CVE-2021-23436 is a critical-severity (CVSS 9.8) CWE-843 vulnerability in immer. 2 public exploit references exist, so weaponization risk is real. A fix is available for immer — see the affected versions and patch details below.
Prototype Pollution in immer
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-23436 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,386 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.
immernpmDescription
This affects the package immer before 9.0.6. A type confusion vulnerability can lead to a bypass of CVE-2020-28477 when the user-provided keys used in the path parameter are arrays. In particular, this bypass is possible because the condition (p === "proto" || p === "constructor") in applyPatches_ returns false if p is ['proto'] (or ['constructor']). The === operator (strict equality operator) returns false if the operands have different type.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | immer | ≥ 7.0.0&&< 9.0.6 | 9.0.6npm install immer@9.0.6 |
Affected Products
immerimmer_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 immer, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update immer to 9.0.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23436 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.
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-2021-23436 in your dependencies?
Find it across npm, including transitive dependencies.