Discourse: Stored oEmbed HTML injection via allowed iframe

Summary

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.

Timeline

1. Oct 14, 2026, 3:30 PM UTCCVE published
2. Oct 20, 2026, 1:33 AM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: partial

3. Oct 20, 2026, 1:33 AM UTCRecord last updated

Affected versions

• discourse discourse

  fixed in 2026.1.9; From 2026.7.0-latest · fixed in 2026.7.3; From 2026.8.0-latest · fixed in 2026.8.1; From 2026.9.0-latest · fixed in 2026.9.0

• discourseBitnami

  fixed in 2026.1.9; From 2026.7.0 · fixed in 2026.7.3; From 2026.8.0 · fixed in 2026.8.1; From 2026.8.99 · fixed in 2026.9.0

References 5