First off, we received a bulletin from Counterpane, a company which monitors SNMP on its clients' machines, saying that the vulnerability does not appear to have been exploited yet. So basically, you're playing 'beat the clock' with the blackhat community, and chances are that if you act soon, you'll win.
Second, this is a developing story to which we haven't got any final answers and for which our recommendations are both preliminary and incomplete, being cobbled together from numerous bulletins and a bit of homebrew tinkering.
Our inbox has been filling steadily with advisories, so we will update this item periodically as developments warrant. If you don't find the information you're looking for below, check back later.
Obviously, your quickest and surest fix is going to be disabling SNMP if you don't have to run it. Indeed, disabling unnecessary network services is a normal part of system hygeine, so this is a good opportunity to take the time and do a thorough job of it.
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.
It's unlikely that you've got this service enabled without knowing it, but not impossible. The very vulnerable SNMPv1 is shipped with Windows 9X, NT and 2K, but is not enabled by default. Win-ME has no such service, but it could have been carried over during an upgrade from 9X. Win-XP uses a later version which is also vulnerable.
Linux home users also may have it enabled without realizing it, especially if they've done default server installations which they've never got around to using. Bust open a shell and enter the command ps ax | more to see everything running on your system, and look for snmpd and/or gxsnmp. If you find it running, and have no reason to believe it ought to be running, you can very quickly chmod the executable.
Of course you can re-configure your system or your firewall, but this is the quick-and-dirty way: kill the process using the process id (under 'pid') with the command kill 'process id'. Now note the path to the executable, and enter the command chmod 0 /'dir'/'subdir'/'executable.file'.
You can simply run chmod 1 on the file when your Linux distributor releases a patch, or leave it if you have no use for the service. (I did try this on my own machine and it didn't break anything, though that's no guarrantee that someone else won't run into a snag. Still, it ought to be harmless on simple home-user installations.)
For Windows users, the procedure depends on your version:
Win-9X users can go to Control Panel/Network, and look for Microsoft SNMP Agent. If you find it, remove it.
You can also check the following registry keys
and ensure that snmp.exe is removed from the lists.
For NT, go to Control Panel/Services, and look for SNMP. If you find it, select it and click Stop; then go to Startup and click Disabled.
For 2K and XP, right click My Computer and go to Manage/Services and then Applications/Services, and look for SNMP. If you find it, select it and click Stop. Go to Startup, and click Disabled.
Microsoft says a patch is in the works. Obviously all the Linux distributors are working on patches as well. At this writing, Red Hat has already released theirs, which are available as RPMs from ftp://updates.redhat.com. ®