PYSEC-2026-2018

Summary

A memory corruption vulnerability that leading to a crash (denial-of-service) and potentially remote code execution (RCE) exists in vLLM versions 0.10.2 and later, in the Completions API endpoint. When processing user-supplied prompt embeddings, the endpoint loads serialized tensors using torch.load() without sufficient validation.

Due to a change introduced in PyTorch 2.8.0, sparse tensor integrity checks are disabled by default. As a result, maliciously crafted tensors can bypass internal bounds checks and trigger an out-of-bounds memory write during the call to to_dense(). This memory corruption can crash vLLM and potentially lead to code execution on the server hosting vLLM.

Details

A vulnerability that can lead to RCE from the completions API endpoint exists in vllm, where due to missing checks when loading user-provided tensors, an out-of-bounds write can be triggered. This happens because the default behavior of torch.load(tensor, weights_only=True) since pytorch 2.8.0 is to not perform validity checks for sparse tensors, and this needs to be enabled explicitly using the torch.sparse.check_sparse_tensor_invariants context manager.

The vulnerability is in the following code in vllm/entrypoints/renderer.py:148

    def _load_and_validate_embed(embed: bytes) -> EngineEmbedsPrompt:
        tensor = torch.load(
            io.BytesIO(pybase64.b64decode(embed, validate=True)),
            weights_only=True,
            map_location=torch.device("cpu"),
        )
        assert isinstance(tensor, torch.Tensor) and tensor.dtype in (
            torch.float32,
            torch.bfloat16,
            torch.float16,
        )
        tensor = tensor.to_dense()

Because of the missing checks, loading invalid prompt...