go-mail has insufficient address encoding when passing mail addresses to the SMTP client

Impact

Due to incorrect handling of the mail.Address values when a sender- or recipient address is passed to the corresponding MAIL FROM or RCPT TO commands of the SMTP client, this could lead to a possible wrong address routing or even to ESMTP parameter smuggling.

Vulnerability details

Instead of making use of the String() method of mail.Address, which takes care of proper escaping and quotation of mail address, we used the Address value of the mail.Address which is the raw value when passing it to our SMTP client.

This meant, if a mail address like this was set: "[email protected]> [email protected]"@example.com for a sender or recipient, instead of the correctly quoted/escaped address, the SMTP client would get the raw value passed which would translate into something like this being passed to the SMTP server: RCPT TO:<[email protected]> [email protected]@example.com>.

Since ORCTP is a valid command for the SMTP server, the mail would be routed to the wrong address. Additionally, other SMTP commands could potientially be smuggled in using this method causing unexpected behaviour.

Exploitation requirements

For successful exploitation of this vulnerability it is required that the user's code is allowing for arbitrary mail address input (i. e. through a web form or similar). If only static mail addresses are used (i. e. in a config file) and the mail addresses in use do not consist of quoted local parts, this should not affect your code.

Patches

The vulnerability has been fixed with PR #496 and the fix has been shipped with the go-mail v0.7.1 release.

Issue #495 holds the full report and discussion.