CVE-2026-91159: Discourse: Stored oEmbed HTML injection via allowed iframe
Discourse is an open-source discussion platform. Prior to 2026.9.0, 2026.8.1, 2026.7.3, and 2026.1.9, oEmbed processing treated an entire attacker-controlled HTML fragment as trusted when the fragment contained an iframe whose source matched the configured allowlist. A user with posting permission who controlled a public webpage and its oEmbed response could combine an allowlisted iframe with unrelated sibling HTML or CSS, causing stored visual overlays or other styled content to appear in post content and enabling misleading or fraudulent user interfaces. Exploitation requires a posting account, control of the fetched webpage and oEmbed response, and an allowlisted iframe source. The issue affects content integrity but does not expose restricted data, disrupt availability, or require interaction with Discourse Content Security Policy. This issue is fixed in versions 2026.9.0, 2026.8.1, 2026.7.3, and 2026.1.9.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Discourseto a version that resolves this vulnerability.Fixed in 2026.9.0 - Upgrade
Upgrade
Discourseto a version that resolves this vulnerability.Fixed in 2026.8.1 - Upgrade
Upgrade
Discourseto a version that resolves this vulnerability.Fixed in 2026.7.3 - Upgrade
Upgrade
Discourseto a version that resolves this vulnerability.Fixed in 2026.1.9
Event History
Frequently Asked Questions
Who can exploit this issue?
An attacker needs a Discourse account with permission to post, control of a public webpage and its oEmbed response, and an iframe source that matches the instance's configured allowlist.
Are default installations affected?
Exploitation depends on the configured iframe allowlist. The attacker must be able to include an iframe whose source is allowed by that configuration.
What is the impact if exploited?
An attacker can store unrelated HTML or CSS alongside an allowlisted iframe, creating visual overlays or other misleading content in posts. The issue affects content integrity; it does not expose restricted data or disrupt availability.
Does Discourse Content Security Policy prevent exploitation?
No. Exploitation does not require interaction with Discourse Content Security Policy.
Which versions contain the fix?
Update to 2026.9.0, 2026.8.1, 2026.7.3, or 2026.1.9, as applicable to the release line in use.