The Poison Null Byte

Some languages use null bytes (0x00) to know when a string terminates, rather than tracking the actual string length (PHP < 5.3.4 is one of these). If a null byte (generally? always? encoded as %00) is included in a string, then everything after that byte is dropped by the interpreter.

Because % characters are themselves special, null bytes need to be encoded in URLs as %2500.

Typically a null byte will either be inserted at the end of a string (to prevent a suffix from being appended or bypass simple path filters) or before a “fake” file extensions (which can cause some file-type checks to pass, again allowing us to download files we’d otherwise be denied access to).