Proper prevention of mail injection in PHP

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']);

Suppose you you want to put the email address of the visitor in the optional header field like so:

$headers = "From: $visitorEmailAddress";

However, if

$visitorEmailAddress

contains

"[email protected]\n\nBCC:[email protected]"

you've made yourself a spam host, opening the door for mail injection. This is a very simple example, but creative spammers and malicious hackers can sneak potentially damaging scripts in your email, since email is sent as a plaintext file. Even attachments are converted plaintext, and they can easily send attachements by adding a mimetype content line.

If your form validation for the FROM and/or TO fields is OK, you have to look at the form validation for the body of the email. I'd strip out the '-=' and '=-' characters, and prevent users from typing plain HTML by using strip_tags().

Try this for a review of various options:

http://www.codeproject.com/Articles/428076/PHP-Mail-Injection-Protection-and-E-Mail-Validatio

It covers several options and tries to explain the benefits and risks of each.