Wide scale SNMP vulnerabilities (posted 2002-03-30)
On February 12th, CERT published "CERTxae Advisory CA-2 002-03 Multiple Vulnerabilities in Many Implementations of the Simple Network Ma nagement Protocol (SNMP)".
Details haven't been published yet, but it seems it is possible to do all kinds of bad things by firing off non-spec SNMPv1 packets to boxes from many vendors.
Cisco has a security advisory about the problem. Cisco has a bad track record when SNMP security is concerned: in older IOS versions there were "hidden" SNMP communities that enabled pretty much anyone to manage the router. It seems this problem has resurfaced in another form: when you create a trap community, this automatically enables processing of incoming SNMP messages for this community, even though this community doesn't provide read or write access. However, this is enough to open the router for denial of service attacks. It is possible to apply an access list to the trap community, but this depends on the order in which the configuration is processed, so it will not survive a reboot.
The only way to be completely secure is to turn off SNMPv1 or filter incoming SNMP packets on the interfaces rather than at the time of SNMP processing. (Remember, this is UDP so the source addresses are easily spoofed.) Upgrading your IOS software image will also do the trick, as soon as they are available. Consult a certified Cisco IOS version specialist to help you find the right one (more than half of the advisory consists of a list of IOS versions).