extension. So maybe the call path is not entirely accurate. Nevertheless, we should focus on `prepend_extension` and its implementation via `_change_extension(prepend=True, ...)`.

We need to assess security. The `_UnsafeExtensionError` class is designed to prevent unsafe extensions. In `_change_extension`, when `prepend` is True and `real_ext` exists, it calls `_UnsafeExtensionError.sanitize_extension(ext, prepend=True)` and then returns `f'{filename}.{ext}{real_ext}'`. The sanitization: for prepend=True, the method checks for '/' or '\' in extension and raises if present. If not prepend (i.e., replace_extension), it also performs rpartition('.') to extract the last part and checks against allowed extensions, and also maps 'bin' to 'unknown_video'. For prepend=True, it only checks for path traversal characters ('/' or '\'). It does not check the extension against the allowed list. So if ext contains something like '..' or malicious input, could it cause path traversal? The '/' and '\' check should prevent path separators, but could something like '..' without slash still be dangerous? In the context of constructing a filename, `os.path.splitext` splits the filename into name and real_ext (including the dot), but then the return with prepend is `f'{filename}.{ext}{real_ext}'`. If ext contains a dot like '..', the resulting string could be something like `name...ext`? That doesn't cause directory traversal. For traversal, you'd need slashes or backslashes, which are caught. So the prepend path seems safe because slashes are blocked and just appending between dots doesn't create directory changes.

However, is there any vulnerability in `_change_extension` due to the condition check? The function logic:

```python
def _change_extension(prepend, filename, ext, expected_real_ext=None):
    name, real_ext = os.path.splitext(filename)

    if not expected_real_ext or real_ext[1:] == expected_real_ext:
        filename = name
        if prepend and real_ext:
            _UnsafeExtensionError.sanitize_extension(ext, prepend=True)
            return f'{filename}.{ext}{real_ext}'

    return f'{filename}.{_UnsafeExtensionError.sanitize_extension(ext)}'
```

The vulnerability might be that when prepend=True and real_ext exists and expected_real_ext is either None or matches, the function uses sanitize with prepend=True and then returns. But if expected_real_ext is provided and does NOT match real_ext, it goes to the final return line, which uses sanitize without prepend. In that case, the final return line constructs filename (which is the original filename variable, not the name variable!) and appends extension after sanitization (non-prepend). But note: In the `if not expected_real_ext or real_ext[1:] == expected_real_ext` block, they reassign `filename = name`. Outside that block, `filename` is still the original full filename (with extension). So if expected_real_ext mismatches, the function returns `f'{filename}.{_UnsafeExtensionError.sanitize_extension(ext)}'` where `filename` still has its original extension? Actually: `filename` is the parameter, they reassign `filename = name` only inside the if block. If the condition fails, they do not reassign, so `filename` remains the original string with the extension. Then they do `f'{filename}.{sanitize(ext)}'`. This would result in double extension? For example, if filename = 'video.mp4', and we prepend with ext='audio' but expected_real_ext='webm' (mismatch), the result would be 'video.mp4.audio'. That may not be intended, but is it a security issue? The sanitize for non-prepend checks the extension against allowed list, so ext must be safe.

The vulnerability might be related to the fact that `_UnsafeExtensionError.sanitize_extension` when prepend=True only blocks '/' and '\', but does not block other potentially dangerous parts like '..'? However, as reasoned, a .. without slash would just be part of the filename part between dots, so no traversal. The resulting file would be created in the same directory as intended? For example, if filename = '/path/video.mp4', ext = '..', real_ext = '.mp4', result = '/path/video...mp4'. That's a valid filename with multiple dots, no directory change. So safe.

But there might be a vulnerability due to Unicode normalization or other tricks that bypass the slash check? For example, some OS might interpret other characters as path separators? But the check is for literal slash and backslash. Could there be alternative representations? Usually not a typical threat in Python, but on Windows, maybe other separators? But Python's os.path operations typically handle forward slash and backslash, and the check catches both. So that seems fine.

Wait: The `_change_extension` function is used by both `prepend_extension` and `replace_extension`. The vulnerability might be in the `replace_extension` when called with an unsafe extension. But we are auditing `prepend_extension`. The call path suggests `prepend_extension` is called from `_prepare_filename`, which might involve user-controlled data for `ext`. The `ext` parameter in `prepend_extension` is whatever is passed; could it be attacker-controlled? In context of yt-dlp, the extension could come from the website or extracted metadata, which might be manipulated. So we need to consider if an attacker can provide an `ext` that bypasses sanitization to cause security issues (like writing arbitrary file, LFI, RCE via crafted filename, etc.). The allowed list in the `_UnsafeExtensionError.sanitize_extension` is relatively strict, but only applied in non-prepend path. In prepend path, only slash/backslash are blocked. That means an attacker could provide an extension like 'exe' or 'sh' or something that, when prepended before the real extension, results in a dangerous file? For example, filename='video.mp4', ext='exe.' (with a trailing dot?) Then result = 'video.exe..mp4'? Actually the format is `{filename}.{ext}{real_ext}`. `real_ext` includes the dot e.g. '.mp4'. So if ext='exe.', then result = 'video.exe..mp4'. The double dot is fine. If ext='exe' (no dot), result = 'video.exe.mp4'. That would be a file named 'video.exe.mp4', which on Windows might be dangerous if the OS recognized the .exe extension? Typically Windows looks at the last extension, so .mp4 would be the extension, not .exe. But there could be cases where the .exe is treated as a double extension and might be executed if associated? For instance, a file named 'video.exe.mp4' might be opened by the media player associated with .mp4, not executed. So unlikely RCE.

However, there is a known security issue in yt-dlp related to filename extension handling: GHSA-79w7-vh3h-8g4j (referenced in the code comment for `_UnsafeExtensionError`). That advisory likely dealt with unsafe extensions being used to write arbitrary files or scripts. The mitigation includes an allowlist of extensions. But the prepend feature might not have the same restriction, which could be a bypass: An attacker could use `prepend_extension` to inject a dangerous extension in the middle, and the final real extension might be something allowed but the OS might still interpret the prepended part as the extension due to how some programs or shells handle double extensions? For example, if the prepended extension is '.exe' and the real ext is '.mp4', the file might be named 'video.exe.mp4'. If a user double-clicks it, Windows might treat it as an executable if the .exe is recognized as the executable extension despite the .mp4? Actually Windows uses the last extension to determine file type, but there is a known trick with double extensions to disguise executables: e.g., 'file.txt.exe' is an executable because the last .exe is the real extension. But if it's 'file.exe.txt', Windows will show the .txt extension and may open with notepad but still the file could be an executable if the content is executable? No, file type is associated with the extension .txt. So RCE unlikely. However, if the real_ext is empty? For example, filename has no extension. Then `real_ext` is empty string. In the prepend branch, the condition `if prepend and real_ext:` checks for truthiness of real_ext. If real_ext is empty (falsy), it does not enter that branch, instead it falls through to the final return line `return f'{filename}.{_UnsafeExtensionError.sanitize_extension(ext)}'` (with no prepend, and now the filename variable is still the original filename, which is ? wait, it reassigns filename = name only if the condition `if not expected_real_ext or real_ext[1:] == expected_real_ext:` is true. If real_ext is empty, real_ext[1:] is empty string, so if expected_real_ext is None or empty, the condition is true? expected_real_ext default is None, so condition true. Then filename = name (name is the filename without extension, which is the whole filename). Then if prepend and real_ext: real_ext falsy, so skips. Then falls through to final return line? Actually the structure is:

```python
if not expected_real_ext or real_ext[1:] == expected_real_ext:
    filename = name
    if prepend and real_ext:
        ... return ...
    # no return here, falls through to the end
return f'{filename}.{_UnsafeExtensionError.sanitize_extension(ext)}'
```

So when expected_real_ext matches (or None) and prepend is True but real_ext is empty, it does nothing inside the if block, then goes to the final return, which uses the non-prepend sanitization. So the extension will be sanitized with the full allowed list check. That seems safe.

But what if real_ext is something like '.exe' and we prepend with 'txt'? The prepend branch will be taken: `prepend and real_ext` true, so it calls `sanitize_extension(ext, prepend=True)` which only blocks slashes. 'txt' passes, returns 'txt', result = 'name.txt.exe'. That's a double extension. If the OS treats the last .exe as the real extension, the file is executable. The real_ext is from the original filename, which could be something like '.exe' if the user requested a format or output template that includes .exe? Typically, yt-dlp controls the extension based on the format selected. The original filename's real_ext comes from the output template or the media's extension, which is usually a safe media extension. But could an attacker influence the real_ext to be something dangerous? The `_prepare_filename` function likely uses `prepend_extension` to add an extra extension before the existing one, e.g., to insert a language code or format id. The `filename` parameter is presumably the output path, and `ext` is something like 'f248' or 'en'. The attacker might control the output template? The output template is usually user-controlled, but the extension might be derived from the template. However, in the `_prepare_filename` call path, `filename` is a path that includes an extension determined by the format? The attack scenario might be that an attacker provides a malicious `ext` argument to `prepend_extension` that contains path traversal or an unsafe extension that bypasses the allowlist, and then the resulting filename is used to open a file for writing. Since the prepend path only blocks slashes, an attacker could provide an `ext` like '..' or something else? But without slash, no traversal. Could they provide an extension that includes a null byte or something? The code doesn't seem to sanitize for null bytes, but Python strings are null-safe; os.path operations typically don't interpret null bytes, and writing a file with null byte in name will either raise an error or truncate on some systems? Not a typical vulnerability.

I recall that the referenced advisory GHSA-79w7-vh3h-8g4j was about arbitrary file write via unsafe extension. The mitigation added `_UnsafeExtensionError` with an allowlist. The prepend case might be an oversight: if prepend=True, the extension is only checked for slashes, but not allowed list. So an attacker could inject an extension that is not in the allowed list, and it could be a malicious extension like 'desktop'? But 'desktop' is in the allowed list anyway. The concern is that the attacker could use an extension that is dangerous on some OS, like 'lnk', 'url', 'deskthemepack', etc. The allowed list is quite extensive and includes many. The prepend path doesn't enforce the allowed list, so an attacker could use any extension, e.g., '.exe' or '.sh' or something that, when combined with the real extension, might still cause execution under certain conditions? Or they could use a long extension with special characters that bypass slash detection but cause buffer overflow or something? Unlikely.

The real vulnerability might be that the `prepend_extension` feature could be used to bypass the allowlist for the *final* extension? But the final real extension remains whatever it was originally, and the prepended part is not the final extension. The file after prepending has the same final extension. So the OS would still use the original real_ext to determine file type. So injecting a dangerous extension in the middle is not a direct RCE. However, there are scenarios where the file could be interpreted based on the full name, e.g., a file named 'video.php.mp4' might be executed as PHP if uploaded to a server that misconfigures? But that's not a client-side threat for a media downloader. The downloader will save the file locally. On the local filesystem, 'video.php.mp4' is just an mp4 file. No RCE.

But there might be a path traversal issue if `ext` contains "/" or "\", which is covered. What about if `ext` contains ".." and the real_ext is empty? That would bypass? If real_ext is empty, prepend path is not taken, it falls to the final return with sanitize (non-prepend). That sanitize does the allowlist check and also checks for slashes. ".." doesn't contain slashes, but does it pass the allowlist? The extension ".." would not be in the allowed set, so it would raise _UnsafeExtensionError, which is caught by the decorator. So that is safe. So prepend extension only poses a risk if real_ext is non-empty, and the attacker provides an ext that contains path traversal but no slashes? For directory traversal, you need slashes, so not possible.

Could there be a case where the `filename` argument contains directory parts, and the `prepend_extension` operation inadvertently causes writing to a different directory due to the dot handling? Unlikely.

Wait, there's a subtlety: `os.path.splitext(filename)` splits on the last dot. The `name` is everything before the last dot, `real_ext` is the last dot and extension. In the prepend branch, they do `f'{filename}.{ext}{real_ext}'`. But `filename` was reassigned to `name` (so without the original extension). So resulting path is `{name}.{ext}{real_ext}`. That's safe. However, if `filename` contained multiple dots, name could still contain dots. For example, filename = '/path/archive.tar.gz', splitext gives name='/path/archive.tar', real_ext='.gz'. prepend with ext='bak' results in '/path/archive.tar.bak.gz'. This seems fine.

So I'm leaning towards no vulnerability. But the prompt says "This sample may come from a noisy retrieval or transformation stage, so reason from the supplied code and context rather than assuming the label." It might be a false positive. However, I should consider that the vulnerability might be that `prepend_extension` doesn't sanitize the `ext` with the allowlist, and the `ext` could be something like '.' (a dot) which, when used in `f'{filename}.{ext}{real_ext}'` could result in 'video..mp4' which is harmless, but could it be used to truncate the filename? For instance, ext = '' (empty). Then result = 'video..mp4'? Actually f'{name}.{ext}{real_ext}' with ext='' gives 'video..mp4'. That's a double dot but still valid. No security issue.

Maybe the vulnerability is that `prepend_extension` could be used to create a filename with a literal new line or control character that might be misinterpreted by the shell if the filename is passed to a command. But that's not a vulnerability in yt-dlp itself; that's a user's responsibility. The extension sanitization mainly prevents writing to files with dangerous extensions that could be executed. The prepend feature might allow writing a file that, when later opened by the user, could cause harm? For example, if the file is named "video.lnk.mp4", Windows might treat it as a shortcut file? Actually .lnk is the extension for shortcuts, but the final extension is .mp4, so Windows will treat it as .mp4. The .lnk part is just part of the name. So not a shortcut. However, there is a known trick where you can have a file like "file.txt.lnk" but it's really a .lnk. Here we have .lnk.mp4, which is .mp4, not .lnk. So no.

What about if the real_ext is something like '.desktop' which is a recognized desktop entry file on Linux? The prepend might add something before it, still ending with .desktop, still a desktop file. But an attacker could already create a .desktop file if they control the output template? The prepend extension doesn't change the final extension, so it doesn't increase the risk of creating a malicious .desktop file beyond what the user could already do by choosing that extension. So no.

Thus, the lack of allowlist check in prepend mode does not seem to introduce a vulnerability. The security boundary is that yt-dlp should not write files with extensions that could cause unintended execution when the user opens them, given that the extension can be attacker-influenced. With prepend, the final extension is still controlled by the template/format and is supposed to be safe (since it's from allowed media types). The prepended part is just an additional identifier, not the final extension. So allowing arbitrary characters there is okay as long as path traversal is prevented.

However, I need to check the call path: `YoutubeDL.process_info` -> `YoutubeDL._prepare_filename` -> `prepend_extension` -> `replace_extension`. Wait, `prepend_extension` does not call `replace_extension`; maybe the call path is showing that `_prepare_filename` uses `prepend_extension` and then later `replace_extension`. But the chain might be misrepresented. The candidate code includes `prepend_extension` and `replace_extension` definitions. The `_change_extension` function is the underlying implementation. The call path may be incomplete. However, the candidate function is `prepend_extension`. The task: "Audit the following candidate code to determine whether it presents a real vulnerability." So we judge `prepend_extension` itself and its behavior in context.

Could there be a vulnerability in the interaction between `prepend_extension` and `replace_extension`? For example, if `_prepare_filename` first calls `prepend_extension` then later `replace_extension`, the result might bypass allowlist because the intermediate filename has a prepended extension that is unsafe? But the code shown doesn't include that logic. We can only reason from the supplied code and context.

Maybe the vulnerability is related to the `_UnsafeExtensionError.sanitize_extension` method when prepend=True, it only checks for slashes, but what about backslash? It checks for both '\\' and '/'. That's okay. It does not check for '.' or '..', but as argued, that's not traversal. However, on Windows, a filename could be something like 'nul', 'con', 'aux' etc which are reserved device names. Could `ext` be set to "con" or "nul" and cause issues? The prepend path would produce 'video.con.mp4', which is not a reserved name because it's not exactly "con". The reserved names must match exactly without extension. So not a problem.

What about if `ext` contains a colon? On Windows, colons are forbidden in filenames except for drive letters. But the sanitize doesn't block colon. If ext='foo:bar', the result would be 'name.foo:bar.mp4'. That would cause an error when trying to create the file, not a security issue. Not a vulnerability.

What about double dots like '..' with prepend? That would be 'name...mp4' – safe.

Could there be a case where `ext` is taken from an untrusted source (e.g., a website's mime type or something) and is used as `ext` without validation, and an attacker can supply a string that includes path separators that bypass the slash check due to encoding? The check is simple string contains '/' or '\'. If the input is unicode, the check still works. But there are alternative path separators? On Unix, only '/' matters. On Windows, both '/' and '\' matter, and also '\\?'? Not really. So it's safe.

Therefore, it seems there is no vulnerability.

But before concluding, let's double-check the `_change_extension` function's logic carefully for any edge cases that might lead to directory traversal. Look at the use of `filename` variable reassignment:

```python
def _change_extension(prepend, filename, ext, expected_real_ext=None):
    name, real_ext = os.path.splitext(filename)

    if not expected_real_ext or real_ext[1:] == expected_real_ext:
        filename = name
        if prepend and real_ext:
            _UnsafeExtensionError.sanitize_extension(ext, prepend=True)
            return f'{filename}.{ext}{real_ext}'

    return f'{filename}.{_UnsafeExtensionError.sanitize_extension(ext)}'
```

Suppose `prepend` is True, `real_ext` is not empty, and `expected_real_ext` condition is true (or None). It returns with prepend. The `filename` on the return line is the `name` (without original extension). So final path is `name + '.' + ext + real_ext`. That's safe.

If `prepend` is True, but `real_ext` is empty (e.g., filename has no extension), the condition `if prepend and real_ext:` fails, so it does not return early. The function continues to the final return line with `filename` still being the `name` (since it was reassigned). But wait: the assignment `filename = name` happened inside the if block, so `filename` is now the name (original without extension). So the final return uses `filename` (which is name) and `sanitize(ext)` (non-prepend). That results in `name + '.' + sanitized_ext`. That is safe because the sanitization includes allowlist.

If `expected_real_ext` is provided and does NOT match `real_ext[1:]`, then the if block is skipped entirely, `filename` remains the original full filename. Then final return uses the full filename and appends sanitized ext. That leads to double extension, but still safe because sanization is done.

But what if `prepend` is True and `real_ext` exists, but `expected_real_ext` is provided and mismatches? Then we go to final return, which uses original filename (with extension) and sanitize (non-prepend). The result is `filename + '.' + ext`, i.e., double extension. For example, filename='video.mp4', ext='audio', expected_real_ext='webm' mismatches -> result='video.mp4.audio'. The extension 'audio' would be checked against allowed list; if it's not allowed, error is raised. So safe.

Now, is there any path traversal possible if `name` contains directory parts? `name` is derived from `filename` by splitting on last dot. So `name` can contain slashes. The prepend operation preserves the directory structure because it just modifies the last part. For example, `filename = '/path/video.mp4'`, name = '/path/video', real_ext='.mp4'. The result is '/path/video.{ext}.mp4'. That's still within the same directory. No directory change.

What if `ext` is something like ".." but real_ext is empty and expected_real_ext mismatch? That would cause the final return with sanitize, which would raise error because ".." is not allowed. So no.

What if `filename` itself contains ".."? That's a different attack vector, but `prepend_extension` is not responsible for sanitizing the filename; it assumes the filename is already safe. The caller should have sanitized it earlier.

Thus, no vulnerability found.

However, the prompt might be testing for missing sanitization for `..` in prepend mode, but we argue that `..` without slashes is harmless. I should verify: Could `..` be used to cause a directory traversal by moving up a directory? For that, you need a slash before or after the `..`. For example, if ext = ".." and real_ext = ".mp4", result = "name...mp4". That's just a file with triple dots. Not traversal. If ext = ".." and real_ext is empty, but prepend not triggered, final return would raise error. If ext = "../" would contain slash, caught. So no traversal.

But wait, what if `real_ext` itself contains a slash? That would require the original filename to have a slash in the extension, which is unlikely because `os.path.splitext` only splits on the last dot, so if there's a slash after that, it's not possible; a filename with a slash before the extension would have been split into directory and base name at the directory level. For instance, filename='/path/video.txt/mp4' - this has a slash in what might be considered the extension? Actually, the path would be interpreted as directory '/path/video.txt/' and file 'mp4'? Or as '/path/video.txt/mp4' which has no extension after last dot? It's messy. But yt-dlp would not normally produce such filenames. The point is that `real_ext` from splitext will not contain a directory separator because the split is on the last '.' in the final component after the last separator? Actually, `os.path.splitext('/path/video.txt/mp4')` would give name='/path/video.txt/mp4', real_ext='' because there's no dot in the last component. So real_ext cannot contain a separator unless the filename has a dot in a directory name? For example, filename='/path/video.dir/file.mp4' -> name='/path/video.dir/file', real_ext='.mp4'. So real_ext is purely the final dot and letters. So safe.

Therefore, no vulnerability in `prepend_extension`.

But maybe the vulnerability is that `_UnsafeExtensionError.sanitize_extension` with `prepend=True` doesn't check the extension against the allowlist, and an attacker could provide an extension like "json" which is allowed anyway? All allowed extensions are safe. The risk would be if they provide an extension that is not allowed, but the file's real extension might be something that, combined with the malicious prepended part, could trick the system? Not plausible.

The advisory GHSA-79w7-vh3h-8g4j likely addressed an issue where the final extension could be controlled by an attacker and the allowlist was introduced to prevent dangerous extensions. The prepend feature might have been overlooked and could allow an attacker to control the middle part, but that doesn't change the final extension, so it doesn't introduce an executable file. However, could it be used to create a file with a name that includes HTML or script code that could cause XSS if the file index is viewed? Unlikely.

I will conclude that the code is not vulnerable.

But let's think about the call path: `YoutubeDL.process_info` -> `YoutubeDL._prepare_filename` -> `prepend_extension` -> `replace_extension`. Maybe `prepend_extension` is used to add a temporary extension like ".part" before the real extension, and then later `replace_extension` is used to change it. However, the chain shown might be that `_prepare_filename` calls `prepend_extension`, and inside that, it might call `replace_extension`? The code I see doesn't show that. Could there be a version where `prepend_extension` calls `replace_extension` internally? Not in the provided code. So maybe the call path is mistaken.

Given the provided context, I must base my assessment solely on the candidate function and the relevant code shown. It doesn't show a vulnerability. I'll write an analysis stating that no vulnerability exists, with reasoning about the sanitization, path traversal checks, and overall safety.

However, the prompt might be expecting me to identify that `prepend_extension` does not validate the extension against the allowlist, which could be a bypass if the real_ext is something like empty and expected_real_ext is used to force the non-prepend path? But I need to re-evaluate: if an attacker can control `ext` and wants to write a file with a dangerous extension, they could use `replace_extension` directly. The vulnerability in `prepend_extension` would be if it could be used to achieve something that `replace_extension` cannot. Since `prepend_extension` always keeps the original extension, it cannot change the final extension to something dangerous. So it's not a bypass.

Thus, no vulnerability.

But wait: there was a known CVE or GHSA about this? The comment says: "Mitigation exception for uncommon/malicious file extensions ... Ref: https://github.com/yt-dlp/yt-dlp/security/advisories/GHSA-79w7-vh3h-8g4j" That advisory was about unsafe extensions leading to arbitrary file write and possibly RCE. The mitigation added the allowlist. The fact that `prepend_extension` does not apply the allowlist might be considered a bypass if the prepend extension could result in the file having a dangerous extension *effectively*. For example, some software, when handling double extensions, might ignore the last extension and use the one before as the real one if the last is unknown? That's not standard OS behavior. On Windows, the file type is determined by the last extension. On Linux, it's based on MIME type detection from content, not name. So not an issue.

What about the possibility of writing a file with ".bin" extension that is actually an executable? The allowed list maps "bin" to "unknown_video" in non-prepend, but in prepend mode, "bin" would be allowed as a prepended extension, and the real extension would remain something like ".mp4". Result: "video.bin.mp4". The file would be treated as mp4. So no.

Thus, I think it's safe.

But the prompt might be a trick: the function definition is `prepend_extension = functools.partial(_change_extension, True)` and `replace_extension = functools.partial(_change_extension, False)`. They are used in the codebase. The vulnerability could be that `prepend_extension` and `replace_extension` are both defined but `prepend_extension` does not call `_UnsafeExtensionError.sanitize_extension` with the full allowlist, and this could allow a malicious extension to be inserted into the filename, which, when later processed by another part of the code, could cause an issue. For instance, if a later step does a `replace_extension` on that filename, it might remove the original real_ext and replace it with something else, and the malicious prepended part could then become the extension. But that would require the code to do something like `replace_extension(filename, new_ext)`, which would split on the last dot. With a prepended extension like "exe.mp4", the last extension is ".mp4", so `replace_extension` would replace ".mp4" with new_ext, leaving "video.exe.new_ext". That could be dangerous if new_ext is not sanitized? But new_ext would be sanitized by `replace_extension` because it's non-prepend. So the final extension would be safe. So still safe.

Thus, no vulnerability.

I'll produce a detailed analysis explaining that the prepend extension function properly sanitizes path traversal characters and, while it doesn't restrict the prepended extension to an allowlist, the resulting file's final extension remains the original one, which is expected to be safe. Therefore, no vulnerability.

We need to output a JSON verdict. Based on the analysis, the function is not vulnerable. So "vulnerable" should be false. The other keys like cwe, severity, root_cause, exploitability, attack_preconditions, impact can be set to null or empty, but we need to provide the non_vulnerable_reason and security_invariants etc. According to the instruction: when code is not vulnerable, explain concrete invariants and guards that prevent the suspected issue, and note any residual uncertainty. So we'll fill relevant fields. The call path is given: YoutubeDL.process_info -> YoutubeDL._prepare_filename -> prepend_extension -> replace_extension. Actually the call path says prepend_extension but then mentions replace_extension as callee - that's fine. We'll note that the path uses prepend_extension which calls _change_extension with prepend=True.

I need to output strict JSON:

{
  "vulnerable": false,
  "cwe": null,
  "severity": null,
  "reviewed_region": "Function prepend_extension (partially applied _change_extension with prepend=True)",
  "vulnerable_region": null,
  "root_cause": null,
  "exploitability": null,
  "attack_preconditions": null,
  "impact": null,
  "non_vulnerable_reason": "The prepend operation only inserts an additional extension before the original final extension. The final extension remains unchanged and is not controlled by the attacker; it comes from the original filename. Path separators '/' and '\\' are blocked by _UnsafeExtensionError.sanitize_extension, preventing directory traversal. The final file's type is determined by the original extension, which is already trusted. No arbitrary extension can be set as the final one, and no bypass of the extension allowlist is possible in the prepend path.",
  "security_invariants": ["The original filename extension (real_ext) always remains the final extension.", "The prepended extension cannot contain '/' or '\\'.", "The final file type is never determined by the attacker-supplied ext parameter in prepend mode."],
  "invariant_verification": "Trace: _change_extension prepend=True -> if prepend and real_ext -> sanitize_extension(ext, prepend=True) raises on path separators; then returns f'{filename}.{ext}{real_ext}', preserving real_ext at end. When real_ext is empty, prepend branch not taken, fallback to replace-like behavior with full allowlist check.",
  "residual_risk_notes": "The security advisory GHSA-79w7-vh3h-8g4j is fully mitigated for the prepend path. No residual risk identified with this function as presented.",
  "call_path": "YoutubeDL.process_info -> YoutubeDL._prepare_filename -> prepend_extension -> _change_extension"
}

That should be correct.