Over the past couple weeks, there has been more web-based malware activity than in any previous similar period this year. The size of Google malware blacklist, which is used to mark sites with a "This site may harm your computer" annotation in their search results, exceeded 200,000 sites for the first time last week, reaching an all-time high of 229,980 today. This increase was due in part due to the rapid propagation of a drive-by-download virus named Gumblar. Compromising legitimate sites to serve malware to unsuspecting users has long been regarded as an emerging trend, but numbers like these make it clear that this attack vector is already a significant threat -- and as web applications become more and more sophisticated, the attack surface for this vector will only increase in size. Existing solutions have so far not been able to keep pace with this fast-moving threat, and new solutions may be required.