LinuxSecurity.com: The net-snmp daemon implements the "simple network management protocol". The version 3 of SNMP as implemented in net-snmp uses the length of the HMAC in a packet to verify against a local HMAC for authentication. An attacker can therefore send a SNMPv3 packet with a one byte HMAC and guess the correct first byte of the local HMAC with 256 packets (max).