CVSS 3.1
4.7 mediumCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:NCVE-2026-126075 Active Support: Possible XSS vulnerability in Active Support MessagePack serializer
Medium (4.7) in rails rails. EPSS 0.3% estimated probability of exploitation activity in the next 30 days. Fixed in activesupport 8.1.4.1.
On this page
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.
Severity
- EPSS
- 0.3% estimated probability of exploitation activity in the next 30 days; 27th percentile. Scored 2026-11-13. A forecast for this CVE across the internet, not for your own systems.
- CISA's assessment (SSVC)
- Exploitation: none · Automatable: no · Technical impact: partial, assessed 2026-11-12. From CISA's Vulnrichment in the CVE List: whether it is exploited, whether exploiting it can be automated, and how much control it gives.
- CWE
- CWE-79 Improper Neutralization of Input During Web Page Generation (Cross-site Scripting)
- OWASP Top 10
- A05:2025 Injection
- Published
- Modified
- Assigned by
- GitHub_M (CNA)
- NVD analysis
- Awaiting analysis
- Record from
- the NVD, synced
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Affected
Versions
From the CNA's record in the CVE List.
| Product | Affected |
|---|---|
| rails | >= 8.1, < 8.1.4.1 |
Fix and patched versions
activesupport(RubyGems): fixed in 8.1.4.1
Packages (OSV)
| Advisory | Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|---|
| GHSA-2w9f-fxwj-w2qf | RubyGems | activesupport | >=8.1.0.beta1 <8.1.4.1 | 8.1.4.1 |
Linux distributions
| Distribution | Release | Package | Fixed in | Advisory |
|---|---|---|---|---|
| Ubuntu | 24.04:LTS, 26.04:LTS, 26.10 and 4 more | rails | no fix listed | UBUNTU-CVE-2026-126075 |
From the distributions' data via OSV.dev: Ubuntu Security Notices © Canonical Ltd., CC BY-SA 4.0. Modified: reformatted and linked to CVE records. Credits. On the distributions' own pages: Ubuntu.
Attack patterns (CAPEC)
- CAPEC-63 Cross-Site Scripting (XSS) Likelihood: High · Severity: Very High
- CAPEC-85 AJAX Footprinting Likelihood: High · Severity: Low
- CAPEC-209 XSS Using MIME Type Mismatch Severity: Medium
- CAPEC-588 DOM-Based XSS Likelihood: High · Severity: Very High
- CAPEC-591 Reflected XSS Likelihood: High · Severity: Very High
- CAPEC-592 Stored XSS Likelihood: High · Severity: Very High
The attack patterns MITRE's CAPEC relates to this record's weaknesses, as MITRE names them. Credits.