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CVE-2026-33170: Rails Active Support has a possible XSS vulnerability in SafeBuffer#%

• Severity: medium (CVSS 6.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-03-24 · updated 2026-06-17
• Weakness: CWE-79

Description

Active Support is a toolkit of support libraries and Ruby core extensions extracted from the Rails framework. Prior to versions 8.1.2.1, 8.0.4.1, and 7.2.3.1, `SafeBuffer#%` does not propagate the `@html_unsafe` flag to the newly created buffer. If a `SafeBuffer` is mutated in place (e.g. via `gsub!`) and then formatted with `%` using untrusted arguments, the result incorrectly reports `html_safe? == true`, bypassing ERB auto-escaping and possibly leading to XSS. Versions 8.1.2.1, 8.0.4.1, and 7.2.3.1 contain a patch.

Affected products

• Ruby on Rails: before 7.2.3.1; from 8.0.0 before 8.0.4.1; from 8.1.0 before 8.1.2.1 — fixed in 7.2.3.1, 8.0.4.1, 8.1.2.1
• Rubygems Activesupport: from 8.1.0.beta1 before 8.1.2.1; from 8.0.0.beta1 before 8.0.4.1; before 7.2.3.1; 8.1.0, 8.1.0.beta1, 8.1.0.rc1, 8.1.1, 8.1.2, 8.0.0 (+491) — fixed in 8.1.2.1, 8.0.4.1, 7.2.3.1

How to fix

Upgrade to a fixed version: Ruby on Rails 7.2.3.1, 8.0.4.1, 8.1.2.1; Rubygems Activesupport 8.1.2.1, 8.0.4.1, 7.2.3.1. Check the vendor advisory for mitigations.

References

• https://github.com/rails/rails/commit/50d732af3b7c8aaf63cbcca0becbc00279b215b7
• https://github.com/rails/rails/commit/6e8a81108001d58043de9e54a06fca58962fc2db
• https://github.com/rails/rails/commit/c1ad0e8e1972032f3395853a5e99cea035035beb
• https://github.com/rails/rails/releases/tag/v7.2.3.1
• https://github.com/rails/rails/releases/tag/v8.0.4.1
• https://github.com/rails/rails/releases/tag/v8.1.2.1
• https://github.com/rails/rails/security/advisories/GHSA-89vf-4333-qx8v

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