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CVE-2026-29063: Immutable-Js Immutable Prototype Pollution (CVSS 8.7)

CVE-2026-29063 is a high severity prototype pollution in Immutable-Js Immutable with a CVSS score of 8.7. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Last updated: Sep 10, 2026

Quick answer

Direct answer

CVE-2026-29063 is a high severity prototype pollution in Immutable-Js Immutable with a CVSS score of 8.7. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Risk scores

CVSS base score
8.7HIGH
EPSS probability
1.61%
Higher than 75 percent of all CVEs
CISA KEV
Not listed
CVSS metricWhat it means
Attack vector (AV:N)Network: reachable remotely
Attack complexity (AC:L)Low complexity
Attack requirements (AT:N)No attack requirements
Privileges required (PR:N)No privileges required
User interaction (UI:N)No user interaction
Confidentiality (VC:H)High confidentiality impact
Integrity (VI:N)No integrity impact
Availability (VA:N)No availability impact

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

About Immutable-Js Immutable

CVE-2026-29063 affects Immutable-Js Immutable, a product from Immutable-Js. Before acting, confirm where Immutable-Js Immutable runs in your estate, who owns it and whether it is reachable from the internet, because exposure decides real risk more than the score alone.

What the flaw is

In plain terms: 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.

The weakness class is Prototype Pollution. Attacker input changes the base object of a JavaScript application.

Based on its CVSS vector, the flaw can be reached over the network, needs no login, and needs no user interaction. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Affected versions and timeline

FieldDetail
ProductImmutable-Js Immutable
Affected versionsImmutable from 3.0.0 before 3.8.3; Immutable from 4.0.0 before 4.3.7; Immutable from 5.0.0 before 5.1.5
PublishedMar 6, 2026
Last modifiedSep 10, 2026
Added to CISA KEVNot listed
Assigning CNAsecurity-advisories@github.com

How to detect it

  • Inventory every instance of Immutable-Js Immutable and record the running version against the affected range above.
  • Check external attack surface scans for internet facing instances of the component.
  • Review logs on affected systems for unusual requests, errors or new processes around the vulnerable function.
  • Confirm with an authenticated vulnerability scan once your scanner has a check for this CVE.

How to fix it

  • Apply the fixed version of Immutable-Js Immutable named in the vendor advisory.
  • If patching must wait, restrict network access to the affected service and put it behind authentication or a web application firewall rule.
  • Rescan after the change to prove the exposure is gone, then close the ticket.

CTEM action plan

CTEM stageWhat to do for this CVE
ScopeAdd every business service that depends on Immutable-Js Immutable to the current CTEM cycle.
DiscoverFind all instances, including shadow and cloud deployments, and record versions and exposure.
PrioritizeRank using EPSS and CVSS together with asset exposure and business impact.
ValidateConfirm the flaw is reachable and exploitable in your environment and that compensating controls work.
MobilizeAssign the fix to the asset owner with a due date matched to the priority, then verify closure.

CWE References

Technical References

Frequently asked questions

CVE-2026-29063 is a high severity prototype pollution in Immutable-Js Immutable with a CVSS score of 8.7. Apply the vendor fix and confirm exposure on internet facing and business critical assets first.

Related CVEs

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All Immutable-Js CVEs in 2026
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