CVE-2026-72721: Discourse

Medium severity, CVSS 5.3. EPSS: 0.6% chance of exploitation in the next 30 days.

Discourse is an open-source discussion platform. Prior to 2026.1.6, 2026.5.2, 2026.6.1, and 2026.7.0, Onebox::DomainChecker.is_blocked? compares hostnames and SiteSetting.blocked_onebox_domains entries case-sensitively, allowing an attacker to bypass configured Onebox domain restrictions by changing character casing in a redirect target hostname. This issue is fixed in versions 2026.1.6, 2026.5.2, 2026.6.1, and 2026.7.0.

Affected products

• Discourse Discourse: before 2026.1.6 (fixed in 2026.1.6); from 2026.5.0-latest, before 2026.5.2 (fixed in 2026.5.2); from 2026.6.0-latest, before 2026.6.1 (fixed in 2026.6.1); from 2026.7.0-latest, before 2026.7.0 (fixed in 2026.7.0)

Published 2026-08-10. Last modified 2026-09-08.