vLLM: ReDoS via structured_outputs.regex in the lm-format-enforcer backend (no compile timeout) — missed sibling of GHSA-rwxx-mrjm-wc2m

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-73556: 0.52% estimated exploitation activity in the next 30 days; percentile 43.95 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The fix for GHSA-rwxx-mrjm-wc2m ("ReDoS via structuredoutputs.regex compiled without timeout") wrapped the regex compile in the xgrammar and outlines backends with compileregexwithtimeout (and, for outlines, validateregexisbuildable). The lm-format-enforcer backend was left unguarded: it compiles the attacker-supplied regex with no timeout and no buildability check. A single request with a catastrophic regex hangs the structured-output compile step and stalls the engine worker (denial of service). Affected code (HEAD d6d39c1) vllm/v1/structuredoutput/backendlmformatenforcer.py: - line 110: characterlevelparser = lmformatenforcer.RegexParser(grammarspec) — builds an interegular FSM from the attacker regex synchronously, no timeout. - line 155: validatestructuredoutputrequestlmformatenforcer returns immediately on if soparams.regex: — no validation. Sibling backends that WERE patched by GHSA-rwxx: - backendxgrammar.py:92 → compileregexwithtimeout(...). - backendoutlines.py:65 →…

Published by the source

4 Sep 2026, 21:37 UTC

Last modified by the source

4 Sep 2026, 21:37 UTC

Source last checked

26 Oct 2026, 13:51 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-48jh-3gj7-fg8v

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# Patch Next advisory brief: CVE-2026-73556

Advisory: vLLM: ReDoS via structured_outputs.regex in the lm-format-enforcer backend (no compile timeout) — missed sibling of GHSA-rwxx-mrjm-wc2m
IDs: CVE-2026-73556, GHSA-48jh-3gj7-fg8v
Record: https://patchnext.com/advisories/cve-2026-73556 (revision 1, source checked 26 Oct 2026, 13:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: vllm 0.26.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI vllm: affected < 0.26.0; fixed in 0.26.0

## 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://github.com/advisories/GHSA-48jh-3gj7-fg8v (GitHub-reviewed advisory GHSA-48jh-3gj7-fg8v lists affected ranges: PyPI vllm >=0 <0.26.0)

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