What’s Vulnerable › Vulnerabilities

CVE-2026-72721: Discourse: Onebox Domain Blocklist Bypass via Case-Sensitive Comparison

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-10 · updated 2026-09-08
• Weakness: CWE-178

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.

Affected products

• discourse: before 2026.1.6; from 2026.5.0-latest, < 2026.5.2; from 2026.6.0-latest, < 2026.6.1; from 2026.7.0-latest, < 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
• https://github.com/discourse/discourse/pull/42092
• https://github.com/discourse/discourse/pull/42093
• https://github.com/discourse/discourse/pull/42094
• https://github.com/discourse/discourse/security/advisories/GHSA-3x7x-24rq-h5j6

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