XSS in Discourse

CVE-2026-91159

Discourse is an open-source discussion platform. Prior to 2026.9.0, 2026.8.1, 2026.7.3, and 2026.1.9, oEmbed processing treated an entire attacker-controlled HTML fragment as trusted when the fragment contained an iframe whose source matched the configured allowlist. A user with posting permission who controlled a public webpage and its oEmbed response could combine an allowlisted iframe with unrelated sibling HTML or CSS, causing stored visual overlays or other styled content to appear in post content and enabling misleading or fraudulent user interfaces. Exploitation requires a posting account, control of the fetched webpage and oEmbed response, and an allowlisted iframe source. The issue affects content integrity but does not expose restricted data, disrupt availability, or require interaction with Discourse Content Security Policy. This issue is fixed in versions 2026.9.0, 2026.8.1, 2026.7.3, and 2026.1.9.

Vulnerability class: XSS (Cross-Site Scripting)

Published · last modified .

CVSS v3 metric

CVSS v3 base score 7.7 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:H/A:N.

EPSS exploit prediction

No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.

Affected products

  • Discourse — versions < 2026.1.9, >= 2026.7.0-latest, < 2026.7.3, >= 2026.8.0-latest, < 2026.8.1

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-91159?
CVE-2026-91159 is a high-severity vulnerability in Discourse, classified under Cross-site Scripting. CVSS score: 7.7/10. Published 2026-10-14.
How severe is CVE-2026-91159?
High severity. CVSS v3 base score is 7.7 out of 10.