CERT and numerous vendors are making a major announcement today regarding a DNS protocol vulnerability that may enable cache poisoning of recursive resolvers. From the CERT page:
Recent additional research into [DNS defects and deficiencies] and methods of combining them to conduct improved cache poisoning attacks have yielded extremely effective exploitation techniques. Caching DNS resolvers are primarily at risk--both those that are open (a DNS resolver is open if it provides recursive name resolution for clients outside of its administrative domain), and those that are not. These caching resolvers are the most common target for attackers; however, stub resolvers are also at risk.
We can expect patches from most vendors that will implement randomization of query source ports. According to ISC, source port randomization only increases the difficulty of the attack, but does not entirely prevent it. The best prevention, they say, is to implement DNSSEC. Here are some vendor announcements: The vulnerability was discovered by Dan Kaminsky of IOActive.