A polyglot file is a single file engineered to be accepted as valid by more than one file format parser. For example, one blob may look like an image to one tool, a document or archive to another, or contain alternate structures that different software prefers when reading it.
In cybersecurity, polyglots matter because many defenses trust file extensions, MIME types, or a single parser result. Attackers can use format ambiguity to bypass upload filters, confuse preview or conversion services, hide payloads, or trigger different behavior in different components of a pipeline. Defenders look for this by checking byte-level signatures, validating complete structure, and testing files with multiple parsers before accepting or processing them. The key lesson is simple: a file’s label is not proof of what it really is.