Rails: From 8.1 until 8.1.4.1, ActiveSupport::MessagePack serializes an ActiveSupport::SafeBuffer as its bytes alone
Affects railsSource GCVE Database
Not known to be exploited · CVSS 4.7 Medium · fixed in 8.1.4.1 : update rails when convenient.
Data as of 13 Nov 2026, 22:32 UTC
Thin evidence
One scorer or none| CNA (GitHub_M) | CVSS 3.1 | 4.7 |
|---|
- Patch
- Available – fixed in 8.1.4.1
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:NExploitation evidence
Disclosed: evidence found.
CVE published 10 Nov 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 13 Nov 2026.
What to do
Affected software
| Product | Affected | Fixed in |
|---|---|---|
Ubuntu 26.10railspkg:deb/ubuntu/rails?arch=source&distro=stonkingOther threats | Affected
| Fixed in– |
Ubuntu 26.04 LTSrailspkg:deb/ubuntu/rails?arch=source&distro=resoluteOther threats | Affected
| Fixed in– |
Ubuntu 24.04 LTSrailspkg:deb/ubuntu/rails?arch=source&distro=nobleOther threats | Affected
| Fixed in– |
Ubuntu Pro 22.04 LTSrailspkg:deb/ubuntu/rails?arch=source&distro=esm-apps/jammyOther threats | Affected
| Fixed in– |
Ubuntu Pro 20.04 LTSrailspkg:deb/ubuntu/rails?arch=source&distro=esm-apps/focalOther threats | Affected
| Fixed in– |
Ubuntu Pro 18.04 LTSrailspkg:deb/ubuntu/rails?arch=source&distro=esm-apps/bionicOther threats | Affected
| Fixed in– |
Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.
AI-suggested
A fix is available in Rails version 8.1.4.1. Users should upgrade to this version or later to remediate the vulnerability. If upgrading is not immediately possible, avoid using MessagePack serializers for cookies, cache, or message verification/encryption with mutated SafeBuffer objects containing untrusted input. No other vendor advisories or temporary mitigations are provided.
Description
A vulnerability in Rails ActiveSupport::MessagePack from version 8.1 up to 8.1.4.1 causes ActiveSupport::SafeBuffer objects to be deserialized as html_safe even if mutated, potentially allowing XSS when rendering untrusted input. This affects applications using MessagePack serializers for cookies, cache, or message verification/encryption that store mutated SafeBuffer objects with untrusted data. The issue is fixed in version 8.1.4.1.
References
- ubuntu.com/security/CVE-2026-126075 (opens in a new tab)
- cve.org/CVERecord?id=CVE-2026-126075 (opens in a new tab)
- github.com/rails/rails/security/advisories/GHSA-2w9f-fxwj-w2qf (opens in a new tab)
- github.com/rails/rails/commit/4ca090665976e6fbc471c38689b399f84b43cdc7 (opens in a new tab)
- github.com/rails/rails/releases/tag/v8.1.4.1 (opens in a new tab)