The 1.6.4 release files were contaminated by code that was not meant to be there and could open a security vulnerability on your forum. It only affects those that are running 1.4.4. We are still investigating how our release became contaminated and if we find anything else in the mean time, well be sure to let you know. We advise that you fix the problem as soon as you can. If you have renamed index.php, for example if youre using the portal as your homepage, please remember to update the correct file accordingly.”]
Source: https://blog.mybb.com/2011/10/06/1-6-4-security-vulnerabilit/