UPX provides zero security against a determined cracker. Dedicated unpackers exist ( upx -d can decompress most UPX files). It’s purely for file size and basic obfuscation.
It is impossible to discuss alternatives without mentioning the "little brother" of the Enigma Protector. is a free application virtualization system offered by the same developers. enigma protector alternative free
If you are selling a $5 utility tool, free alternatives are perfect. If you are selling a $500 B2B software suite, buy Enigma (or a competitor). The time you waste building a free license server and debugging false positives will exceed the cost of a commercial license. UPX provides zero security against a determined cracker
int main() VMProtectBegin("MainFunction"); enigma protector alternative free