Prior to 5.2.1, the sendmail transport (Swift_Transport_SendmailTransport) was vulnerable to an arbitrary shell execution if the "From" header came from a non-trusted source and no "Return-Path" is configured. This has been fixed in 5.2.1. If you are using sendmail as a transport, you are encouraged to upgrade as soon as possible.
Symfony 1 has a gadget chain due to vulnerable Swift Mailer dependency that would enable an attacker to get remote code execution if a developer unserialize user input in his project.
The mail transport (aka Swift_Transport_MailTransport) in Swift Mailer before 5.4.5 might allow remote attackers to pass extra parameters to the mail command and consequently execute arbitrary code via a " (backslash double quote) in a crafted e-mail address in the (1) From, (2) ReturnPath, or (3) Sender header.
Sendmail transport arbitrary shell execution.