CERT has told us of an authentication bypass flaw in Net-SNMP and UCD-SNMP.

According to net-snmp:

"The quick technical summary is that the SNMPv3 packet contains a truncated HMAC authentication code. The author that wrote the code very very long ago to check that HMAC code used the length of the packet's version of the HMAC code to do the check. Thus if you send a single byte HMAC code, it'll only check it against the first byte of HMAC output. Thus it's fairly easy to spoof an authenticated SNMPv3 packet."