Bidirectional Communication

What the technique is

Adversaries may use an existing, legitimate external Web service as a means for sending commands to and receiving output from a compromised system over the Web service channel. Compromised systems may leverage popular websites and social media to host command and control (C2) instructions. Those infected systems can then send the output from those commands back over that Web service channel. The return traffic may occur in a variety of ways, depending on the Web service being utilized. For example, the return traffic may take the form of the compromised system posting a comment on a forum, issuing a pull request to development project, updating a document hosted on a Web service, or by sending a Tweet. Popular websites and social media acting as a mechanism for C2 may give a significant amount of cover due to the likelihood that hosts within a network are already communicating with them prior to a compromise. Using common services, such as those offered by Google or Twitter, makes it easier for adversaries to hide in expected noise. Web service providers commonly use SSL/TLS encryption, giving adversaries an added level of protection.

What MITRE says you would watch

• AN0100

  Suspicious processes initiating encrypted HTTPS connections to common web service domains, followed by abnormal data upload behavior or automated posting behavior indicative of C2 bidirectional traffic.

• AN0101

  Non-interactive system processes making encrypted HTTPS connections to well-known web services followed by high outbound traffic volume or scripted upload patterns.

• AN0102

  Scripting engines (e.g., osascript, Python) initiating HTTPS requests to social media or content-sharing platforms, paired with automated response handling indicative of two-way communication.