Active Support: Possible XSS vulnerability in Active Support MessagePack serializer
Summary
| CVE | CVE-2026-126075 |
|---|---|
| State | PUBLISHED |
| Assigner | GitHub_M |
| Source Priority | CVE Program / NVD first with legacy fallback |
| Published | 2026-11-10 21:19:25 UTC |
| Updated | 2026-11-10 21:19:25 UTC |
| 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. |
Risk And Classification
Primary CVSS: v3.1 4.7 MEDIUM from [email protected]
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N
Problem Types: CWE-79 | CWE-79 CWE-79: Improper Neutralization of Input During Web Page Generation
| Version | Source | Type | Score | Severity | Vector |
|---|---|---|---|---|---|
| 3.1 | [email protected] | Secondary | 4.7 | MEDIUM | CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N |
| 3.1 | CNA | DECLARED | 4.7 | MEDIUM | CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N |
CVSS v3.1 Breakdown
Attack Vector
NetworkAttack Complexity
HighPrivileges Required
NoneUser Interaction
RequiredScope
ChangedConfidentiality
LowIntegrity
LowAvailability
NoneCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N
Vendor Declared Affected Products
References
| Reference | Source | Link | Tags |
|---|---|---|---|
| github.com/rails/rails/commit/4ca090665976e6fbc471c38689b399f84b43cdc7 | [email protected] | github.com | |
| github.com/rails/rails/releases/tag/v8.1.4.1 | [email protected] | github.com | |
| github.com/rails/rails/security/advisories/GHSA-2w9f-fxwj-w2qf | [email protected] | github.com | |
| CVE Program record | CVE.ORG | www.cve.org | canonical |
| NVD vulnerability detail | NVD | nvd.nist.gov | canonical, analysis |
No vendor comments have been submitted for this CVE.
There are currently no legacy QID mappings associated with this CVE.