A keylogger is a form of malware or hardware that records the keys pressed by a user on the keyboard. One major threat arising from this is the creation of logs of daily keyboard usage, which allows for the unauthorized monitoring of a user's activity on the computer.

Furthermore, a keylogger may send these logs to an external entity without the knowledge of the user. This leads to a severe security breach where very sensitive and personal information like passwords, emails, and private conversations can be revealed to unauthorized individuals. This compromises the user's privacy and security.