Adversaries may communicate using the Domain Name System (DNS) application layer protocol to avoid detection/network filtering by blending in with existing traffic. Commands to the remote system, and often the results of those commands, will be embedded within the protocol traffic between the client and server. The DNS protocol serves an administrative function in computer networking and thus may be very common in environments. DNS traffic may also be allowed even before network authentication is completed. DNS packets contain many fields and headers in which data can be concealed. Often known as DNS tunneling, adversaries may abuse DNS to communicate with systems under their control within a victim network while also mimicking normal, expected traffic.(Citation: PAN DNS Tunneling)(Citation: Medium DnsTunneling) DNS beaconing may be used to send commands to remote systems via DNS queries. A DNS beacon is created by tunneling DNS traffic (i.e. Protocol Tunneling). The commands may be embedded into different DNS records, for example, TXT or A records.(Citation: OilRig Uses Updated BONDUPDATER to Target Middle Eastern Government) DNS beacons may be difficult to detect because the beacons infrequently communicate with infected devices.(Citation: DNS Beacons) Infrequent communication conceals the malicious DNS traffic with normal DNS traffic. Adversaries may also use DNS over TLS (DoT) or HTTPS (DoH) for resolving C2 servers. This encrypts the DNS requests and allows the malware to lookup C2 servers without being detected by security systems (e.g., DNS filtering based on indicators of compromise).
Use intrusion detection signatures to block traffic at network boundaries. In industrial control environments, network intrusion prevention should be configured so it will not disrupt protocols and communications responsible for real-time functions related to control or safety.
M0937Filter Network TrafficUse network appliances to filter ingress or egress traffic and perform protocol-based filtering. Configure software on endpoints to filter network traffic. Perform inline allow/denylisting of network messages based on the application layer (OSI Layer 7) protocol, especially for automation protocols. Application allowlists are beneficial when there are well-defined communication sequences, types, rates, or patterns needed during expected system operations. Application denylists may be needed if all acceptable communication sequences cannot be defined, but instead a set of known malicious uses can be denied (e.g., excessive communication attempts, shutdown messages, invalid commands). Devices performing these functions are often referred to as deep-packet inspection (DPI) firewalls, context-aware firewalls, or firewalls blocking specific automation/SCADA protocol aware firewalls. (Citation: Centre for the Protection of National Infrastructure February 2005)
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