Summary
Prototype Pollution in immer
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.
Impact
An object is accessed using a type that is incompatible with its actual type, causing the runtime to interpret memory incorrectly. Typical impact: memory safety violations, unexpected behavior, or code execution.
CVE-2021-23436 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (9.0.6); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2021-23436? CVE-2021-23436 is a critical-severity type confusion vulnerability in immer (npm), affecting versions >= 7.0.0, < 9.0.6. It is fixed in 9.0.6. An object is accessed using a type that is incompatible with its actual type, causing the runtime to interpret memory incorrectly.
- How severe is CVE-2021-23436? CVE-2021-23436 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of immer are affected by CVE-2021-23436? immer (npm) versions >= 7.0.0, < 9.0.6 is affected.
- Is there a fix for CVE-2021-23436? Yes. CVE-2021-23436 is fixed in 9.0.6. Upgrade to this version or later.
- Is CVE-2021-23436 exploitable, and should I be worried? Whether CVE-2021-23436 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-23436 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-23436? Upgrade
immerto 9.0.6 or later.