Polyglot Files: How One Byte Sequence Can Fool Two Different File Parsers

A polyglot file is a single byte sequence deliberately structured to satisfy the syntax rules of two or more different file formats simultaneously. This is possible because different formats make different structural assumptions — some parsers only check the beginning of a file, while others, like ZIP, anchor their critical structures near the end. This leaves regions within the file where a second format's structures can coexist without conflicting with the first. Unlike simple extension spoofing, polyglot files genuinely pass content-level validation by multiple parsers, making them a subtle security concern. The phenomenon exploits gaps and tolerances in how file format specifications define required versus ignorable byte regions.