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.

Quoted source text, attributed separately from HOL analysis.