CVE-2026-72721

Description

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.

References

• https://github.com/discourse/discourse/commit/a3e10759fef47f03d450649bb51d9e84f4c68b8f
• https://github.com/discourse/discourse/commit/c2eb6b0e5597d6f28f4be739b83300bc9c69e3cd
• https://github.com/discourse/discourse/commit/caa615c371b1696982ee11ac8f3558f0f8ced584
• https://github.com/discourse/discourse/commit/f503971d311b7b0dfdb751ee8cfd67e8cb99f5c5
• https://github.com/discourse/discourse/pull/42091