{"id":"CVE-2026-126075","state":"PUBLISHED","title":"Active Support: Possible XSS vulnerability in Active Support MessagePack serializer","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.","publishedAt":"2026-11-10T20:50:44.522Z","updatedAt":"2026-11-10T20:50:44.522Z","severity":{"score":4.7,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N","source":"GitHub_M"},"assigner":"GitHub_M","products":["rails rails"],"affected":[{"vendor":"rails","product":"rails","versions":[{"version":">= 8.1, < 8.1.4.1","status":"affected"}]}],"references":["https://github.com/rails/rails/security/advisories/GHSA-2w9f-fxwj-w2qf","https://github.com/rails/rails/commit/4ca090665976e6fbc471c38689b399f84b43cdc7","https://github.com/rails/rails/releases/tag/v8.1.4.1"],"weaknesses":["CWE-79"],"ingestedAt":"2026-11-10T21:14:06.595Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/126xxx/CVE-2026-126075.json","commit":"c5a01f083f4f1bb59d77be7743a509cc9f9b04ca","baselineCommit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-126075","state":"PUBLISHED","datePublished":"2026-11-10T20:50:44.522Z","dateUpdated":"2026-11-10T20:50:44.522Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2026-11-10T17:07:43.284Z"},"containers":{"cna":{"title":"Active Support: Possible XSS vulnerability in Active Support MessagePack serializer","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"rails","product":"rails","versions":[{"version":">= 8.1, < 8.1.4.1","status":"affected"}]}],"references":[{"url":"https://github.com/rails/rails/security/advisories/GHSA-2w9f-fxwj-w2qf","name":"https://github.com/rails/rails/security/advisories/GHSA-2w9f-fxwj-w2qf","tags":["x_refsource_CONFIRM"]},{"url":"https://github.com/rails/rails/commit/4ca090665976e6fbc471c38689b399f84b43cdc7","name":"https://github.com/rails/rails/commit/4ca090665976e6fbc471c38689b399f84b43cdc7","tags":["x_refsource_MISC"]},{"url":"https://github.com/rails/rails/releases/tag/v8.1.4.1","name":"https://github.com/rails/rails/releases/tag/v8.1.4.1","tags":["x_refsource_MISC"]}],"metrics":[{"cvssV3_1":{"attackComplexity":"HIGH","attackVector":"NETWORK","availabilityImpact":"NONE","baseScore":4.7,"baseSeverity":"MEDIUM","confidentialityImpact":"LOW","integrityImpact":"LOW","privilegesRequired":"NONE","scope":"CHANGED","userInteraction":"REQUIRED","vectorString":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N","version":"3.1"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-79","lang":"en","description":"CWE-79: Improper Neutralization of Input During Web Page Generation","type":"CWE"}]}],"providerMetadata":{"shortName":"GitHub_M","orgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateUpdated":"2026-11-10T20:50:44.522Z"},"source":{"advisory":"GHSA-2w9f-fxwj-w2qf","discovery":"UNKNOWN"}}}}}