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No exploitation reportedFix availableAwaiting NVD analysis

CVE-2026-126075

Active Support: Possible XSS vulnerability in Active Support MessagePack serializer

No exploitation reported by the sources we read.

  • Published2026-11-10
  • Fixed in8.1.4.1
  • WeaknessCWE-79

Am I affected?

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Description

Active Support is a collection of utility classes and standard library extensions used by Rails and other Ruby projects. From 8.1 until 8.1.4.1, ActiveSupport::MessagePack serializes an ActiveSupport::SafeBuffer as its bytes alone and deserializes it as a new SafeBuffer, so a buffer that is no longer html_safe? (for example, one mutated in place with gsub!) is restored as html_safe. If such a buffer holds untrusted input and is stored through a MessagePack serializer, rendering it after it is loaded bypasses ERB auto-escaping, possibly leading to XSS. Applications on Rails 8.1 are affected if they use a MessagePack serializer, such as config.action_dispatch.cookies_serializer = :message_pack or :message_pack_allow_marshal, or serializer: :message_pack for Rails.cache, ActiveSupport::MessageVerifier or ActiveSupport::MessageEncryptor, and store a SafeBuffer that was mutated in place with untrusted input. Rails 8.0 and earlier do not serialize SafeBuffer with MessagePack and are not affected. This issue is fixed in version 8.1.4.1.

Timeline

dates in UTC
  1. 2026-11-1020:50CVE publishedCNA

Versions per branch

OSV
activesupport
BranchAffectedFixed in
8.1≥ 8.1.0.beta1, < 8.1.4.18.1.4.1

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Products declared by the CNA1 product

As GitHub_M, the CNA that assigned this CVE, declared them in the CVE record; versions as written.

  • rails
    Affected
    • = >= 8.1, < 8.1.4.1no fix stated

Source: the CNA record (CVE Program), © The MITRE Corporation,CVE Terms of UseCVE recordupdated 2026-11-10

Fix

  1. Update activesupport to 8.1.4.1

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Metasploit modules
none listed
Metasploit Framework

Sources

  • CNACVSS 3.14.72026-11-10
  • CNAweakness (CWE)CWE-792026-11-10
  • NVDweakness (CWE)CWE-79read 2026-11-10
  • OSVaffected packages—
  • GitHub AdvisoriesadvisoryMODERATE2026-11-10

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N
Published
2026-11-10
Last Modified
2026-11-10
NVD status
Received
GitHub Advisory
GHSA-2w9f-fxwj-w2qf

Frequently asked questions

What is CVE-2026-126075?

Active Support is a collection of utility classes and standard library extensions used by Rails and other Ruby projects. From 8.1 until 8.1.4.1, ActiveSupport::MessagePack serializes an ActiveSupport::SafeBuffer as its bytes alone and deserializes it as a new SafeBuffer, so a buffer that is no longer html_safe? (for example, one mutated in place with gsub!) is restored as html_safe. If such a buffer holds untrusted input and is stored through a MessagePack serializer, rendering it after it is loaded bypasses ERB auto-escaping, possibly leading to XSS. Applications on Rails 8.1 are affected if they use a MessagePack serializer, such as config.action_dispatch.cookies_serializer = :message_pack or :message_pack_allow_marshal, or serializer: :message_pack for Rails.cache, ActiveSupport::MessageVerifier or ActiveSupport::MessageEncryptor, and store a SafeBuffer that was mutated in place with untrusted input. Rails 8.0 and earlier do not serialize SafeBuffer with MessagePack and are not affected. This issue is fixed in version 8.1.4.1.

Is CVE-2026-126075 actively exploited?

Active exploitation of CVE-2026-126075 has not been confirmed.

What is the CVSS score for CVE-2026-126075?

CVE-2026-126075 has a CVSS v3 base score of 4.7 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N.

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