# CVE-2021-23436: Prototype Pollution

> Prototype pollution in immer allows attackers to bypass previous security fixes via array-based path parameters, potentially leading to code execution.

- **Severity:** Medium 5.6 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 1 Sep 2021
- **Updated:** 16 Sep 2024
- **Also known as:** GHSA-33f9-j839-rf8h, ROOT-APP-NPM-CVE-2021-23436

## What is CVE-2021-23436?

CVE-2021-23436 is a medium-severity vulnerability in immer, published on 1 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-843 (Access of Resource Using Incompatible Type)

## Which versions are affected by CVE-2021-23436?

2 open source packages ship an affected version, including immer. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `immer` | npm | From 7.0.0 | 9.0.6 |
| `immer` | npm | From 7.0.0 | 9.0.6 |

Products: Immer Project Immer.

## How do I fix CVE-2021-23436?

Upgrade immer to 9.0.6 or later.

## Is CVE-2021-23436 being exploited?

There is no known exploitation of CVE-2021-23436 in the wild and no public exploit code.

## Other vulnerabilities in immer

- [CVE-2021-3757](https://insights.sbomify.com/CVE-2021-3757/): Prototype Pollution in immerjs/immer (high 7.5)
- [CVE-2020-28477](https://insights.sbomify.com/CVE-2020-28477/): Prototype Pollution (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-23436 and fix it if it is affected.

What it is: CVE-2021-23436 is a medium-severity vulnerability in immer, published on 1 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-23436 in the wild and no public exploit code.

Affected packages and fixed versions:
- immer (npm): affected from 7.0.0; fixed in 9.0.6
- immer (npm): affected from 7.0.0; fixed in 9.0.6

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-23436/ (Markdown: https://insights.sbomify.com/CVE-2021-23436/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-23436
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-23436
- OSV: https://osv.dev/vulnerability/CVE-2021-23436

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-23436/
Summaries are generated and may contain mistakes; the official description above is the record.