Active Support: Possible XSS vulnerability in Active Support MessagePack serializer

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Impact `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.

Details The fixed releases serialize a `SafeBuffer` that is not `html_safe?` with a marker and…

Published by the source

10 Nov 2026, 21:27 UTC

Last modified by the source

10 Nov 2026, 21:45 UTC

Source last checked

11 Nov 2026, 07:35 UTC

1. Revision 2 · amended · 11 Nov 2026

   Amended from osv record GHSA-2w9f-fxwj-w2qf

2. Revision 1 · created · 10 Nov 2026

   Published from ghsa record GHSA-2w9f-fxwj-w2qf

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# Patch Next advisory brief: CVE-2026-126075

Advisory: Active Support: Possible XSS vulnerability in Active Support MessagePack serializer
IDs: CVE-2026-126075, GHSA-2w9f-fxwj-w2qf
Record: https://patchnext.com/advisories/cve-2026-126075 (revision 2, source checked 11 Nov 2026, 07:35 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: activesupport 8.1.4.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems activesupport: affected >= 8.1.0.beta1, < 8.1.4.1; fixed in 8.1.4.1

## Suggested inspection (read-only)
- RubyGems activesupport: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on activesupport.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-2w9f-fxwj-w2qf (GitHub-reviewed advisory GHSA-2w9f-fxwj-w2qf lists affected ranges: RubyGems activesupport >=8.1.0.beta1 <8.1.4.1)
- Verified: https://osv.dev/vulnerability/GHSA-2w9f-fxwj-w2qf (OSV record GHSA-2w9f-fxwj-w2qf lists affected ranges: RubyGems activesupport >=8.1.0.beta1 <8.1.4.1 (fixed))

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