CVE-2026-55691

The EmbedVideo Extension is a MediaWiki extension which adds a parser function called #ev and various parser tags for embedding video clips from various video sharing services. Prior to 4.1.0, EmbedHtmlFormatter::toHtml in includes/EmbedService/EmbedHtmlFormatter.php passes the user-supplied class value directly to sprintf while constructing a figure element. A quote in the class value can terminate the class attribute and inject arbitrary HTML attributes or markup into the rendered page. A user able to edit a wiki page can store JavaScript that executes for visitors who render the affected content. This issue is fixed in version 4.1.0.

The EmbedVideo Extension is a MediaWiki extension which adds a parser function called #ev and various parser tags for embedding video clips from various video sharing services. Prior to 4.1.0, EmbedHtmlFormatter::toHtml in includes/EmbedService/EmbedHtmlFormatter.php passes the user-supplied class value directly to sprintf while constructing a figure element. A quote in the class value can terminate the class attribute and inject arbitrary HTML attributes or markup into the rendered page. A user able to edit a wiki page can store JavaScript that executes for visitors who render the affected content. This issue is fixed in version 4.1.0.

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 adds a figure with a class that is substituted in. This value is provided to sprintf here, 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.

The EmbedVideo Extension is a MediaWiki extension which adds a parser function called #ev and various parser tags for embedding video clips from various video sharing services. Prior to 4.1.0, EmbedHtmlFormatter::toHtml in includes/EmbedService/EmbedHtmlFormatter.php passes the user-supplied class value directly to sprintf while constructing a figure element. A quote in the class value can terminate the class attribute and inject arbitrary HTML attributes or markup into the rendered page. A user able to edit a wiki page can store JavaScript that executes for visitors who render the affected content. This issue is fixed in version 4.1.0.

The EmbedVideo Extension is a MediaWiki extension which adds a parser function called #ev and various parser tags for embedding video clips from various video sharing services. Prior to 4.1.0, EmbedHtmlFormatter::toHtml in includes/EmbedService/EmbedHtmlFormatter.php passes the user-supplied class value directly to sprintf while constructing a figure element. A quote in the class value can terminate the class attribute and inject arbitrary HTML attributes or markup into the rendered page. A user able to edit a wiki page can store JavaScript that executes for visitors who render the affected content. This issue is fixed in version 4.1.0.

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 adds a figure with a class that is substituted in. This value is provided to sprintf here, 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.