Active Support: Possible XSS vulnerability in Active Support MessagePack serializer
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 htmlsafe? (for example, one mutated in place with gsub!) is restored as htmlsafe. 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.actiondispatch.cookiesserializer = :messagepack or :messagepackallowmarshal, or serializer: :messagepack 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 htmlsafe? with a marker and deserialize it as a String; a SafeBuffer that is htmlsafe? 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…
Assessment
- CVSS
- 4.7 (v3.1)
- Severity
- Medium
- EPSS
- 0.00336% chance of exploitation in the next 30 days
- Known exploited
- Not listed in CISA KEV
- Exploit availability
- Nothing found in Exploit-DB, Metasploit, Nuclei or public PoC repositories
- Published
- 10 November 2026
- Last updated
- 10 November 2026
- Affected
- activesupport
- Weakness
- CWE-79
CVSS vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N
At the source
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
- https://github.com/rubysec/ruby-advisory-db/blob/master/gems/activesupport/GHSA-2w9f-fxwj-w2qf.yml
- https://github.com/advisories/GHSA-2w9f-fxwj-w2qf
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