The issue allows command injection via the PhpWiki 1.3.10 SyntaxHighlighter plugin in the Project Wiki component. This occurs because the proc open PHP function is used with a syntax value in its first argument, which can be controlled by an authenticated Tuleap user, including shell metacharacters. For example, a line like '<?plugin SyntaxHighlighter syntax=`c;id`>' can be used to execute the id command.