CVE-2026-73556
Received Received - Intake

Regex Denial of Service in vLLM Structured Output Engine

Publication date: 2026-08-13

Last updated on: 2026-08-13

Assigner: GitHub, Inc.

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.

CVSS Scores

EPSS Scores

Probability:
Percentile:

Meta Information

Published
2026-08-13
Last Modified
2026-08-13
Generated
2026-08-13
AI Q&A
2026-08-13
EPSS Evaluated
N/A
NVD
CVE-2026-73556
EUVD
EUVD-2026-58063

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
vllm-project vllm 0.26.0

Helpful Resources

Exploitability

CWE
CWE Icon
KEV
KEV Icon
CWE ID Description
CWE-1333 The product uses a regular expression with an inefficient, possibly exponential worst-case computational complexity that consumes excessive CPU cycles.
CWE-400 The product does not properly control the allocation and maintenance of a limited resource.

Attack-Flow Graph

AI Quick Actions

Instant insights powered by AI
Executive Summary

This is a Regular Expression Denial of Service (ReDoS) vulnerability in the vLLM project's lm-format-enforcer backend. It allows an unauthenticated request to submit a malicious regex pattern that consumes excessive CPU resources, stalling the structured-output engine and causing a denial of service.

Impact Analysis

If exploited, this vulnerability can cause system slowdowns or crashes by consuming all available CPU resources with a single malicious request. This reduces service availability and may require manual intervention to restore normal operation.

Mitigation Strategies

Upgrade vLLM to version 0.26.0 or later to apply the regex timeout protection and validation fixes in the lm-format-enforcer backend.

If using the lm-format-enforcer backend, ensure it is configured with compile_regex_with_timeout and early regex pattern validation as implemented in the fix.

Chat Assistant

Ask questions about this CVE
Hi! I’m here to help you understand CVE-2026-73556. Ask me anything about the vulnerability, its impact, or mitigation strategies.
0/70

EPSS Chart