CVE-2026-55574: vLLM Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-rwxx-mrjm-wc2m
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CVSS 7.5
CWE-1333
Published
Advisory Summary

CVE-2026-55574 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-1333: Inefficient Regular Expression Complexity) affecting vLLM libraries in the PyPI ecosystem. The issue involves redos via structured_outputs.regex compiled without timeout in xgrammar and outlines backends. Remediation is available by upgrading affected packages to version 0.24.0.

CVE-2026-55574 at a Glance

CVE Identifier CVE-2026-55574
GHSA Identifier GHSA-rwxx-mrjm-wc2m
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-1333: Inefficient Regular Expression Complexity
Affected Product vLLM
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-55574

To remediate this vulnerability, upgrade all affected vLLM dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

vllm fixed: 0.24.0 
pip install vllm>=0.24.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show vllm

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-55574, inspect your installed packages using pip:

pip show vllm

Analyze the terminal output to locate any version of vLLM in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 0.24.0), your application is affected.

Note: The pip show vllm command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-1333: Inefficient Regular Expression Complexity, where the product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential. In vLLM, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

## Summary 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: ```python ctx = self.compiler.compile_regex(grammar_spec) ``` `backend_outlines.py:299–330` — structural checks only, no complexity analysis: ```python 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: ```python oc.Index(regex_string, vocabulary.inner) ``` ## Impact Denial of service — one request with an adversarial regex pattern hangs an inference worker indefinitely. ## Remediation 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.

CVE-2026-55574 Affected vLLM Versions

1 component(s)
Fixed Versions:
0.24.0
Product / Package Ecosystem Affected Versions Fixed Version
vllm
pypi/vllm
PyPI
< 0.24.0
0.24.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/vllm-project/vllm/security/advisories/GHSA-rwxx-mrjm-wc2m
NVD — CVE-2026-55574 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-55574
vLLM Source Repository REPOSITORY
https://github.com/vllm-project/vllm/pull/45118
Security Fix Commit FIX COMMIT
https://github.com/vllm-project/vllm/commit/2b3006076c5e9bc4cda9e03e3641388de3c5c286
vLLM Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/vllm/PYSEC-2026-2304.yaml
vLLM Source Repository REPOSITORY
https://github.com/vllm-project/vllm

Frequently Asked Questions

What is CVE-2026-55574?
CVE-2026-55574 is a security vulnerability in vLLM involving ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends under CWE-1333: Inefficient Regular Expression Complexity.
Which vLLM versions are affected?
Affected versions include releases prior to 0.24.0 across affected packages: vllm.
What version fixes CVE-2026-55574?
Upgrading to 0.24.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is vllm affected?
Yes, vllm is affected in releases prior to 0.24.0. Upgrade your dependency to 0.24.0 or later.
What is the CVSS score of CVE-2026-55574?
CVE-2026-55574 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-55574?
CVE-2026-55574 is classified under CWE-1333: Inefficient Regular Expression Complexity (The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.).
How can I check PyPI dependencies for CVE-2026-55574?
Run the dependency tree command: 'pip show vllm'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-rwxx-mrjm-wc2m",
  "modified" : "2026-07-17T17:10:37Z",
  "published" : "2026-07-17T17:10:37Z",
  "aliases" : [ "CVE-2026-55574" ],
  "summary" : "vLLM: ReDoS via structured_outputs.regex compiled without timeout in xgrammar and outlines backends",
  "details" : "## Summary\n\nThe `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.\n\n## Root Cause\n\n`backend_xgrammar.py:91` — no timeout:\n```python\nctx = self.compiler.compile_regex(grammar_spec)\n```\n\n`backend_outlines.py:299–330` — structural checks only, no complexity analysis:\n```python\ndef validate_regex_is_buildable(regex: str) -> None:\n    sre_parse.parse(regex)   # AST parse only — does not detect exponential patterns\n    _check_unsupported(...)  # blocks lookarounds/backrefs, not nested quantifiers\n```\n\n`backend_outlines.py:64` — no timeout:\n```python\noc.Index(regex_string, vocabulary.inner)\n```\n\n## Impact\n\nDenial of service — one request with an adversarial regex pattern hangs an inference worker indefinitely.\n\n## Remediation\n\nWrap `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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "vllm"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.24.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/vllm-project/vllm/security/advisories/GHSA-rwxx-mrjm-wc2m"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-55574"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/vllm-project/vllm/pull/45118"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/vllm-project/vllm/commit/2b3006076c5e9bc4cda9e03e3641388de3c5c286"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/vllm/PYSEC-2026-2304.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/vllm-project/vllm"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1333" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-17T17:10:37Z",
    "nvd_published_at" : "2026-07-06T21:16:57Z"
  }
}