CVE-2026-34753: vLLM affected by Server-Side Request Forgery (SSRF) in `download_bytes_from_url `

Summary

A Server Side Request Forgery (SSRF) vulnerability in downloadbytesfromurl allows any actor who can control batch input JSON to make the vLLM batch runner issue arbitrary HTTP/HTTPS requests from the server, without any URL validation or domain restrictions.

This can be used to target internal services (e.g. cloud metadata endpoints or internal HTTP APIs) reachable from the vLLM host.

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Details

Vulnerable component

The vulnerable logic is in the batch runner entrypoint vllm/entrypoints/openai/runbatch.py, function downloadbytesfromurl:

runbatch.py Lines 442-482 async def downloadbytesfromurl(url: str) -> bytes: """ Download data from a URL or decode from a data URL.

Args: url: Either an HTTP/HTTPS URL or a data URL (data:...;base64,...)

Returns: Data as bytes """ parsed = urlparse(url)

# Handle data URLs (base64 encoded) if parsed.scheme == "data": # Format: data:...;base64,<base64data> if "," in url: header, data = url.split(",", 1) if "base64" in header: return base64.b64decode(data) else: raise ValueError(f"Unsupported data URL encoding: {header}") else: raise ValueError(f"Invalid data URL format: {url}")

# Handle HTTP/HTTPS URLs elif parsed.scheme in ("http", "https"): async with ( aiohttp.ClientSession() as session, session.get(url) as resp, ): if resp.status != 200: raise Exception( f"Failed to download data from URL: {url}. Status: {resp.status}" ) return await resp.read()

else: raise ValueError( f"Unsupported URL scheme: {parsed.scheme}. " "Supported schemes: http, https, data" )

Key properties:

- The function only parses the URL to dispatch on the scheme (data, http, https). - For http / https, it directly calls session.get(url) on the provided string. - There is no validation of: - hostname or IP address, - whether the target is internal or external, - port number, - path, query, or redirect target. - This is in contrast to the multimodal media path (MediaConnector), which implements an explicit domain allowlist. downloadbytesfromurl does not reuse that protection.

URL controllability

The url argument is fully controlled by batch input JSON via the fileurl field of BatchTranscriptionRequest / BatchTranslationRequest.

1. Batch request body type:

runbatch.py Line 67-80 class BatchTranscriptionRequest(TranscriptionRequest): """ Batch transcription request that uses fileurl instead of file.

This class extends TranscriptionRequest but replaces the file field with fileurl to support batch processing from audio files written in JSON format. """

fileurl: str = Field( ..., description=( "Either a URL of the audio or a data URL with base64 encoded audio data. " ), )

runbatch.py Line 98-111 class BatchTranslationRequest(TranslationRequest): """ Batch translation request that uses fileurl instead of file.

This class extends TranslationRequest but replaces the file field with fileurl to support batch processing from audio files written in JSON format. """

fileurl: str = Field( ..., description=( "Either a URL of the audio or a data URL with base64 encoded audio data. " ), )

There is no restriction on the domain, IP, or port of fileurl in these models.

1. Batch input is parsed directly from the batch file:

runbatch.py Line 139-179 class BatchRequestInput(OpenAIBaseModel): ... url: str body: BatchRequestInputBody @fieldvalidator("body", mode="plain") @classmethod def checktypeforurl(cls, value: Any, info: ValidationInfo): url: str = info.data["url"] ... if url == "/v1/audio/transcriptions": return BatchTranscriptionRequest.modelvalidate(value) if url == "/v1/audio/translations": return BatchTranslationRequest.modelvalidate(value)

runbatch.py Line 770-781 logger.info("Reading batch from %s...", args.inputfile)

# Submit all requests in the file to the engine "concurrently". responsefutures: list[Awaitable[BatchRequestOutput]] = [] for requestjson in (await readfile(args.inputfile)).strip().split("\n"): # Skip empty lines. requestjson = requestjson.strip() if not requestjson: continue

request = BatchRequestInput.modelvalidatejson(requestjson)

The batch runner reads each line of the input file (args.inputfile), parses it as JSON, and constructs a BatchTranscriptionRequest / BatchTranslationRequest. Whatever fileurl appears in that JSON line becomes batchrequestbody.fileurl.

1. fileurl is passed directly into downloadbytesfromurl:

runbatch.py Line 610-623 def wrapper(handlerfn: Callable): async def transcriptionwrapper( batchrequestbody: (BatchTranscriptionRequest | BatchTranslationRequest), ) -> ( TranscriptionResponse | TranscriptionResponseVerbose | TranslationResponse | TranslationResponseVerbose | ErrorResponse ): try: # Download data from URL audiodata = await downloadbytesfromurl(batchrequestbody.fileurl)

So the data flow is:

1. Attacker supplies JSON line in the batch input file with arbitrary body.fileurl. 2. BatchRequestInput / BatchTranscriptionRequest / BatchTranslationRequest parse that JSON and store fileurl verbatim. 3. maketranscriptionwrapper calls downloadbytesfromurl(batchrequestbody.fileurl). 4. downloadbytesfromurl’s HTTP/HTTPS branch issues aiohttp.ClientSession().get(url) to that attacker-controlled URL with no further validation.

This is a classic SSRF pattern: a server-side component makes arbitrary HTTP requests to a URL string taken from untrusted input.

Comparison with safer code

The project already contains a safer URL-handling path for multimodal media in vllm/multimodal/media/connector.py, which demonstrates the intent to mitigate SSRF via domain allowlists and URL normalization:

connector.py Lines 169-189 def loadfromurl( self, url: str, mediaio: MediaIO[M], , fetchtimeout: int | None = None, ) -> M: # type: ignore[type-var] urlspec = parseurl(url)

if urlspec.scheme and urlspec.scheme.startswith("http"): self.asserturlinallowedmediadomains(urlspec)

connection = self.connection data = connection.getbytes( urlspec.url, timeout=fetchtimeout, allowredirects=envs.VLLMMEDIAURLALLOWREDIRECTS, )

return mediaio.loadbytes(data)

and:

connector.py Lines 158-167 def asserturlinallowedmediadomains(self, urlspec: Url) -> None: if ( self.allowedmediadomains and urlspec.hostname not in self.allowedmediadomains ): raise ValueError( f"The URL must be from one of the allowed domains: " f"{self.allowedmediadomains}. Input URL domain: " f"{urlspec.hostname}" )

downloadbytesfromurl does not reuse this allowlist or any equivalent validation, even though it also fetches user-provided URLs.