DNSpionage and how to mitigate DNS tunneling

This blog post is a summary of this week’s Information Security News put together by our Security Incident Response Team (SIRT).

Cisco Talos has published details regarding an APT campaign using DNS redirection and a malware they call DNSpionage. The malware supports both regular HTTP and also DNS tunneling as a way of communicating back with the attackers.

The DNS redirection part of the attack was done by compromising nameservers, and then pointing hostnames under the nameservers control to IPs of the attackers choosing. The attacker used LetsEncrypt and was in that way able to set up perfectly valid HTTPS copies of any sites.

DNS tunneling is where data are encapsulated within a DNS query and its reply, often using base64 encoding. As long as a server is able to perform domain name lookups it is able to exfiltrate data in this manner. This can also be used, with some preparation, if you find yourself in an airports WIFI or such, to proxy legitimate traffic and bypass and “signup”-requirement the WIFI might have.

This covert channel can be hard to detect, if the malware minimize the bandwidth used. If used as a proxy for larger amounts of data it will be possible to detect a significant change in the amount of DNS-queries and the size of the queries. A modern IDS or next generation firewall should be able to detect this out of the box today. Another way of mitigating is to use the split horizon DNS concept, resolving internal IPs normally, but external IPs resolving to a proxy server that can have the capability of checking the DNS information further.

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