vLLM: ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends

## Summary The `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. ## Root Cause `backend_xgrammar.py:91` , no timeout: ```python ctx = self.compiler.compile_regex(grammar_spec) ``` `backend_outlines.py:299–330` , structural checks only, no complexity analysis: ```python def validate_regex_is_buildable(regex: str) -> None: sre_parse.parse(regex) # AST parse only , does not detect exponential patterns _check_unsupported(...) # blocks lookarounds/backrefs, not nested quantifiers ``` `backend_outlines.py:64` , no timeout: ```python oc.Index(regex_string, vocabulary.inner) ``` ## Impact Denial of service , one request with an adversarial regex pattern hangs an inference worker indefinitely. ## Remediation Wrap `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.

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Severity from

GitHub (reviewed advisory)

Weakness

CWE-1333

Also known as

CVE-2026-55574, PYSEC-2026-2304

• github.com/vllm-project/vllm/security/advisories/GHSA-rwxx-mrjm-wc2m
• nvd.nist.gov/vuln/detail/CVE-2026-55574
• github.com/vllm-project/vllm/pull/45118
• github.com/vllm-project/vllm/commit/2b3006076c5e9bc4cda9e03e3641388de3c5c286
• github.com/pypa/advisory-database/tree/main/vulns/vllm/PYSEC-2026-2304.yaml
• github.com/vllm-project/vllm