{"id":"PYSEC-2026-3933","summary":"vLLM: ReDoS via structured_outputs.regex in the lm-format-enforcer backend (no compile timeout) — missed sibling of GHSA-rwxx-mrjm-wc2m","details":"### Summary\n\nThe 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).\n\n### Affected code (HEAD d6d39c1)\n\n`vllm/v1/structured_output/backend_lm_format_enforcer.py`:\n- line 110: `character_level_parser = lmformatenforcer.RegexParser(grammar_spec)` — builds an `interegular` FSM from the attacker regex synchronously, **no timeout**.\n- line 155: `validate_structured_output_request_lm_format_enforcer` returns immediately on `if so_params.regex:` — **no validation**.\n\nSibling backends that WERE patched by GHSA-rwxx:\n- `backend_xgrammar.py:92` → `compile_regex_with_timeout(...)`.\n- `backend_outlines.py:65` → `compile_regex_with_timeout(...)` (plus `validate_regex_is_buildable`).\n\nlm-format-enforcer uses the same `interegular` DFA-construction primitive the advisory cites for the outlines backend.\n\n### Reproduction (runtime-verified against the sink)\n\nThe sink `lmformatenforcer.RegexParser(\u003cregex\u003e)` was exercised directly (this is exactly what the backend calls):\n\n```\nbaseline  '[0-9]{3}'          -\u003e 0.0002 s\nattacker  '(a{1,300}){300}'   -\u003e DID NOT COMPLETE in 20 s (one core pegged at 100% in interegular FSM construction)\n```\n\nEnd-to-end: start `vllm serve \u003cmodel\u003e --structured-outputs-config '{\"backend\":\"lm-format-enforcer\"}'`, then `POST /v1/completions` with `{\"structured_outputs\":{\"regex\":\"(a{1,300}){300}\"}, ...}`. The request never returns; because grammar compile runs in the engine's structured-output path, concurrent requests stall = worker-level DoS. The identical request against the outlines backend is bounded by `compile_regex_with_timeout` and returns a clean error.\n\n### Impact\n\nUnauthenticated denial of service (vLLM ships with no authentication by default). One request pegs a CPU core and blocks the structured-output engine path.\n\n**Reachability precondition:** the operator must have selected `backend=lm-format-enforcer` via `--structured-outputs-config` (the default is `auto` → xgrammar). This is the same opt-in tier as the outlines backend that GHSA-rwxx already covered.\n\n### Suggested remediation\n\nRoute the lm-format-enforcer regex compile (`backend_lm_format_enforcer.py:110`) through the same `compile_regex_with_timeout` guard already applied to the xgrammar and outlines backends, and reject un-buildable / oversized patterns in `validate_structured_output_request_lm_format_enforcer`.","aliases":["CVE-2026-73556","GHSA-48jh-3gj7-fg8v"],"modified":"2026-09-10T12:15:13.663911452Z","published":"2026-09-10T09:45:00.190897Z","references":[{"type":"WEB","url":"https://github.com/vllm-project/vllm/security/advisories/GHSA-48jh-3gj7-fg8v"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-73556"},{"type":"WEB","url":"https://github.com/vllm-project/vllm/pull/47595"},{"type":"WEB","url":"https://github.com/vllm-project/vllm/commit/c9a788eedc412acceaa5112e0d44624b49841577"},{"type":"PACKAGE","url":"https://github.com/vllm-project/vllm"},{"type":"WEB","url":"https://github.com/vllm-project/vllm/releases/tag/v0.26.0"},{"type":"PACKAGE","url":"https://pypi.org/project/vllm"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-48jh-3gj7-fg8v"}],"affected":[{"package":{"name":"vllm","ecosystem":"PyPI","purl":"pkg:pypi/vllm"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.26.0"}]}],"versions":["0.0.1","0.1.0","0.1.1","0.1.2","0.1.3","0.1.4","0.1.5","0.1.6","0.1.7","0.10.0","0.10.1","0.10.1.1","0.10.2","0.11.0","0.11.1","0.11.2","0.12.0","0.13.0","0.14.0","0.14.1","0.15.0","0.15.1","0.16.0","0.17.0","0.17.1","0.18.0","0.18.1","0.19.0","0.19.1","0.2.0","0.2.1","0.2.1.post1","0.2.2","0.2.3","0.2.4","0.2.5","0.2.6","0.2.7","0.20.0","0.20.1","0.20.2","0.21.0","0.22.0","0.22.1","0.23.0","0.24.0","0.25.0","0.25.1","0.3.0","0.3.1","0.3.2","0.3.3","0.4.0","0.4.0.post1","0.4.1","0.4.2","0.4.3","0.5.0","0.5.0.post1","0.5.1","0.5.2","0.5.3","0.5.3.post1","0.5.4","0.5.5","0.6.0","0.6.1","0.6.1.post1","0.6.1.post2","0.6.2","0.6.3","0.6.3.post1","0.6.4","0.6.4.post1","0.6.5","0.6.6","0.6.6.post1","0.7.0","0.7.1","0.7.2","0.7.3","0.8.0","0.8.1","0.8.2","0.8.3","0.8.4","0.8.5","0.8.5.post1","0.9.0","0.9.0.1","0.9.1","0.9.2"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/vllm/PYSEC-2026-3933.yaml"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"}]}