vLLM: DoS via unbounded chat_template Jinja2 rendering exhausts the renderer thread pool

Summary

vLLM renders chat templates by passing the resolved Jinja2 source to tokenizer.apply_chat_template(), which compiles and .render()s it in an ImmutableSandboxedEnvironment. The sandbox prevents code-execution escapes but imposes no wall-clock timeout, no output-size cap, and no nested-loop or binop limit. The render runs in a ThreadPoolExecutor whose max_workers defaults to 1, and CPython worker threads cannot be interrupted. An authenticated API client (or a malicious model repo loaded with trust_remote_code=False) can therefore submit a single short template and permanently wedge all chat-style endpoints (/v1/chat/completions, /v1/responses, /tokenize, /pooling-chat, /invocations-chat) for every tenant until the process is restarted. A string-multiplication variant instead OOM-kills the API-server process. Because of a separately-reported (GHSA-jmjw-q4x3-hmc2) gate omission on /v1/responses, the remote authenticated attacker vector works in default vllm serve configuration with no operator opt-in.

Impact

A single authenticated request of <100 bytes permanently denies all chat-style inference (/v1/chat/completions, /v1/responses, /tokenize, /pooling-chat, /invocations-chat) to every tenant of the API-server replica until the operator restarts the process. The string-multiplication variant OOM-kills the process. In a multi-tenant deployment this is a cross-tenant denial-of-service from any one tenant. From the malicious-model angle, a model repo whose tokenizer_config.json carries the hostile template wedges every replica that loads it on the first legitimate chat request, with trust_remote_code=False.

Preconditions: any non-harmony model (model_type != gpt_oss); renderer_num_workers at its default of 1; for the default-config authenticated API attacker vector, --chat-template must be unset (the default because most HF models ship their own). Blast radius: API-server process, per replica. Engine/worker processes unaffected by the CPU variant (event loop survives); affected by the memory variant (process OOM). Persistence: until process restart. Survives client disconnect.

This advisory was fixed in MR50300.