CVE-2020-26311: Useragent Denial of Service Vulnerability

MODERATE
Advisory Identifier: GHSA-mgfv-m47x-4wqp
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CVSS 7.5
CWE-1333
Published
Advisory Summary

CVE-2020-26311 is a MODERATE (CVSS 7.5) denial of service vulnerability (CWE-1333: Inefficient Regular Expression Complexity) affecting Useragent libraries in the npm ecosystem. The issue involves useragent regular expression denial of service vulnerability. Remediation is available in the latest vendor maintenance releases.

CVE-2020-26311 at a Glance

CVE Identifier CVE-2020-26311
GHSA Identifier GHSA-mgfv-m47x-4wqp
Severity MODERATE
CVSS v3.1 Base Score 7.5
CWE Classification CWE-1333: Inefficient Regular Expression Complexity
Affected Product Useragent
Package Ecosystem npm
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-2020-26311

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

useragent fixed: Latest secure release 
npm install useragent@Latest secure release
Verify Direct and Transitive Dependencies

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

npm ls useragent

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2020-26311, inspect your project dependencies using npm:

npm ls useragent

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

Note: The npm ls useragent command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

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 Useragent, 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

Useragent is a user agent parser for Node.js. All versions as of time of publication contain one or more regular expressions that are vulnerable to Regular Expression Denial of Service (ReDoS). ## PoC ```js async function exploit() { const useragent = require(\"useragent\"); // Create a malicious user-agent that leads to excessive backtracking const maliciousUserAgent = 'Mozilla/5.0 (' + 'X'.repeat(30000) + ') Gecko/20100101 Firefox/77.0'; // Parse the malicious user-agent const agent = useragent.parse(maliciousUserAgent); // Call the toString method to trigger the vulnerability const result = await agent.device.toString(); console.log(result); } await exploit(); ```

CVE-2020-26311 Affected Useragent Versions

1 component(s)
Product / Package Ecosystem Affected Versions Fixed Version
useragent
npm/useragent
npm
≤ 2.3.0

References

NVD — CVE-2020-26311 NVD
https://nvd.nist.gov/vuln/detail/CVE-2020-26311
Useragent Source Repository REPOSITORY
https://github.com/3rd-Eden/useragent/issues/167
Security Fix Commit FIX COMMIT
https://github.com/3rd-Eden/useragent/commit/4c3ee79358bea72d88fe78ac98f4f861db40b89b
Useragent Source Repository REPOSITORY
https://github.com/3rd-Eden/useragent
Useragent Source Repository REPOSITORY
https://github.com/3rd-Eden/useragent/blob/ffa906f923183c85fbb9e6c90f19345e2bd3c52a/lib/regexps.js#L5568
GitHub Advisory / Wiki ADVISORY
https://securitylab.github.com/advisories/GHSL-2020-312-redos-useragent

Frequently Asked Questions

What is CVE-2020-26311?
CVE-2020-26311 is a security vulnerability in Useragent involving useragent Regular Expression Denial of Service vulnerability under CWE-1333: Inefficient Regular Expression Complexity.
Which Useragent versions are affected?
Affected versions include releases prior to current release patches across affected packages: useragent.
What version fixes CVE-2020-26311?
Upgrading to current release patches (or the latest patched release for your respective branch) resolves the vulnerability.
Is useragent affected?
Yes, useragent is affected in releases prior to Latest secure release. Upgrade your dependency to Latest secure release or later.
What is the CVSS score of CVE-2020-26311?
CVE-2020-26311 has a CVSS v3.1 base score of 7.5 (MODERATE) 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-2020-26311?
CVE-2020-26311 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 npm dependencies for CVE-2020-26311?
Run the dependency tree command: 'npm ls useragent'. 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-mgfv-m47x-4wqp",
  "modified" : "2025-09-03T15:17:53Z",
  "published" : "2024-10-26T21:30:47Z",
  "aliases" : [ "CVE-2020-26311" ],
  "summary" : "useragent Regular Expression Denial of Service vulnerability",
  "details" : "Useragent is a user agent parser for Node.js. All versions as of time of publication contain one or more regular expressions that are vulnerable to Regular Expression Denial of Service (ReDoS).\n\n## PoC\n```js\nasync function exploit() {\n   const useragent = require(\\\"useragent\\\");\n\n   // Create a malicious user-agent that leads to excessive backtracking\n   const maliciousUserAgent = 'Mozilla/5.0 (' + 'X'.repeat(30000) + ') Gecko/20100101 Firefox/77.0';\n\n   // Parse the malicious user-agent\n   const agent = useragent.parse(maliciousUserAgent);\n\n   // Call the toString method to trigger the vulnerability\n   const result = await agent.device.toString();\n   console.log(result);\n}\n\nawait exploit();\n```",
  "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/E:U/U:Green"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "useragent"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "2.3.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2020-26311"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/3rd-Eden/useragent/issues/167"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/3rd-Eden/useragent/commit/4c3ee79358bea72d88fe78ac98f4f861db40b89b"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/3rd-Eden/useragent"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/3rd-Eden/useragent/blob/ffa906f923183c85fbb9e6c90f19345e2bd3c52a/lib/regexps.js#L5568"
  }, {
    "type" : "ADVISORY",
    "url" : "https://securitylab.github.com/advisories/GHSL-2020-312-redos-useragent"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1333" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2024-10-28T15:01:50Z",
    "nvd_published_at" : "2024-10-26T21:15:14Z"
  }
}