PHPMailer 6.4.1 and earlier contain a vulnerability that can result in untrusted code being called (if such code is injected into the host project's scope by other means). If the $patternselect parameter to validateAddress() is set to 'php' (the default, defined by PHPMailer::$validator), and the global namespace contains a function called php, it will be called in preference to the built-in validator of the same name. Mitigated in PHPMailer 6.5.0 by denying the use of simple strings as validator function names.

Reference:
https://www.huntr.dev/bounties/1-PHPMailer/PHPMailer/

Upstream patch:
https://github.com/PHPMailer/PHPMailer/commit/45f3c18dc6a2de1cb1bf49b9b249a9ee36a5f7f3