back door

A computer system feature -- which may be (a) an unintentional flaw, (b) a mechanism deliberately installed by the system's creator, or (c) a mechanism surreptitiously installed by an intruder -- that provides access to a system resource by other than the usual procedure and usually is hidden or otherwise not well-known. (See: maintenance hook. Compare: Trojan Horse.)

Example: A way to access a computer other than through a normal login. Such an access path is not necessarily designed with malicious intent; operating systems sometimes are shipped by the manufacturer with hidden accounts intended for use by field service technicians or the vendor's maintenance programmers.

A feature of a cryptographic system that makes it easily possible to break or circumvent the protection that the system is designed to provide.

Example: A feature that makes it possible to decrypt cipher text much more quickly than by brute-force cryptanalysis, without having prior knowledge of the decryption key.