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Vulnerability · Published Aug 10, 2026 · Assigned by GitHub_M

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

Medium 5.3

In plain English

A medium-severity security flaw in discourse from discourse. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 48% of all known vulnerabilities).

Severity (CVSS 3.1)5.3Medium
Exploit odds (30 days)0.6%top 51.9% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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 (as reported)

VendorProductVersions
discourse discourse < 2026.1.6>= 2026.5.0-latest, < 2026.5.2>= 2026.6.0-latest, < 2026.6.1>= 2026.7.0-latest, < 2026.7.0

Weakness: CWE-178 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

References