A good example of collaborative filtering is CloudMark, a service providing plug-ins to Outlook and Outlook Express users. Additionally, Outlook 2003 includes a collaborative filtering service operated by Microsoft. And SpamCop uses collaborative filtering techniques to create a transport-level filter as described above.
Protecting against Malware
Where spam is a (major) nuisance, malicious code, in the form of viruses and worms, is a danger to data and networks everywhere. Not only do worms running amok clog mail servers and networks, causing unintentional denial-of-service attacks, but their payload can range from benign to malicious, hitting all points in between. The most recent rash of worms seems to have been aimed at compromising computers all over the Internet to use as spam cannons, sending out streams of spam using stolen resources. Other worms and viruses are used to allow human attackers remote access to victims computers, for fun or profit (or both). These attackers can see and steal sensitive information stored on the compromised machines.
Malware propagates in every known fashion: email and network attacks via buffer overflows being the most common methods.
A short explanation buffer overflows: buffer overflows exploit programming errors to make programs behave in ways they were not intended. To explain simply, when a program takes input it places that input in predetermined places in the computer's memory. If a programmer was sloppy and forgot to check the size of the input was small enough to fit in its intended resting place, a buffer overflow can result where the input will overflow the place it was intended to fill and overwrite another portion of memory. In some cases, attackers can carefully craft this overflowing input so that actually alter instructions inside the program. From there it's a (relatively) simple step to hijack the program and use it for whatever purpose the attacker sees fit.