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…
| CVE ID | CVE-2026-55574 |
| Vendor | Unknown Vendor |
| Affected Product | Unknown Product |
| Vulnerability Type | Security Vulnerability |
| CVSS Score | 7.5 (HIGH) |
| EPSS Score | 0.3% probability of exploitation in the next 30 days |
| Actively Exploited | ❌ No known exploitation |
| Patch Status | Pending Vendor Disclosure |
| Reported By | CYBERDUDEBIVASH SENTINEL APEX Intelligence (via sentinel_apex) |
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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.
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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)
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