CVE-2026-126075

### 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 deserialize it as a `String`; a `SafeBuffer` that is `html_safe?` keeps the existing format. Versions without the fix cannot read an unsafe buffer serialized by the fixed releases, so during a rolling deploy a cookie or cache entry holding one could fail to load on a process that has not been upgraded. ### Workarounds Applications that cannot upgrade can convert a `SafeBuffer` to a `String` with `to_str` before storing it in a value that a MessagePack serializer writes. ### Releases The fixed releases are available at the normal locations. Patches for those who cannot upgrade at once: - 8.1: https://github.com/rails/rails/commit/4ca090665976e6fbc471c38689b399f84b43cdc7.patch ### Credit This issue was responsibly reported by HackerOne researcher [jcwilk](https://hackerone.com/jcwilk).