A script is FUD to an antivirus when it can infect a target machine and operate without being noticed on that machine by that AV. This usually entails a script that is simple, small, and precise

To know how to write a FUD script, one must understand what the targeted antivirus is actually looking for. If the script contains events such as Hook_Keyboard(), File_Delete(), or File_Copy(), it’s very likely it wil be picked up by antivirus scanners, so these events are not used. Further, FUD scripts will often mask function names with common names used in the industry, rather than naming them things like fToPwn1337(). A talented attacker might even break up his or her files into smaller chunks, and then hex edit each individual file, thereby making it even more unlikely to be detected.

As antivirus software becomes more and more sophisticated, attackers become more sophisticated in response. Antivirus software such as McAfee is much harder to fool now than it was 10 years ago. However, there are talented hackers everywhere who are more than capable of writing fully undetectable scripts, and who will continue to do so. Virus protection is very much a cat and mouse game.