high · 8.6 Patched

StarCitizenWiki Extension Embed Video: Stored XSS via unsanitized class passed to template

CVE-2026-55691 · GHSA-7h5p-637f-jfr7 · published 19 Jun 2026

How serious is it

  • Severity: high, CVSS score 8.6 out of 10.
  • No exploit prediction (EPSS) score yet.
  • Not on the CISA list of exploited vulnerabilities.
  • No public exploit in Exploit-DB.

Is your app affected by CVE-2026-55691?

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Who is affected

  • starcitizenwiki/embedvideo (Packagist) <= 4.0.0

How to check

  1. Run composer show starcitizenwiki/embedvideo in your project to see the installed version, including copies pulled in by other packages.
  2. Or paste your lockfile into the checker: it lists every package with a known vulnerability, this one included.

How to fix

Copies a prompt for Claude Code, Cursor or Codex: it checks your lockfiles for this and upgrades it.
  1. Upgrade starcitizenwiki/embedvideo to 4.1.0 or later, commit the lockfile and redeploy.
  2. If a vulnerable version was live, check your logs and rotate the secrets that server could read.

The advisory

### Summary The user supplied class value is fed directly into the sprintf call that creates HTML. You can add a quote to escape the class and then inject arbitrary html/javascript to the final output. ### Details The template [here](https://github.com/StarCitizenWiki/mediawiki-extensions-EmbedVideo/blob/a573a16d925ee0ea0d34b360856dc8ab0b88f822/includes/EmbedService/EmbedHtmlFormatter.php#L138) adds a figure with a class that is substituted in. This value is provided to sprintf [here](https://github.com/StarCitizenWiki/mediawiki-extensions-EmbedVideo/blob/a573a16d925ee0ea0d34b360856dc8ab0b88f822/includes/EmbedService/EmbedHtmlFormatter.php#L156), an unescaped version of the class supplied by the user. ``` $template = <<<HTML <figure class="%s" data-service="%s" %s %s> <div class="embedvideo-wrapper" %s>%s%s%s</div>%s </figure> HTML; ``` ### PoC Note the double quote immediately following the single quote to escape the class attribute in the template: ``` <youtube class='" onmouseover="alert(document.domain)' id="dQw4w9WgXcQ">dQw4w9WgXcQ</youtube> ``` ### Impact Arbitrary HTML can be inserted into the DOM by any user on any page, allowing for JavaScript to be executed.

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References

Merged from GitHub Advisory Database. Last updated 23 Oct 2026. All vulnerabilities · RSS · JSON

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