Could you advise me how do I go about preventing email injection in PHP mail() without losing original message data? E.g. if I need to allow user to use \r\n, To, CC etc, so I do not want to completely strip them away from the message - I still want them delivered, but without adding any additional headers or somehow allowing mail injection to happen.
Most of the advices on internet suggest stripping that data away completely - but I do not want to do that.
I am sending plain text (non HTML) messages through PHP mail() function.
What would you advise?
To filter valid emails for use in the recipient email field, take a look at filter_var():
$email = filter_var($_POST['recipient_email'], FILTER_VALIDATE_EMAIL);
if ($email === FALSE) {
echo 'Invalid email';
exit(1);
}
This will make sure your users only supply singular, valid emails, which you can then pass to the mail() function. As far as I know, there's no way to inject headers through the message body using the PHP mail() function, so that data shouldn't need any special processing.
Update:
According to the documentation for mail(), when it's talking directly to an SMTP server, you will need to prevent full stops in the message body:
$body = str_replace("\n.", "\n..", $body);
Update #2:
Apparently, it's also possible to inject via the subject, as well, but since there is no FILTER_VALIDATE_EMAIL_SUBJECT, you'll need to do the filtering yourself:
$subject = str_ireplace(array("\r", "\n", '%0A', '%0D'), '', $_POST['subject']);