CVE-2026-55574 — vLLM: ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends

Affected: vllm-project vllm

vLLM is a high-throughput and memory-efficient inference and serving engine for LLMs. Prior to 0.24.0, the structured_outputs.regex API parameter passes a user-supplied regular expression string directly to the grammar compiler backends with no compilation timeout; in the xgrammar backend the string reaches the regex compiler with no guard, and in the outlines backend the validation step blocks structural issues such as lookarounds and backreferences but performs no complexity analysis, so a pattern with nested quantifiers passes all checks and causes exponential state-space expansion, allowing a single request containing an adversarial regex to hang an inference worker indefinitely and deny service. This issue is fixed in version 0.24.0.

GHSA-rwxx-mrjm-wc2m — vLLM: ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends

Affected: PyPI: vllm

## 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.

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CVE-2026-55574 — vLLM: ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends

GHSA-rwxx-mrjm-wc2m — vLLM: ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends