DNS Tunneling

Synopsis

Description

10 KB or more were sent encoded in subdomain names during a 10-minute window. All subdomains queried were under a single suspicious domain.

DNS tunneling encodes data in DNS queries and responses, allowing an attacker to bypass firewalls and proxies to reach his or her command and control server, even when HTTP/S traffic is blocked.

The endpoint may be remotely controlled by an attacker, and/or an attacker may have exfiltrated data from it. This detector is not supported when networking events arrive solely from Cortex XDR Linux agents.

Attacker's Goals

Communicate with malware running on your network to control malware activities, perform software updates on the malware, or to take inventory of infected machines.

Investigative actions

• Verify that the source device or process is not an approved security solution.
  
• Verify if the DNS query types are non-standard. DNS tunnels use uncommon query types that enable encoding of more data. Examples include: INIT, PRIVATE, NULL, SRV, KEY, and TXT.
  
• If the affected endpoint is operating Windows, verify that the DNS traffic is coming from svchost.exe and search for other processes that ran when the alert triggered. On Windows, the DNS requests go through svchost.exe.
  
• Verify the responses per DNS query. Many responses per query may indicate a tool being downloaded.
  
• Verify the destination domain details and compare the number of endpoints in your network that access the domain over time to see if this is an uncommonly contacted domain.
  
• Verify the source web-browser traffic to determine if the process was generated by user action, if the user did not initiate the traffic it can be indicative of malicious activity.
  
• Verify non-DNS traffic to the domain. Any traffic except DNS queries to the destination domain may indicate a legitimate domain and not used solely for command-and-control or data exfiltration.