New exploit taints patched DNS servers, claims researcher

Patches meant to fix a flaw in the Internet's Domain Name System do not completely protect the web's traffic cop from attack, a Russian researcher has claimed.


Patches meant to fix a flaw in the Internet's Domain Name System do not completely protect the web's traffic cop from attack, a Russian researcher has claimed.

The head of the non-profit organisation that maintains the most commonly used DNS software, however, said there was little to worry about.

In a blog, Evgeniy Polyakov said he had created an exploit able to insert bogus routing information into systems running the most-up-to-date version of BIND (Berkeley Internet Name Domain), the popular open source software that runs a majority of the web's DNS servers.

BIND 9.5.0-P2 was released on 2 Aug as a follow-on to the initial patch issued July 8, the day that researcher Dan Kaminsky announced the DNS flaw and a coordinated patching effort by several vendors, including Internet Systems Consortium, which maintains BIND, Microsoft, and Cisco Systems.

Both the 8 July and 2 Aug BIND updates added source port randomisation in the name server to reduce the likelihood of "cache poisoning," the term used to describe attacks that attempt to reroute users' requests for legitimate sites to fakes created to dupe them into entering confidential information.

Polyakov claimed that his exploit was able to insert rogue instructions into a DNS server running BIND 9.5.0-P2, although it took a pair of attacking PCs connected to the server via a Gigabit Ethernet (GigE) network connection about 10 hours to pull off the attack.

"Attack took about half of the day, a bit less than 10 hours," Polyakov claimed on his blog. "So, if you have a GigE LAN, any Trojaned machine can poison your DNS during one night."

Paul Vixie, the president of ISC, said the threat posed by Polyakov's exploit was small potatoes compared to the ease with which attackers can poison the caches of unpatched DNS servers. "While I think it's bad that anybody who can hammer you at GigE speed for ten hours can poison your cache, it's not a threat to the real world the way 11 seconds at 10-megabit was," Vixie said.

"Any DNS server with a host-based firewall can implement a 100 percent effective mitigation for the Polyakov attack, and it's possible that an upstream/outboard firewall could also be made to do it," Vixie said. "At some point ISC will have to put logic like this into BIND, of course, but protecting against the Polyakov attack is like synflood protection in that it's a rate-limit problem."

According to other entries posted to his blog, Polyakov began working on an exploit for the DNS flaw around 29 July.

Although Kaminsky withheld details of the flaw when he first disclosed the bug early last month, other security researchers parsed the vulnerability from the scant information they had. In a presentation at the Black Hat security conference last week where he outlined the flaw and provided more information, Kaminsky noted that others had reproduced the vulnerability within five days, but kept quiet.

On 21 July, however, Thomas Dullien, chief executive at German security company Zynamics, took a stab at the flaw and posted his best guesses about its details and how it might be exploited. Two days later, working attack code able to poison unpatched DNS servers went public.

For his part, ISC's Vixie urged a move toward a more secure routing system for the internet, a call that some in the most technical areas of the internet have voiced for years. "It's long past time for Secure DNS, which is a combination of TSIG+TKEY, SIG(0), and DNSSEC. End to end crypto authentication," said Vixie.

"In the time it would take for this Russian's attack to work over your 512K DSL line (2.2 years, I heard?) we could completely secure the DNS or at least the parts of DNS whose operators gave a rat's ass about security," Vixie concluded.

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