Improper Handling of Syntactically Invalid Structure

When software expects data in a specific format—like JSON, XML, protocol messages, or file structures—it must rigorously validate that incoming input matches that exact syntax. If the code assumes the input is always well-formed and doesn't implement proper error handling for malformed data, attackers can exploit this by sending deliberately invalid structures. This can trigger unhandled exceptions, cause the application to crash, or bypass security checks that rely on correct parsing. To prevent this, developers should implement strict validation at all data entry points, using well-tested parsers and libraries instead of custom logic. Always define clear error-handling routines that safely reject invalid input without exposing internal details, and design systems to fail securely—defaulting to a denied state—when encountering unexpected syntax. Regular fuzz testing with invalid inputs helps uncover these hidden parsing weaknesses before attackers do.