### 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.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:L
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