CVE-2021-4279 is a critical-severity (CVSS 9.8) CWE-1321 vulnerability in fast-json-patch. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for fast-json-patch — see the affected versions and patch details below.
Starcounter-Jack JSON-Patch Prototype Pollution vulnerability
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.
fast-json-patchnpmDescription
A vulnerability has been found in Starcounter-Jack JSON-Patch up to 3.1.0 and classified as problematic. This vulnerability affects unknown code. The manipulation leads to improperly controlled modification of object prototype attributes ('prototype pollution'). The attack can be initiated remotely. The exploit has been disclosed to the public and may be used. Upgrading to version 3.1.1 is able to address this issue. The name of the patch is 7ad6af41eabb2d799f698740a91284d762c955c9. It is recommended to upgrade the affected component. VDB-216778 is the identifier assigned to this vulnerability.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | fast-json-patch | all versions | 3.1.1npm install fast-json-patch@3.1.1 |
Affected Products
json-patchstarcounter-jackResearch 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 fast-json-patch, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fast-json-patch to 3.1.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-4279 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-2021-4279 in your dependencies?
Find it across npm, including transitive dependencies.