An anti-virus signature is a unique pattern used to detect viruses, malware, and other such problematic software.
Types of signatures:
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Static signatures: fixed patterns within malware’s binary files such as a string or series of bytes.
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Hash signatures: MD5, SHA256, … hashes of known malicious files.
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Heuristic signatures: looks for traits within malware such as permissions and layout of program sections, and other characteristics that may differ from “normal”, benign files.
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Behavioral signatures: look for unusual system calls, network traffic, file accesses, etc.