PHPMailer
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PHPMailer is a popular library for sending email from PHP, including WordPress, though renamed as class-phpmailer.php.
Security
PHPMailer publishes a list of security updates at https://github.com/PHPMailer/PHPMailer/blob/master/SECURITY.md
In 2016-12, a remote code execution security vulnerability was reported [1][2]. According to the security updates linked above, it was fixed in version 5.2.18.