Malware’s Next Move: DNS

Thursday, March 29, 2012 @ 03:03 PM gHale

The domain-name service (DNS) protocol may increasingly help criminals communicate with compromised systems.

More malware would hide its commands and exfiltrated data in DNS packets, said Ed Skoudis, a senior security consultant with InGuardians at last month’s RSA Conference. The advantage for malware writers is even if a company bars a potentially infected computer from contacting the Internet, malware could send DNS requests to a local server, which would then act as a proxy, bypassing defenses.

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“With DNS as a command-and-control channel, as long as the internal machine can resolve names on the Internet, then there is command and control,” Skoudis said during the conference. “A machine could be blocked by sending outbound connections, so, instead, the machine sends a request to its internal DNS server, and that DNS server forwards onto the Internet, ultimately getting to the bad guy’s server.”

The tactic does not see much use these days, but usage could begin to pick up. Skoudis gave two malware attacks that used a DNS channel as part of major breaches. Two minor attacks — PowPow and Wibimo — used the technique as well, according to Dell SecureWorks.

Around 5 percent of attacks contain some sort of DNS signaling, according to researchers at Internet Identity, a firm that specializes in securing Internet infrastructure and information. People don’t watch DNS, so it is the kind of communication that can slip through the cracks.

The covert or malicious use of DNS for communications typically falls into two categories, said Gunter Ollmann, vice president of research at threat-protection firm Damballa. Tunneling uses the DNS port 53 to bypass firewalls that typically leave the port open so as not to interfere with domain lookups. The second technique hides data in DNS packets and uses the domain-name system infrastructure to transport the information to the destination chosen by the attacker.

It’s the latter method that sees more use, Ollmann said. Today, most command-and-control traffic carries over the Hypertext Transfer Protocol (HTTP), but as defenders find ways of blocking the traffic, attackers will look for other possibilities.

“The bad guys are becoming more aware of how DNS is a weak spot in our defenses,” he said. “And as other defensive technologies have improved inside enterprise networks, that particular door — port 53 — looks more attractive.”

Detecting DNS is not that difficult, however. Sending communications over the domain-name system typically results in anomalous volumes of information, which is fairly easy to detect if a company is watching DNS. Ironically, the domain-name service security extensions, or DNSSEC, could make shipping data over DNS easier to hide. Normal DNS traffic has a limit of 512 bytes, while DNSSEC packets have no limit, so the protocol could exfiltrate data from an unwary company’s servers.

Companies need to be watching their DNS traffic to make sure online thieves are not communicating with compromised systems or stealing data from the corporate network, researchers said. While logging all traffic is too onerous, companies should watch out for DNS traffic issued to parts of the world where they do no business and use traffic monitoring to look for anomalies.

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