---
id: CVE-2026-72721
title: Discourse is an open-source discussion platform
summary: >-
  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
  attac…
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N'
cwe:
  - CWE-178
published: '2026-08-10'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T20:54:37.790'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-72721'
references:
  - url: >-
      https://github.com/discourse/discourse/commit/a3e10759fef47f03d450649bb51d9e84f4c68b8f
    label: security-advisories@github.com
  - url: >-
      https://github.com/discourse/discourse/commit/c2eb6b0e5597d6f28f4be739b83300bc9c69e3cd
    label: security-advisories@github.com
  - url: >-
      https://github.com/discourse/discourse/commit/caa615c371b1696982ee11ac8f3558f0f8ced584
    label: security-advisories@github.com
  - url: >-
      https://github.com/discourse/discourse/commit/f503971d311b7b0dfdb751ee8cfd67e8cb99f5c5
    label: security-advisories@github.com
  - url: 'https://github.com/discourse/discourse/pull/42091'
    label: security-advisories@github.com
  - url: 'https://github.com/discourse/discourse/pull/42092'
    label: security-advisories@github.com
  - url: 'https://github.com/discourse/discourse/pull/42093'
    label: security-advisories@github.com
  - url: 'https://github.com/discourse/discourse/pull/42094'
    label: security-advisories@github.com
  - url: >-
      https://github.com/discourse/discourse/security/advisories/GHSA-3x7x-24rq-h5j6
    label: security-advisories@github.com
tags:
  - nvd
epss: 0.00585
epssPercentile: 0.45669
ingestedAt: '2026-09-08T21:11:12.272Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.