CVE-2021-4279 is a critical-severity (CVSS 9.8) CWE-1321 vulnerability in fast-json-patch. 1 public exploit reference exists, so weaponization risk is real. O3 Security confirms whether CVE-2021-4279 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Starcounter-Jack JSON-Patch Prototype Pollution vulnerability
Real-World Exposure
fast-json-patchReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects npm packages — download data is not available via public APIs for these ecosystems.
Description
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 | 3.1.1 |
Research 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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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.
How O3 protects you
O3 pinpoints whether CVE-2021-4279 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-4279. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-4279 in your dependencies?
O3 detects CVE-2021-4279 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.