Summary
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.
Root Cause
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)
Impact
Denial of service, one request with an adversarial regex pattern hangs an inference worker indefinitely.
A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.
CVE-2026-55574 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.24.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
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.
Frequently Asked Questions
- What is CVE-2026-55574? CVE-2026-55574 is a high-severity inefficient regular expression (ReDoS) vulnerability in vllm (pip), affecting versions < 0.24.0. It is fixed in 0.24.0. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
- How severe is CVE-2026-55574? CVE-2026-55574 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of vllm are affected by CVE-2026-55574? vllm (pip) versions < 0.24.0 is affected.
- Is there a fix for CVE-2026-55574? Yes. CVE-2026-55574 is fixed in 0.24.0. Upgrade to this version or later.
- Is CVE-2026-55574 exploitable, and should I be worried? Whether CVE-2026-55574 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-55574 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-55574? Upgrade
vllmto 0.24.0 or later.