Vulnerability · Published Aug 13, 2026 · Assigned by GitHub_M

CVE-2026-73556vLLM: ReDoS via structured_outputs.regex in the lm-format-enforcer backend (no compile timeout) — missed sibling of CVE-2026-55574

Medium 5.3

In plain English

A medium-severity uncontrolled resource consumption in vllm from vllm-project. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 44% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.3Medium

Exploit odds (30 days)0.5%top 55.9% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

vLLM is an inference and serving engine for large language models. Prior to 0.26.0, the structured_outputs.regex parameter in vllm/v1/structured_output/backend_lm_format_enforcer.py is passed to lmformatenforcer.RegexParser without compile_regex_with_timeout or validation in validate_structured_output_request_lm_format_enforcer, allowing an unauthenticated /v1/completions request against the lm-format-enforcer backend to consume a CPU core and stall the structured-output engine path with a catastrophic regular expression. This issue is fixed in version 0.26.0.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
PyPI	vllm	0.26.0	GHSA-48jh-3gj7-fg8v
PyPI	vllm	0.26.0	PYSEC-2026-3933

Affected products (as reported)

Vendor	Product	Versions
vllm-project	vllm	< 0.26.0

Weakness: CWE-400, CWE-1333 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

References

https://github.com/vllm-project/vllm/security/advisories/GHSA-48jh-3gj7-fg8v x_refsource_CONFIRM
https://github.com/vllm-project/vllm/pull/47595 x_refsource_MISC
https://github.com/vllm-project/vllm/commit/c9a788eedc412acceaa5112e0d44624b49841577 x_refsource_MISC
https://github.com/vllm-project/vllm/releases/tag/v0.26.0 x_refsource_MISC