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The SNMP fiasco: steps you need to take
If you're stuck with it, then you can filter ports 7/udp 161/udp 161/tcp 162/udp 162/tcp 199/udp 199/tcp 391/udp 391/tcp 705/tcp 1993/udp and 1993/tcp. Obviously, if you need to keep any of these open you'll leave a vulnerability, but by shutting off the ones you don't need you'll make it easier to monitor for suspicious activity.
Meanwhile, check with your vendor for specific advice and patch availability relevant to your router/switch/hub/server/OS/etc. All the vendors are currently working on patches, and it may be only a matter of hours before yours makes one available if it hasn't yet. The most comprehensive single source of general information and workaround suggestions is the CERT Advisory.