CVE-2021-23406 is a critical-severity (CVSS 9.8) vulnerability in pac-resolver. 2 public exploit references exist, so weaponization risk is real. A fix is available for pac-resolver — see the affected versions and patch details below.
Code Injection in pac-resolver
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-23406 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.
pac-resolvernpmdegeneratornpmDescription
This affects the package pac-resolver before 5.0.0. This can occur when used with untrusted input, due to unsafe PAC file handling. NOTE: The fix for this vulnerability is applied in the node-degenerator library, a dependency written by the same maintainer.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | pac-resolver | all versions | 5.0.0npm install pac-resolver@5.0.0 |
| 📦npm | degenerator | all versions | 3.0.1npm install degenerator@3.0.1 |
Affected Products
pac-resolverpac-resolver_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 pac-resolver, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update pac-resolver to 5.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23406 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2021-23406 in your dependencies?
Find it across npm, including transitive dependencies.