{"id":"GHSA-2w9f-fxwj-w2qf","summary":"Active Support: Possible XSS vulnerability in Active Support MessagePack serializer","details":"### Impact\n`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.\n\nApplications 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.\n\n### Details\nThe 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.\n\n### Workarounds\nApplications that cannot upgrade can convert a `SafeBuffer` to a `String` with `to_str` before storing it in a value that a MessagePack serializer writes.\n\n### Releases\nThe fixed releases are available at the normal locations.\n\nPatches for those who cannot upgrade at once:\n- 8.1: https://github.com/rails/rails/commit/4ca090665976e6fbc471c38689b399f84b43cdc7.patch\n\n### Credit\nThis issue was responsibly reported by HackerOne researcher [jcwilk](https://hackerone.com/jcwilk).","aliases":["CVE-2026-126075"],"modified":"2026-11-10T21:45:04.318644879Z","published":"2026-11-10T21:27:12Z","database_specific":{"github_reviewed_at":"2026-11-10T21:27:12Z","nvd_published_at":null,"cwe_ids":["CWE-79"],"severity":"MODERATE","github_reviewed":true},"references":[{"type":"WEB","url":"https://github.com/rails/rails/security/advisories/GHSA-2w9f-fxwj-w2qf"},{"type":"WEB","url":"https://github.com/rails/rails/commit/4ca090665976e6fbc471c38689b399f84b43cdc7"},{"type":"PACKAGE","url":"https://github.com/rails/rails"},{"type":"WEB","url":"https://github.com/rails/rails/releases/tag/v8.1.4.1"},{"type":"WEB","url":"https://github.com/rubysec/ruby-advisory-db/blob/master/gems/activesupport/GHSA-2w9f-fxwj-w2qf.yml"}],"affected":[{"package":{"name":"activesupport","ecosystem":"RubyGems","purl":"pkg:gem/activesupport"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"8.1.0.beta1"},{"fixed":"8.1.4.1"}]}],"versions":["8.1.0","8.1.0.beta1","8.1.0.rc1","8.1.1","8.1.2","8.1.2.1","8.1.3","8.1.3.1","8.1.4"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/11/GHSA-2w9f-fxwj-w2qf/GHSA-2w9f-fxwj-w2qf.json"}}],"schema_version":"1.9.2","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N"}]}