http://blog.whmcs.com/?t=85887
WHMCS 5.3.5 is now available to download.
This update addresses 4 specific issues identified following the 5.3.4 release.
You may download either an incremental patch set (only for 5.3.4 upgrades) or the full file version (valid for upgrading any previous version or for new installs) via our Downloads Page.
Download: http://download.whmcs.com/
Changelog: http://changelog.whmcs.com/WHMCS_V5.3
Release Notes: http://docs.whmcs.com/Version_5.3_Release_Notes
Note to 5.3.3 Users
If you are running Version 5.3.3 currently, and do not wish to use the full version to upgrade, then you can apply the 5.3.4 incremental patch set followed by the 5.3.5 incremental patch set. Both of these are made available via the Downloads page.
IMPORTANT: For each incremental patch set you wish to apply, remember to visit the Admin area and confirm the appropriate version number is displayed before applying a subsequent incremental patch set.