CVE-2026-91123: Discourse

High severity, CVSS 7.2. EPSS: 0.3% chance of exploitation in the next 30 days.

Discourse is an open-source discussion platform. Prior to 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0, the iframe src traversal guard did not treat literal backslashes as path separators after decoded dot segments. A crafted source could therefore pass an allowed_iframes subpath check while browser URL normalization moved the iframe outside the intended allowed path. The resulting iframe could load content from a location that the administrator did not allow. This issue is fixed in versions 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0.

Affected products

• Discourse Discourse: before 2026.8.0 (fixed in 2026.8.0); from 2026.7.0-latest, before 2026.7.2 (fixed in 2026.7.2); from 2026.6.0-latest, before 2026.6.3 (fixed in 2026.6.3); from 2026.1.0-latest, before 2026.1.8 (fixed in 2026.1.8)

Published 2026-09-24. Last modified 2026-09-24.