3.4 The TLS alert protocol

The alert protocol is there to allow signals to be sent between peers. These signals are mostly used to inform the peer about the cause of a protocol failure. Some of these signals are used internally by the protocol and the application protocol does not have to cope with them (e.g. GNUTLS_A_CLOSE_NOTIFY), and others refer to the application protocol solely (e.g. GNUTLS_A_USER_CANCELLED). An alert signal includes a level indication which may be either fatal or warning (under TLS1.3 all alerts are fatal). Fatal alerts always terminate the current connection, and prevent future re-negotiations using the current session ID. All supported alert messages are summarized in the table below.

The alert messages are protected by the record protocol, thus the information that is included does not leak. You must take extreme care for the alert information not to leak to a possible attacker, via public log files etc.