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Fencer Vulns
medium 6.1

CVE-2026-33170 json

Rails Active Support has a possible XSS vulnerability in SafeBuffer#%

published 2026-03-23 7 mo ago modified 2026-09-10 59 d ago also EUVD-2026-14624 , GHSA-89vf-4333-qx8v

cvss
6.1
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 27%
public exploits
0
none known

Summary

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.

Patch / fix

  • rubygems activesupport 7.2.3.1, 8.0.4.1, 8.1.2.1
  • activesupport_project activesupport 7.2.3.1, 8.0.4.1, 8.1.2.1
  • rails activesupport 7.2.3.1, 8.0.4.1, 8.1.2.1
  • rubyonrails activesupport 7.2.3.1, 8.0.4.1, 8.1.2.1
  • rails Debian 12 2:6.1.7.10+dfsg-1~deb12u3
  • rails Debian 13 2:7.2.2.2+dfsg-2~deb13u2
  • rails Debian 14 2:7.2.3.1+dfsg-1

debian advisories DEBIAN-CVE-2026-33170

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-33170 updated cvss 5.4

Rails: Active Support: Active Support: Cross-Site Scripting (XSS) due to improper HTML safety flag propagation in SafeBuffer#%

  • Red Hat Satellite 6 rubygem-activesupport
    affected: workaround, no fix available “Fix deferred”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
7 mo ago

Exploit prediction

0.34% chance of exploitation in the next 30 days, higher than 27% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 5.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N
CNA 4.0 5.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
NVD 3.1 6.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Affected packages

ecosystem package affected fixed in
rubygems activesupport
>=8.1.0.beta1, <8.1.2.1
>=8.0.0.beta1, <8.0.4.1
<7.2.3.1
8.1.2.1
8.0.4.1
7.2.3.1

Affected products

  • rails activesupport

Classification

Weakness
  • CWE-79 Improper Neutralization of Input During Web Page Generation
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

The maintainers of rails/rails published their own advisory on GitHub. Reviewed sources take precedence over it.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.