DNS hacked again

Amit Klein recently released details on DNS server cache poisoning attacks that affect both BIND (Berkeley Internet Name Domain) and Windows DNS servers. It goes to show that every time you think a problem with a well-known protocol or service has been solved, it may not be.

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DNS has been with us since 1983 – nearly as long as the Internet. And although DNS RFCs have come and gone, DNS is still very similar to its original specifications. Certainly it has grown in feature set and complication, but it still has the same underlying security problems it did when it was invented by Paul Mockapetris. The biggest problem is the lack of default authentication. Several security mechanisms have been created for DNS with varying degrees of success (and failure) to solve the authentication problem, but it is still relatively easy to fake a DNS packet to either a DNS server or an unwitting client.

Klein's last find involved two discoveries, both of which allow important parts of a DNS server packet to be forged with trivial effort. The first implementation error involves the DNS UDP source port. Although it should be randomized to prevent forging, it turns out that the source port never changes the whole time the DNS server is up and running. The second, and more important, problem is the trivial predictability of the transaction ID value. Both errors allow DNS server packet information to be predicted and forged.

An attacker can send a malicious Web page link and induce an end-user to click on the link. The clicked link sends off a DNS client query, which can be forged, sending the end-user to a bogus location. DNS has been found vulnerable in the same way before. In fact, Klein laments, "It is saddening to realize that 10-15 years after the dangers of predictable DSN transaction ID were discovered" that DNS software is still susceptible to transaction ID exploitation.

Klein reported his findings to BIND's caretakers, the Internet Software Consortium (ISC), in late May and to Microsoft in April. Both the ISC and Microsoft have released patches or updated software. Thanks are due to Amit Klein for his research and responsible disclosure.

Overall, Microsoft's DNS implementation has been relatively secure. The last major security update to Windows DNS was in Windows 2000 SP2 and SP4, as well as Windows Server 2003 (nearly five years ago). BIND is the most popular version of DNS server software used on the Internet, and its overall security track record has been a bit more active over the years, as one would expect with more popular software. BIND versions 8.x and 9.x have had at least six different vulnerabilities published.