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CVE-2026-91120: Discourse: Stored HTML injection in video notification emails

• Severity: medium (CVSS 5.4 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-24 · updated 2026-09-24
• Weakness: CWE-79

Description

Discourse is an open-source discussion platform. Prior to 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0, provider-controlled video titles in lazy video embeds could be reparsed as HTML when Discourse generated notification emails or chat summaries. A user with standard posting privileges could create a post for an enabled YouTube, Vimeo, or TikTok provider whose title contained markup, and the email and chat-summary rendering path would place that markup into the output instead of treating it solely as text. Recipients using HTML-capable email clients could see injected content, potentially including event handlers, but the issue did not directly expose a Discourse browser session or affect the forum page itself. Lazy video embeds must be enabled, which is the default configuration. This issue is fixed in versions 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0.

Affected products

• discourse: before 2026.8.0; from 2026.7.0-latest, < 2026.7.2; from 2026.6.0-latest, < 2026.6.3; from 2026.1.0-latest, < 2026.1.8

References

• https://github.com/discourse/discourse/commit/6bf1cdcb3ae1aa146e1fd953a62046bf4a1f143f
• https://github.com/discourse/discourse/commit/8338919df4ecbd0fb85314b762772cc033b764e3
• https://github.com/discourse/discourse/commit/e7de8285203a2edf85beff3b951e66aeb7e5dc76
• https://github.com/discourse/discourse/commit/ed8b47df80bcc9f0bd70773fc399c9386f9d52dd
• https://github.com/discourse/discourse/pull/42882
• https://github.com/discourse/discourse/security/advisories/GHSA-h7cg-2vww-m45c

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