CVE-2026-29063

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 this means for your business

It affects Immutable-Js Immutable. It matters if your company, or a supplier that handles your data, runs it.
Code showing how to exploit it has been published, but real attacks have not been confirmed.
An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
FIRST's prediction model gives it a 1.0% chance of attack attempts being seen in the next 30 days, ranking above 60% of all known flaws.

What to do

1Check whether your company or your suppliers run Immutable-Js Immutable, and which version. The affected versions are listed further down this page.
2If you do, apply the vendor's fix. A patch or vendor advisory has been published.

Not sure if your company is exposed?

Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.

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Scoring

CVSS: 9.8 (v3.1)
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS v4: 8.7
Weakness: CWE-1321
Assigned by: security-advisories@github.com

Dates

Published: 2026-03-06
Last modified: 2026-09-10
Sources: NVD

Affected products

Immutable-Js Immutable3.0.0 - 3.8.3, 4.0.0 - 4.3.7, 5.0.0 - 5.1.5

As listed in the NVD configuration data. Not a statement about your estate.