GHSA-2W9F-FXWJ-W2QF
Vulnerability from github – Published: 2026-11-10 21:27 – Updated: 2026-11-10 21:27Impact
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.
{
"affected": [
{
"package": {
"ecosystem": "RubyGems",
"name": "activesupport"
},
"ranges": [
{
"events": [
{
"introduced": "8.1.0.beta1"
},
{
"fixed": "8.1.4.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-126075"
],
"database_specific": {
"cwe_ids": [
"CWE-79"
],
"github_reviewed": true,
"github_reviewed_at": "2026-11-10T21:27:12Z",
"nvd_published_at": null,
"severity": "MODERATE"
},
"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).",
"id": "GHSA-2w9f-fxwj-w2qf",
"modified": "2026-11-10T21:27:12Z",
"published": "2026-11-10T21:27:12Z",
"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"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N",
"type": "CVSS_V3"
}
],
"summary": "Active Support: Possible XSS vulnerability in Active Support MessagePack serializer"
}
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