PHP mail form is being abused I'm nowhere near a PHP expert, but I have dabbled a bit. I can make things work just fine, but I am terrible at figuring out what is wrong. I was informed by our server admin that we have a mail script that is being abused, and I am unsure of how to remedy the issue.

I am looking for help to see how someone is using this form to send out spam mail. The issue is that we got a bounce-back email sent from an address that doesn't exist to a hotmail address using this form. As far as I know, this form ONLY sends email to the $recipient which is neither this non-existent email or this Hotmail. I am trying to figure out a way to secure this form so that individuals cannot abuse this form and send mail to others through it.

&lt;?php
$name = check_input($_POST['name'],"Enter your name.");
$email = check_input($_POST['email'],"Email address is required.");
$company = check_input($_POST['company']);
$phone = check_input($_POST['phone']);
$message = check_input($_POST['message'],"Please don't send us a blank message! Let us know what you're thinking.");

if (!preg_match("/([\w\-]+\@[\w\-]+.[\w\-]+)/",$email))
{
    show_error("E-mail address is not valid.");
}

$messageBody ="Hello!

Your contact form has been submitted by:

Name: $name
E-mail: $email
Company: $company
Phone: $phone

Message:
$message

End of message
";

$recipient = "someone@example.com"; // removed real email for SE
$subject = "[Contact Form - Site Name]"; // removed site name for SE

$mailheaders = "From: someone@example.com\n"; // removed real email for SE
$mailheaders .= "Reply-To:". $_POST['email']."\n\n";

mail($recipient, $subject, $messageBody, $mailheaders);

header('Location: http://www.example.com/contact/thankyou.html'); //removed real address for SE
exit();

function check_input($data, $problem='')
{
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    if ($problem &amp;&amp; strlen($data) == 0)
    {
        show_error($problem);
    }
    return $data;
}

function show_error($mailError)
{
?&gt;
// error message in here, removed for SE as unrelated to problem
&lt;?php
exit();
}
?&gt;


This is the message my server admin sent me:

-----Original Message-----
From: MAILER-DAEMON@ss2.site-hosts.com
[mailto:MAILER-DAEMON@ss2.site-hosts.com]
Sent: Monday, August 04, 2014 2:47 PM
To: anonymous@ssasi.site-hosts.com
Subject: failure notice

Hi. This is the qmail-send program at ss2.site-hosts.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

&lt;wsinwjfjqxv@hotmail.com&gt;:
65.55.37.104 does not like recipient.
Remote host said: 550 Requested action not taken: mailbox unavailable Giving up on 65.55.37.104.

--- Below this line is a copy of the message.

Return-Path: &lt;anonymous@ssasi.site-hosts.com&gt;
Received: (qmail 6348 invoked by uid 2523); 4 Aug 2014 14:47:01 -0500
X-Qmail-Scanner-Diagnostics: from
199.192.201.219.rdns.continuumdatacenters.com by ss2.site-hosts.com (envelope-from &lt;anonymous@ssasi.site-hosts.com&gt;, uid 2020) with qmail-scanner-2.10st
 (clamdscan: 0.98.4/19261. mhr: 1.0. spamassassin: 3.3.2. perlscan:
2.10st.  
 Clear:RC:1(199.192.201.219):. 
 Processed in 0.024127 secs); 04 Aug 2014 19:47:01 -0000
Received: from 199.192.201.219.rdns.continuumdatacenters.com (HELO
ssasi.site-hosts.com) (199.192.201.219)
  by ss2.site-hosts.com with ESMTPA; 4 Aug 2014 14:47:01 -0500
Received: (qmail 15903 invoked by uid 10024); 4 Aug 2014 14:47:00 -0500
X-Qmail-Scanner-Diagnostics: from  by ssasi.site-hosts.com (envelope-from &lt;anonymous@ssasi.site-hosts.com&gt;, uid 48) with qmail-scanner-2.10st
 (clamdscan: 0.98.4/19261. mhr: 1.0. spamassassin: 3.3.2. perlscan:
2.10st.  
 Clear:RC:1(127.0.0.1):. 
 Processed in 0.013521 secs); 04 Aug 2014 19:47:00 -0000
Date: 4 Aug 2014 14:47:00 -0500
Message-ID: &lt;20140804194700.15894.qmail@ssasi.site-hosts.com&gt;
From: anonymous@ssasi.site-hosts.com
To: wsinwjfjqxv@hotmail.com
Subject: 
X-PHP-Originating-Script: 10021:vnbMail.php
Your script is vulnerable to a header-splitting attack.  Due to the poor design of PHP's mail() function, it is actually quite easy to introduce that kind of security hole.

In summary, if…


any part of the mail headers consists of user-supplied input,
and you didn't make any effort to prohibit newlines or escape that input,


then you will have a program that can send mail to any recipient of the attacker's choice.

Validation problems

As @EmanuelePaolini points out, you do make an attempt to validate $_POST['email']:

$email = check_input($_POST['email'],"Email address is required.");
# …

if (!preg_match("/([\w\-]+\@[\w\-]+.[\w\-]+)/",$email))
{
    show_error("E-mail address is not valid.");
}


However, the regular expression is not anchored with ^ and $ at each end.  Therefore, the code confirms that $_POST['email'] contains an e-mail address, but it fails to enforce that it contains nothing but a single e-mail address.

On the other hand, the regular expression is also too strict: it rejects many valid characters that can legally appear in an e-mail address.  Read RFC 5322 Sec 3.4 for the specification, or this summary of the older RFC 822 standard.