Affected: vllm-project vllm
vLLM is an inference and serving engine for large language models. Prior to 0.26.0, the structured_outputs.regex parameter in vllm/v1/structured_output/backend_lm_format_enforcer.py is passed to lmformatenforcer.RegexParser without compile_regex_with_timeout or validation in validate_structured_output_request_lm_format_enforcer, allowing an unauthenticated /v1/completions request against the lm-format-enforcer backend to consume a CPU core and stall the structured-output engine path with a catastrophic regular expression. This issue is fixed in version 0.26.0.
Affected: PyPI: vllm
### Summary
The fix for `GHSA-rwxx-mrjm-wc2m` ("ReDoS via `structured_outputs.regex` compiled without timeout") wrapped the regex compile in the **xgrammar** and **outlines** backends with `compile_regex_with_timeout` (and, for outlines, `validate_regex_is_buildable`). The **lm-format-enforcer** backend was left unguarded: it compiles the attacker-supplied regex with no timeout and no buildability check. A single request with a catastrophic regex hangs the structured-output compile step and stalls the engine worker (denial of service).
### Affected code (HEAD d6d39c1)
`vllm/v1/structured_output/backend_lm_format_enforcer.py`: - line 110: `character_level_parser = lmformatenforcer.RegexParser(grammar_spec)` — builds an `interegular` FSM from the attacker regex synchronously, **no timeout**. - line 155: `validate_structured_output_request_lm_format_enforcer` returns immediately on `if so_params.regex:` — **no validation**.
Sibling backends that WERE patched by GHSA-rwxx: - `backend_xgrammar.py:92` → `compile_regex_with_timeout(...)`. - `backend_outlines.py:65` → `compile_regex_with_timeout(...)` (plus `validate_regex_is_buildable`).
lm-format-enforcer uses the same `interegular` DFA-construction primitive the advisory cites for the outlines backend.
### Reproduction (runtime-verified against the sink)
The sink `lmformatenforcer.RegexParser(<regex>)` was exercised directly (this is exactly what the backend calls):
``` baseline '[0-9]{3}' -> 0.0002 s attacker…