CVE-2026-91120
Deferred Deferred - Pending Action

XSS in Discourse via Lazy Video Embed Titles

Publication date: 2026-09-24

Last updated on: 2026-09-24

Assigner: GitHub, Inc.

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.

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Meta Information

Published
2026-09-24
Last Modified
2026-09-24
Generated
2026-09-24
AI Q&A
2026-09-24
EPSS Evaluated
N/A
NVD
CVE-2026-91120
EUVD
EUVD-2026-86304

Affected Vendors & Products

Showing 4 associated CPEs
Vendor Product Version / Range
discourse discourse to 2026.1.8 (inc)
discourse discourse to 2026.6.3 (inc)
discourse discourse to 2026.7.2 (inc)
discourse discourse to 2026.8.0 (inc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-79 The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

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Executive Summary

This vulnerability in Discourse allows a user with standard posting privileges to inject HTML markup into notification emails or chat summaries by crafting a video title with embedded markup for enabled providers like YouTube, Vimeo, or TikTok. The system fails to properly sanitize the title, treating it as HTML instead of plain text.

Detection Guidance

This vulnerability can be detected by checking the Discourse version installed on your system. If your version is below 2026.1.8, 2026.6.3, 2026.7.2, or 2026.8.0, it is vulnerable. Run commands like 'discourse version' or check the version in the admin panel.

Impact Analysis

If you receive Discourse notifications via HTML-capable email clients, an attacker could inject malicious content like event handlers or misleading text into these messages. This could lead to phishing attempts or misinformation, but does not directly compromise your Discourse session or forum pages.

Mitigation Strategies

Immediately update Discourse to version 2026.1.8, 2026.6.3, 2026.7.2, or 2026.8.0 or later. Disable lazy video embeds if not required, as this feature is enabled by default and is a prerequisite for exploitation.

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