We have received an important security advisory for WHMCS as shown below.

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WHMCS Security Advisory for 4.x, 5.0, 5.1
http://blog.whmcs.com/?t=72051
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WHMCS has released new patches for the 4 series, 5.0 and 5.1 minor releases.
These updates provide targeted changes to address security concerns with the
WHMCS product. If you are not running the WHMCS 5.2, you are highly encouraged
to update immediately.

WHMCS has rated these updates as including critical or important security
impacts. Information on security ratings is available at
http://docs.whmcs.com/Security_Levels

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Releases
++++++++++++
The following full-release version of WHMCS have been published and address all
known vulnerabilities:
5.1.6

The latest public releases of WHMCS are available inside our members area at
https://www.whmcs.com/members/clientarea.php

++++++++++++++++++++++++++++++++++++
Security Issue Information
++++++++++++++++++++++++++++++++++++
The resolved security issue was identified by Dinesh Kumar Mohanty of Ultra Web
Solutions Private Limited, India. 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 1 vulnerability in WHMCS version 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 5.0, and
5.1. Additional, supplemental information is scheduled to be released May 28th,
2013.

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Mitigation
++++++++++++

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WHMCS Version 4.x
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Download and apply the appropriate patch files to protect against these
vulnerabilities.

Patch files for affected versions of the 4.x series are located on the WHMCS
site as itemized below.

4.0 series: http://www.whmcs.com/download/258/v403patch
4.1 series: http://www.whmcs.com/download/262/v413patch
4.2 series: http://www.whmcs.com/download/266/v422patch
4.3 series: http://www.whmcs.com/download/270/v432patch
4.4 series: http://www.whmcs.com/download/274/v443patch
4.5 series: http://www.whmcs.com/download/278/v454patch

To apply the patch, simply download the appropriate patch file specific to the
WHMCS version you are running, extract the contents, and upload the files from
the /whmcs/ folder to your installation.

No install or upgrade process is required.

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WHMCS Version 5.x
------------------------
Download and apply the appropriate full-version or patch of WHMCS to protect
against these vulnerabilities.

Full-version and patches for the affected version of the 5.x series are located
in the WHMCS members area download section, under your license details.

v5.0.5 (patch only)
v5.1.6 (full-version and patch)

When updating from v5.0.4 or v5.1.5, the upgrade process is not required. To
apply the full-version or patch, simply download the appropriate file specific
to the WHMCS version you are running, extract the contents, and upload the files
from the /whmcs/ folder to your installation.

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WHMCS Limited
www.whmcs.com

- Support: http://support.whmcs.com/
- Documentation: http://docs.whmcs.com/
- Members Area: http://www.whmcs.com/members/
If you need any further information or assistance please open a support ticket.