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High, CVSS 8.7amended

vLLM: ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends

CVE-2026-55574GHSA-rwxx-mrjm-wc2m

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (vllm 0.24.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • PyPI vllm

    Affected
    < 0.24.0
    Fixed in
    0.24.0

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2026-55574: 0.58% estimated exploitation activity in the next 30 days; percentile 47.92 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary The structuredoutputs.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 compileregex() with no guard. In the outlines backend, validateregexisbuildable() 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 backendxgrammar.py:91 — no timeout: backendoutlines.py:299–330 — structural checks only, no complexity analysis: backendoutlines.py:64 — no timeout: Impact Denial of service — one request with an adversarial regex pattern hangs an inference worker indefinitely. Remediation Wrap compileregex() and oc.Index() calls in a thread with a deadline (e.g., 5 seconds). Add complexity analysis to validateregexisbuildable() to detect nested quantifier patterns before compilation.

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-rwxx-mrjm-wc2m

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-rwxx-mrjm-wc2m

Agent brief

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# Patch Next advisory brief: CVE-2026-55574

Advisory: vLLM: ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends
IDs: CVE-2026-55574, GHSA-rwxx-mrjm-wc2m
Record: https://patchnext.com/advisories/cve-2026-55574 (revision 3, source checked 26 Oct 2026, 13:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: vllm 0.24.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI vllm: affected < 0.24.0; fixed in 0.24.0

## Suggested inspection (read-only)
- `python -m pip show vllm`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on vllm.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-rwxx-mrjm-wc2m (OSV record GHSA-rwxx-mrjm-wc2m lists affected ranges: PyPI vllm >=0 <0.24.0 (fixed))
- Verified: https://github.com/advisories/GHSA-rwxx-mrjm-wc2m (GitHub-reviewed advisory GHSA-rwxx-mrjm-wc2m lists affected ranges: PyPI vllm >=0 <0.24.0)

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