DNS a Malware Vector of Choice

Thursday, March 1, 2012 @ 03:03 PM gHale

The number of malware threats that receive instructions from attackers through DNS should increase, and most companies are not currently scanning for such activity on their networks, security experts said.

There are channels attackers use for communicating with their botnets, ranging from traditional ones like TCP, IRC and HTTP to more unusual ones like Twitter feeds, Facebook walls and even YouTube comments.

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It is possible to detect and block most malware-generated traffic that passes through these channels at the network level by firewalls or intrusion prevention systems.

However, that’s not the case for DNS (Domain Name System) and attackers are taking advantage of that, said Ed Skoudis, founder of Counter Hack Challenges and SANS fellow, during a presentation at the RSA Conference 2012 on new attack techniques at the conference.

The DNS protocol normally sees use for a precise critical function — the translation of host names into IP addresses and vice-versa. Because of this, DNS traffic doesn’t go through the filtering process or an inspection by traffic monitoring solutions. It just flows freely through most networks.

As DNS queries pass from one DNS server to another until they reach the authoritative servers for the respective domains, network-level IP blocklists are useless at blocking them.

Skoudis has seen malware that receives instructions via DNS responses involved in two large-scale breaches that resulted in the compromise of millions of accounts. He expects more attackers to adopt this stealthy technique in the following months.

The infected computer doesn’t even need to have outbound connectivity. As long as it can resolve the host name through a local DNS server that performs recursive lookups on the Internet, it can communicate with attackers, Skoudis said.

Logging all DNS queries that pass through a local server is impractical because it can lead to serious performance issues. However, using a network sniffer to capture samples periodically for analysis can be a solution, Skoudis said.

Network administrators should look for unusually long queries or responses that contain weird names and encoded data, the security expert said. However, attackers might split the responses in smaller chunks.

Identical queries that repeat every few minutes can also be an indication of DNS command and control activity, because infected computers will periodically check for new commands.

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