Files with fun extension can be most recently found as files encrypted and affected by one of the variants of Jigsaw ransomware.
FUN file extension - Jigsaw (variant) ransomware affected file
What is fun file? How to open fun files?
File type specification:
File extension fun seems to be also used for one of the variants of Jigsaw ransomware that encrypts users files and demands bitcoins as ransom. This variant utilizes a background featuring the Joker and Batman.
Note: The self-titled Fake Jigsaw Ransomware also uses this suffix.