XSS in Discourse

CVE-2026-91121

Discourse is an open-source discussion platform. Prior to 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0, attacker-controlled upload filenames used in chat message excerpts were rendered as unescaped HTML. A user able to upload a file and send chat content could place markup in a filename that was then interpreted by chat channel lists, chat summary emails, pinned message bars, reply previews, thread previews, and other excerpt renderers. The issue allowed trusted-HTML injection that altered rendered excerpt content, but JavaScript execution was not demonstrated with default Content Security Policy settings. The advisory states that no confidentiality, integrity, or availability impact was identified, while its structured scoring metadata separately assigns low integrity impact. Sites that disable or relax the default CSP have increased exposure. This issue is fixed in versions 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0.

Vulnerability class: XSS (Cross-Site Scripting)

Published · last modified .

CVSS v3 metric

CVSS v3 base score 5.0 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:L/A:N.

EPSS exploit prediction

EPSS: 0.003 (17.6th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 18th percentile — 17.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-91121: held from 0.003 to 0.003.

EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: 0.000

Affected products

  • Discourse — versions < 2026.8.0, >= 2026.1.0-latest, < 2026.1.8, >= 2026.6.0-latest, < 2026.6.3

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-91121?
CVE-2026-91121 is a medium-severity vulnerability in Discourse, classified under Cross-site Scripting. CVSS score: 5.0/10. Published 2026-09-24.
How severe is CVE-2026-91121?
Medium severity. CVSS v3 base score is 5.0 out of 10.