vLLM: Denial of Service via Unbounded Frame Count in video/jpeg Base64 Processing

Summary

The VideoMediaIO.load_base64() method at vllm/multimodal/media/video.py:51-62 splits video/jpeg data URLs by comma to extract individual JPEG frames, but does not enforce a frame count limit. The num_frames parameter (default: 32), which is enforced by the load_bytes() code path at line 47-48, is completely bypassed in the video/jpeg base64 path. An attacker can send a single API request containing thousands of comma-separated base64-encoded JPEG frames, causing the server to decode all frames into memory and crash with OOM.

Details

Vulnerable code

# video.py:51-62
def load_base64(self, media_type: str, data: str) -> tuple[npt.NDArray, dict[str, Any]]:
    if media_type.lower() == "video/jpeg":
        load_frame = partial(self.image_io.load_base64, "image/jpeg")
        return np.stack(
            [np.asarray(load_frame(frame_data)) for frame_data in data.split(",")]
            #                                                       ^^^^^^^^^^
            # Unbounded split — no frame count limit
        ), {}
    return self.load_bytes(base64.b64decode(data))

The load_bytes() path (line 47-48) properly delegates to a video loader that respects self.num_frames (default 32). The load_base64("video/jpeg", ...) path bypasses this limit entirely — data.split(",") produces an unbounded list and every frame is decoded into a numpy array.

video/jpeg is part of vLLM's public API

video/jpeg is a vLLM-specific MIME type, not IANA-registered. However it is part of the public API surface:

• encode_video_url() at vllm/multimodal/utils.py:96-108 generates data:video/jpeg;base64,... URLs
• Official test suites at tests/entrypoints/openai/test_video.py:62 and tests/entrypoints/test_chat_utils.py:153 both use this format

Memory amplification

Each JPEG frame decodes to a full numpy array. For 640x480 RGB images, each frame is ~921 KB decoded. 5000 frames = ~4.6 GB. np.stack() then creates an additional copy. The compressed JPEG payload is small (~100 KB for 5000 frames) but decompresses to gigabytes.

Data flow

POST /v1/chat/completions
  → chat_utils.py:1434   video_url type → mm_parser.parse_video()
  → chat_utils.py:872    parse_video() → self._connector.fetch_video()
  → connector.py:295     fetch_video() → load_from_url(url, self.video_io)
  → connector.py:91      _load_data_url(): url_spec.path.split(",", 1)
                          → media_type = "video/jpeg"
                          → data = "<frame1>,<frame2>,...,<frame10000>"
  → connector.py:100     media_io.load_base64("video/jpeg", data)
  → video.py:54          data.split(",")  ← UNBOUNDED
  → video.py:55-57       all frames decoded into numpy arrays
  → video.py:56          np.stack([...])  ← massive combined array → OOM

connector.py:91 uses split(",", 1) which splits on only the first comma. All remaining commas stay in data and are later split by video.py:54.

Comparison with existing protections