FUD (Fully undetectable) it is a method of hacking. Every successful hack was done and perfectly execute because of two reasons.
- Social engineering
- Loop hole in system.
Day by day hacking attacks get more power and systems are also patches for stop the attack.
Some antivirus can detect harmful trojan, rat etc then it will be block by the antivirus and atlast the attack was failed.
For avoiding this situation hacker's sent Trojan, rat by encrypts method or they will attach the file with another file, whenever user open the file the malicious/Trojan was automatically install on the victim's computer and antivirus can't detected the file cause of encryption. So the file is Fully Undetectable.