CVE-2026-126075 Active Support: Possible XSS vulnerability in Active Support MessagePack serializer
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2026-126075?
CVE-2026-126075 is a medium-severity vulnerability in rails, published on 10 Nov 2026.
Official 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.
Weakness: CWE-79
Which versions are affected by CVE-2026-126075?
1 open source package ship an affected version, including activesupport.
| Package | Affected | Fixed in |
|---|---|---|
activesupport RubyGems | From 8.1.0.beta1 | 8.1.4.1 |
How do I fix CVE-2026-126075?
Upgrade activesupport to 8.1.4.1 or later.
Is CVE-2026-126075 being exploited?
There is no known exploitation of CVE-2026-126075 in the wild and no public exploit code.
Sources
Also known as GHSA-2w9f-fxwj-w2qf, UBUNTU-CVE-2026-126075.