Affected: vllm-project vllm

vLLM is an inference and serving engine for large language models. Prior to 0.28.0, request bodies for Chat Completions and Responses can set media_io_kwargs.video.video_backend to pynvvideocodec, and MediaConnector.fetch_video forwards that choice to VideoMediaIO even when startup configuration selected a software decoder. The engine's _reserve_mm_ipc_gpu_memory logic budgets decoder memory only from static configuration, so the request-selected VIDEO_LOADER_REGISTRY backend can create a CUDA context, decoder surfaces, and decoded-frame allocations that were not removed from the engine's KV-cache budget. An attacker able to submit video requests to a video-capable GPU deployment with PyNvVideoCodec installed can exhaust shared GPU memory, causing request failures, worker crashes, or denial of service. The first release containing the fix is version 0.28.0.

Affected: PyPI: vllm

## Summary

Current vLLM `main` lets an inference request choose the PyNvVideoCodec GPU video decoder through `media_io_kwargs.video.video_backend`, but engine GPU memory reservation is computed only from static startup configuration and `VLLM_VIDEO_LOADER_BACKEND`. If the server starts with the default OpenCV/software backend and no `--mm-ipc-gpu-memory-gb` budget, a client can still route a video request into the PyNvVideoCodec path after startup, causing frontend CUDA-context, decoder-surface, and decoded-frame GPU allocations that were not carved out of the engine KV-cache budget.

## Technical Details

The vulnerable boundary is the split between request-time media decoding choices in the API server and startup-time memory budgeting in the engine worker. Request bodies for Chat Completions and Responses expose `media_io_kwargs`, and those values are forwarded to the shared media connector. For video inputs, `MediaConnector.fetch_video()` copies `self.media_io_kwargs["video"]` into `video_io_kwargs`, only setting a model-derived backend when `video_backend` is absent. `VideoMediaIO.__init__()` then consumes `video_backend` from those kwargs and loads that backend from `VIDEO_LOADER_REGISTRY`.

The relevant request-side source path is:

```python video_io_kwargs = dict(self.media_io_kwargs.get("video", {})) if "video_backend" not in video_io_kwargs and ( video_backend := get_video_loader_backend_for_processor(video_processor) ): video_io_kwargs["video_backend"] =…