WHMCS has released new patches for the 4.5, 5.0, 5.1, and 5.2 minor releases. These updates provide targeted changes to address security concerns with the WHMCS product. You are highly encouraged to update immediately.
WHMCS has rated these updates as including critical or important security impacts.
Releases
The following full-release versions of WHMCS have been published and address all known vulnerabilities:
5.2.6
The latest public releases of WHMCS are available inside members area at WHMCS.
WHMCS has been updated to 5.2.6 in Softaculous as well. If you have Softaculous installed on your server you can upgrade to the latest version of WHMCS via Softaculous.
PLEASE NOTE: The 4.5 series reached End Of Life as of June 30th 2013. WHMCS is aware that some customers have not moved to an LTS version due to the newness of the LTS policy. The related 4.5 patch release published along with this Security Advisory is provided as a courtesy to those customers. From this point forward, there will be no more patches provided for 4.5 or any other release that has reached EOL.
There is no reason to believe that these vulnerabilities are known to the public. As such, WHMCS will only release limited information regarding the vulnerabilities at this time.
Once sufficient time has passed to allow WHMCS customers to update their installed software, WHMCS will release additional information regarding the nature of the security issue.
These Targeted Security Releases and Patches address 9 vulnerabilities in WHMCS versions 4.5, 5.0, .5.1, and 5.2.
Source : http://www.whmcs.com