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Worms (posted 2003-08-24)

The worm situation seems to be getting worse, but fortunately worm makers fail to exploint the full potential of the vulnerabilities they use. For instance, we were all waiting to see what would happen to the Windows Update site when the "MS Blaster" worm was going to attack it on august 16th. But nothing happened. I read that Microsoft took the site offline to avoid problems, having no intention to bring it back online again, but obviously this information was incorrect because they're (back?) online now.

However, Microsoft received help from another worm creator who took it upon him/herself to fix the security hole that Blaster exploits, by first exploiting the same vulnerability, then removing Blaster and finally downloading Microsoft's patch. But in order to help scanning, the new worm (called "Nachi") first pings potential target, leading to huge ICMP floods in some networks, although others didn't see much traffic generated by the new worm. Nachi uses an uncommong ICMP echo request packet size of 92 bytes, which makes it possible to filter the worm without having to block all ping traffic. See Cisco's recommendations. It seems TNT dial-up concentrators have a hard time handling this traffic and reboot periodically. The issue seems to be lack of memory/CPU to cache all the destinations the worm tries to contact, just like what happened with the MS SQL worm earlier this year and others before it.

Then there's the Sobig worm. This one uses email to spread, which has two advantages: the users actually see the worm so they can't ignore the issue and the network impact is negligible as the spread speed is limited by mail server capacity. Unfortunately, this one forges the source with email addresses found on the infected computer, which means "innocent bystanders" receive the blame for sending out the worm. Interestingly enough, I haven't received a single copy of either this worm or its backscatter so far, even though it seems to be rather aggressive, many people report receiving lots (even thousands) of copies.

Then on friday there was a new development as it was discovered that Sobig would contact 20 IP addresses at 1900 UTC, presumably to receive new instructions/malicious code. 19 of the addresses were offline by this time, and the remaining one immediately became heavily congested. So far, nobody has been able to determine what was supposed to happen, but presumably, it didn't.

There was some discussion on the NANOG list about whether it's a good idea to block TCP/UDP ports to help stop or slow down worms. The majority of those who posted their opinion on the subject feel that the network shouldn't interfere with what users are doing, unless the network itself is at risk. This means temporary filters when there is a really aggressive worm on the loose, but not permanently filtering every possible vulnerable service. However, a sizable minority is in favor of this. But apart from philosophical preferences, it makes little sense to do this as the more you filter, the bigger the impact for legitimate users, and it has been well-established that worms manage to bypass filters and firewalls, presumably through VPNs or because people bring in infected laptops and connect them to the internal network.

Something to look forward to: with IPv6, there are so many addresses (even in a single subnet) that simply generating random addresses and see if there is a vulnerable host there isn't a usable approach. On the other hand, I've already seen scans on a virtual WWW server which means that the scanning happened using the DNS name rather than the server's IP address, so it's unlikely that the IPv6 internet will remain completely wormfree, even if things won't be as bad as they're now in IPv4.