How do polymorphic and metamorphic malware work, and why do they bypass signature analysis?

Polymorphic and metamorphic malware represent advanced types of threats created specifically to evade traditional antiviruses that rely on signature analysis. The signature method looks for specific invariant sequences of bytes in files, known as hashes or unique fragments of virus code. Malware creators have learned to change this outer shell of programs so that antivirus databases cannot recognize them.

Polymorphic viruses use a complex mechanism to encrypt their main body. Each time a new file or system is infected, the virus generates a new random encryption key and completely rewrites its decoder. As a result, the file looks completely different on the outside, and its hash sum changes, although the malicious functionality itself remains the same. The antivirus sees a new signature each time and misses the threat.

Metamorphic viruses go even further, as they do not just encrypt themselves, but completely rewrite their own source or machine code upon each propagation. They can change the order of instructions, replace some commands with equivalent ones, add useless junk code, and change processor registers. To counter such threats, modern defense uses heuristic analysis, behavioral monitoring, and artificial intelligence technologies.