Group-Office is an enterprise customer relationship management and groupware tool. Prior to versions 6.8.167, 25.0.102, and 26.0.27, the Contact/labels JMAP endpoint in GroupOffice accepts a fully attacker-controlled template string (tpl parameter) that is passed without restriction to TemplateParser::parse(). When the template contains an [assign] block whose expression includes a {{...}} variable reference, the engine calls eval() on the expanded expression after running it through a validateExpression() guard. The guard double-quotes unrecognized tokens, but PHP 7+ allows a string enclosed in parentheses to be invoked as a function ("system")("id") is valid PHP and calls system("id"). A single authenticated HTTP request is sufficient to execute arbitrary OS commands as the web server user (www-data). This issue has been patched in versions 6.8.167, 25.0.102, and 26.0.27.