CBC-MAC
A message authentication code built on CBC mode that is only safe for messages of a fixed length.
CBC-MAC runs the message through a block cipher in CBC mode and keeps only the last block as the tag. The construction is simple, and it was used for years in standards such as ISO/IEC 9797-1. It has a serious flaw: if messages can have different lengths, an attacker who has seen a valid tag can often forge one for a longer message.
The fix is CMAC, specified in NIST SP 800-38B, which adds a key-dependent final transformation and handles variable lengths safely.