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CVE-2026-126075VulnerabilityPublished Updated

Rails: From 8.1 until 8.1.4.1, ActiveSupport::MessagePack serializes an ActiveSupport::SafeBuffer as its bytes alone

Affects railsSource GCVE Database

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Authoritative record:CVE-2026-126075· Medium 4.7

Not known to be exploited · CVSS 4.7 Medium · fixed in 8.1.4.1 : update rails when convenient.

Data as of 13 Nov 2026, 22:32 UTC

CVSS 3.1 base score
4.7Medium
How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none
CVSS scores by source
CNA (GitHub_M)CVSS 3.14.7
Patch
Available – fixed in 8.1.4.1
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 10 Nov 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 13 Nov 2026.

What to do

Affected software

ProductAffectedFixed in
Ubuntu 26.10railspkg:deb/ubuntu/rails?arch=source&distro=stonkingOther threatsAffected
  • =2:7.2.3+dfsg-2
  • =2:7.2.3.2+dfsg-2
Fixed in–
Ubuntu 26.04 LTSrailspkg:deb/ubuntu/rails?arch=source&distro=resoluteOther threatsAffected
  • =2:7.2.2.1+dfsg-7
  • =2:7.2.2.2+dfsg-2
  • =2:7.2.3+dfsg-2
Fixed in–
Ubuntu 24.04 LTSrailspkg:deb/ubuntu/rails?arch=source&distro=nobleOther threatsAffected
  • =2:6.1.7.3+dfsg-2build1
  • =2:6.1.7.3+dfsg-3
Fixed in–
Ubuntu Pro 22.04 LTSrailspkg:deb/ubuntu/rails?arch=source&distro=esm-apps/jammyOther threatsAffected
  • =2:6.0.3.7+dfsg-2
  • =2:6.1.4.1+dfsg-8ubuntu2
  • =2:6.1.4.1+dfsg-8ubuntu2+esm1
Fixed in–
Ubuntu Pro 20.04 LTSrailspkg:deb/ubuntu/rails?arch=source&distro=esm-apps/focalOther threatsAffected
  • =2:5.2.2.1+dfsg-1ubuntu1
  • =2:5.2.3+dfsg-3
  • =2:5.2.3+dfsg-3ubuntu0.1~esm1
Fixed in–
Ubuntu Pro 18.04 LTSrailspkg:deb/ubuntu/rails?arch=source&distro=esm-apps/bionicOther threatsAffected
  • 2:4.2.9-2 – 2:4.2.10-0ubuntu4+esm2
5 affected builds
Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A fix is available in Rails version 8.1.4.1. Users should upgrade to this version or later to remediate the vulnerability. If upgrading is not immediately possible, avoid using MessagePack serializers for cookies, cache, or message verification/encryption with mutated SafeBuffer objects containing untrusted input. No other vendor advisories or temporary mitigations are provided.

Description

A vulnerability in Rails ActiveSupport::MessagePack from version 8.1 up to 8.1.4.1 causes ActiveSupport::SafeBuffer objects to be deserialized as html_safe even if mutated, potentially allowing XSS when rendering untrusted input. This affects applications using MessagePack serializers for cookies, cache, or message verification/encryption that store mutated SafeBuffer objects with untrusted data. The issue is fixed in version 8.1.4.1.

References

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