CVE-2026-29063: Immutable-Js Immutable

Critical severity, CVSS 9.8. EPSS: 1.6% chance of exploitation in the next 30 days.

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.

Affected products

• Immutable-Js Immutable: from 3.0.0, before 3.8.3 (fixed in 3.8.3); from 4.0.0, before 4.3.7 (fixed in 4.3.7); from 5.0.0, before 5.1.5 (fixed in 5.1.5)

Published 2026-03-06. Last modified 2026-09-10.