{"id":"CVE-2026-55574","aliases":["GHSA-rwxx-mrjm-wc2m","PYSEC-2026-2304"],"url":"https://o3.security/vulnerability/CVE-2026-55574","summary":"vLLM: ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends","details":"## Summary\n\nThe `structured_outputs.regex` API parameter passes a user-supplied regex string directly to grammar compiler backends with no compilation timeout. In the xgrammar backend, the string reaches `compile_regex()` with no guard. In the outlines backend, `validate_regex_is_buildable()` blocks structural issues (lookarounds, backreferences) but provides zero protection against exponential DFA state-space explosion. Patterns like `(a+)+b` pass all checks and hang the inference worker.\n\n## Root Cause\n\n`backend_xgrammar.py:91` — no timeout:\n```python\nctx = self.compiler.compile_regex(grammar_spec)\n```\n\n`backend_outlines.py:299–330` — structural checks only, no complexity analysis:\n```python\ndef validate_regex_is_buildable(regex: str) -> None:\n    sre_parse.parse(regex)   # AST parse only — does not detect exponential patterns\n    _check_unsupported(...)  # blocks lookarounds/backrefs, not nested quantifiers\n```\n\n`backend_outlines.py:64` — no timeout:\n```python\noc.Index(regex_string, vocabulary.inner)\n```\n\n## Impact\n\nDenial of service — one request with an adversarial regex pattern hangs an inference worker indefinitely.\n\n## Remediation\n\nWrap `compile_regex()` and `oc.Index()` calls in a thread with a deadline (e.g., 5 seconds). Add complexity analysis to `validate_regex_is_buildable()` to detect nested quantifier patterns before compilation.","published":"2026-07-06T20:05:31.003Z","modified":"2026-08-24T18:49:06.401046718Z","cvss":null,"epss":{"score":0.00583,"percentile":0.46395,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"vllm","fixedVersion":"0.24.0"}],"fix":{"url":"https://github.com/vllm-project/vllm/commit/2b3006076c5e9bc4cda9e03e3641388de3c5c286","label":"vllm-project/vllm@2b30060"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55574.json"},{"type":"ADVISORY","url":"https://github.com/vllm-project/vllm/security/advisories/GHSA-rwxx-mrjm-wc2m"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55574"},{"type":"FIX","url":"https://github.com/vllm-project/vllm/commit/2b3006076c5e9bc4cda9e03e3641388de3c5c286"},{"type":"FIX","url":"https://github.com/vllm-project/vllm/pull/45118"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/vllm/PYSEC-2026-2304.yaml"},{"type":"PACKAGE","url":"https://github.com/vllm-project/vllm"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-24T18:49:06.401046718Z"}}