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CVE-2026-55691 – Stored XSS – EmbedVideo MediaWiki Extension < 4.1.0

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Summary

CVE-2026-55691 is a stored cross-site scripting (XSS) vulnerability in the EmbedVideo extension for MediaWiki, maintained by StarCitizenWiki. The extension passes a user-supplied class value directly into a sprintf-built HTML <figure> element without escaping, allowing an attacker to break out of the attribute and inject arbitrary HTML/JavaScript that executes for any visitor viewing the wiki page. The issue carries a CVSS v3.1 score of 8.6 (High) and requires no authentication to exploit.

Technical details

  • Root cause: EmbedHtmlFormatter::toHtml (in includes/EmbedService/EmbedHtmlFormatter.php) substitutes the user-controlled class parameter into an HTML template via sprintf without sanitizing or escaping it.
  • Trigger conditions: A user who can edit a wiki page supplies a class value containing a quote character to a video-embedding tag (e.g., <youtube>), which terminates the class attribute early and permits injection of additional HTML attributes/markup, such as an onmouseover event handler.
  • Attack vector: Network-based, no privileges beyond the ability to edit a wiki page, no user interaction required beyond a victim viewing the affected page (AV:N/AC:L/PR:N/UI:N).
  • Impact: Stored JavaScript executes in the browser context of any visitor who renders the malicious page content, resulting in high confidentiality impact and limited integrity/availability impact (session hijacking, credential theft, or further compromise of the wiki session).

Affected software

  • EmbedVideo MediaWiki Extension (StarCitizenWiki) — all versions up to and including 4.0.0

Severity

  • CVSS v3.1 Base Score: 8.6 (High)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:L

Mitigation and recommended actions

  • Immediate: Upgrade the EmbedVideo extension to version 4.1.0 or later, which fixes the root cause by properly escaping the class value before insertion into HTML output.
  • If no patch can be applied immediately: Restrict page-editing privileges to trusted users only, and review existing wiki content for embedded video tags containing suspicious class attributes (e.g., quote characters or embedded event handlers) as a temporary detective control until the upgrade can be applied.

References

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