How to sanitze user input in PHP before mailing?

Question

I have a simple PHP mailer script that takes values from a form submitted via POST and mails them to me:

<?php
$to = "me@example.com";

$name = $_POST['name'];
$message = $_POST['message'];
$email = $_POST['email'];

$body  =  "Person $name submitted a message: $message";
$subject = "A message has been submitted";

$headers = 'From: ' . $email;

mail($to, $subject, $body, $headers);

header("Location: http://example.com/thanks");
?>

How can I sanitize the input?

Answer 1

Sanitize the post variable with filter_var().

Example here. Like:

echo filter_var($_POST['email'], FILTER_SANITIZE_EMAIL);   

Answer 2

Since you're not building an SQL query or anything here, the only relevant validation that I can see for those inputs is an email validation for $_POST["email"], and maybe an alphanumeric filter on the other fields if you really want to limit the scope of what the message can contain.

To filter the email address, simply use filter_var:

$email = filter_var($email, FILTER_SANITIZE_EMAIL);

As per Frank Farmer's suggestion, you can also filter out newlines in the email subject:

$subject = str_replace(array("\r","\n"),array(" "," "),$subject);

Answer 3

As others have noted, filter_var is great. If it's not available, add this to your toolchest.

The $headers variable is particularly bad security-wise. It can be appended to and cause spoofed headers to be added. This post called Email Injection discusses it pretty well.

filter_var is great, but another way to assure that something is an email address and not something bad is to use an isMail() function. Here's one:

function isEmail($email) {
    return preg_match('|^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]{2,})+$|i', $email);
};

So to use this, you could do:

if (isset($_POST['email']) && isEmail($_POST['email'])) {
    $email = $_POST['email'] ;
} else {
    // you could halt execution here, set $email to a default email address
    // display an error, redirect, or some combination here,
}

In terms of manual validation, limiting the length using substr(), running strip_tags() and otherwise limiting what can be put in.

Answer 4

You need to remove any newlines from input provided by users in $headers, which gets passed to mail() ($email in your case)! See Email injection.

PHP should take care of sanitizing $to and $subject, but there are versions of PHP with bugs (Affected are PHP 4 <= 4.4.6 and PHP 5 <= 5.2.1, see MOPB-34-2007).

Answer 5

You can use the code from artlung's answer above to validate email..

I use this kind of code to prevent header injection ..

// define some mail() header's parts and commonly used spam code to filter using preg_match
$match = "/(from\:|to\:|bcc\:|cc\:|content\-type\:|mime\-version\:|subject\:|x\-mailer\:|reply\-to\:|\%0a|\%0b)/i";

// check if any field's value containing the one or more of the code above
if (preg_match($match, $name) || preg_match( $match, $message) || preg_match( $match, $email)) {

// I use ajax, so I call the string below and send it to js file to check whether the email is failed to send or not
echo "failed";

// If you are not using ajax, then you can redirect it with php header function i.e: header("Location: http://example.com/anypage/");

// stop the script before it reach or executing the mail function
die();

}

The mail()'s header filtering above is too strict, since some users may be using the filtered strings in their message without any intention to hijack your email form, so redirect it to a page that is explaining what kind of strings that is not allowed in the form or explain it on your form page.