Fallback Channels

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Fallback Channels
Technique
ID T1008
Tactic Command and Control
Platform Windows Server 2003, Windows Server 2008, Windows Server 2012, Windows XP, Windows 7, Windows 8, Windows Server 2003 R2, Windows Server 2008 R2, Windows Server 2012 R2, Windows Vista, Windows 8.1, Linux, Windows 10, MacOS, OS X
Data Sources Packet capture, Netflow/Enclave netflow, Malware reverse engineering, Process use of network, Process monitoring
Requires Network Yes

Adversaries may use fallback or alternate communication channels if the primary channel is compromised or inaccessible in order to maintain reliable command and control and to avoid data transfer thresholds.

Examples

  • Lazarus Group malware SierraAlfa sends data to one of the hard-coded C2 servers chosen at random, and if the transmission fails, chooses a new C2 server to attempt the transmission again.1
  • BISCUIT malware contains a secondary fallback command and control server that is contacted after the primary command and control server.2
  • BlackEnergy has the capability to communicate over a backup channel via plus.google.com.3
  • CHOPSTICK can switch to a new C2 channel if the current one is broken.4
  • Derusbi uses a backup communication method with an HTTP beacon.5
  • DustySky has two hard-coded domains for C2 servers; if the first does not respond, it will try the second.6
  • JHUHUGIT tests if it can reach its C2 server by first attempting a direct connection, and if it fails, obtaining proxy settings and sending the connection through a proxy, and finally injecting code into a running browser if the proxy method fails.7
  • MiniDuke uses Google Search to identify C2 servers if its primary C2 method via Twitter is not working.8
  • Mis-Type first attempts to use a Base64-encoded network protocol over a raw TCP socket for C2, and if that method fails, falls back to a secondary HTTP-based protocol to communicate to an alternate C2 server.9
  • NETEAGLE will attempt to detect if the infected host is configured to a proxy. If so, NETEAGLE will send beacons via an HTTP POST request; otherwise it will send beacons via UDP/6000.10
  • S-Type primarily uses port 80 for C2, but falls back to ports 443 or 8080 if initial communication fails.9
  • SslMM has a hard-coded primary and backup C2 string.11
  • WinMM is usually configured with primary and backup domains for C2 communications.11
  • The C2 server used by XTunnel provides a port number to the victim to use as a fallback in case the connection closes on the currently used port.4

Mitigation

Network intrusion detection and prevention systems that use network signatures to identify traffic for specific adversary malware can be used to mitigate activity at the network level. Signatures are often for unique indicators within protocols and may be based on the specific protocol used by a particular adversary or tool, and will likely be different across various malware families and versions. Adversaries will likely change tool C2 signatures over time or construct protocols in such a way as to avoid detection by common defensive tools.12

Detection

Analyze network data for uncommon data flows (e.g., a client sending significantly more data than it receives from a server). Processes utilizing the network that do not normally have network communication or have never been seen before are suspicious. Analyze packet contents to detect communications that do not follow the expected protocol behavior for the port that is being used.12

References