vLLM: Incomplete CVE-2025-62164 remediation can be bypassed by concurrent prompt parts

## Executive Summary The follow-up protection for CVE-2025-62164 is incomplete at vLLM revision `26587f9519e22a5c4549ead7595ad9ca3229c4fd`. It wraps serialized prompt-embedding reconstruction and dense conversion in `torch.sparse.check_sparse_tensor_invariants()`, but PyTorch 2.11.0 implements that context with save/enable/restore operations over process-global state. Two prompt-embedding parts in one `/v1/chat/completions` request are gathered concurrently on the event loop's default executor. When one context exits before the other loads its tensor, it can restore the global flag to `False` while the second part remains inside its guard. In a deterministic run against hash-verified source from the affected revision, the actual target loader rejected an invalid sparse payload as a negative control. The frozen chat tracker then scheduled benign and malicious parts on distinct `asyncio_0` and `asyncio_1` threads. The benign context exited, the malicious loader observed the invariant flag disabled, and `torch.load(weights_only=True)` reconstructed indices `[[10], [10]]` for a declared shape of `[3, 3]`. The run intercepted the target's `to_dense()` call before it operated on the invalid tensor. This primary trigger requires `--enable-prompt-embeds`, which is default-off, but it does **not** require `renderer_num_workers > 1`, a multimodal model, or `--enable-mm-embeds`. API authentication is optional in the stock server: middleware is installed only when CLI or environment API keys are supplied. The lab proves bypass of the follow-up guard, invalid sparse reconstruction, and guarded-sink reachability. Crash and memory-corruption consequences are conditional on the behavior documented by the published CVE. ## Background CVE-2025-62164 / [GHSA-mrw7-hf4f-83pf](https://github.com/vllm-project/vllm/security/advisories/GHSA-mrw7-hf4f-83pf) concerns client-controlled serialized `prompt_embeds` reaching `torch.load(weights_only=True)` and an invalid sparse tensor reaching `to_dense()`. The advisory attributes memory corruption, denial of service, and potential code execution to that historical unsafe operation. The remediation chronology matters for duplicate handling: - PR [#27204](https://github.com/vllm-project/vllm/pull/27204), merge commit [`58fab50d82838d5014f4a14d991fdb9352c9c84b`](https://github.com/vllm-project/vllm/commit/58fab50d82838d5014f4a14d991fdb9352c9c84b) on 2025-10-22, introduced the default-off `enable_prompt_embeds` gate. It did not add the sparse-invariant context. - Commit [`84e23d103d3483f944780d0d42bcf0993fd27e3a`](https://github.com/vllm-project/vllm/commit/84e23d103d3483f944780d0d42bcf0993fd27e3a) on 2025-12-15, titled `additional protection for CVE-2025-62164 (#30649)`, added the process-global sparse-invariant context around load, type check, and dense conversion. - Refactor commit [`f0a1c8453ad1c664c8a04c83fe545195fcd556eb`](https://github.com/vllm-project/vllm/commit/f0a1c8453ad1c664c8a04c83fe545195fcd556eb) on 2026-01-31 moved the guarded loader into `vllm/renderers/embed_utils.py` while preserving the same context. - Chat content-part commit [`14043dfecd35dd2f12b4d51eb9fa166184a0ca0f`](https://github.com/vllm-project/vllm/commit/14043dfecd35dd2f12b4d51eb9fa166184a0ca0f) on 2026-05-01 introduced `prompt_embeds` chat parts and the concurrent one-request schedule described here. This report therefore does not present the malformed sparse payload or `to_dense()` sink as new. It reports a distinct concurrency root cause and trigger: unsynchronized save/enable/restore of the process-global follow-up guard, reachable through the later multi-part chat scheduler. The affected revision pins PyTorch 2.11.0 in `pyproject.toml:10`. ## Vulnerability Details The target's `safe_load_prompt_embeds` performs the guarded operation in `vllm/renderers/embed_utils.py:16-39`: ```python with torch.sparse.check_sparse_tensor_invariants(): tensor = torch.load( BytesIO(pybase64.b64decode(embed, validate=Tr

CVSS 4.0

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

Severity from

GitHub (reviewed advisory)

Weakness

CWE-362

Also known as

CVE-2026-73557, PYSEC-2026-3938

• github.com/vllm-project/vllm/security/advisories/GHSA-pr7f-p5mw-fc87
• nvd.nist.gov/vuln/detail/CVE-2026-73557
• github.com/vllm-project/vllm/pull/48583
• github.com/vllm-project/vllm/commit/793cf79c89d4049124e756915468ac30318f2e50
• github.com/vllm-project/vllm
• github.com/vllm-project/vllm/releases/tag/v0.26.0