PyPI · vllm
vLLM: ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends
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.
backend_xgrammar.py:91 — no timeout:
ctx = self.compiler.compile_regex(grammar_spec)
backend_outlines.py:299–330 — structural checks only, no complexity analysis:
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:
oc.Index(regex_string, vocabulary.inner)
Denial of service — one request with an adversarial regex pattern hangs an inference worker indefinitely.
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.
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