Cloud Threats Memo: Understanding the Dead Drop Resolver Technique

If I asked you what the common ways to exploit a cloud app for malicious purposes are, I bet your answer would probably be either to use it to distribute malicious content (such as malware or phishing pages), or to host the command and control (C2) infrastructure. In reality another frequent technique is the dead drop resolver, where a legitimate service is abused by threat actors to host the information related to the C2 infrastructure rather than the C2 infrastructure itself.

In this scenario the malware payload connects to the cloud application, and instead of contacting the command and control directly, it obtains the updated list of C2 servers where the connection can be eventually established. This makes the malicious infrastructure more resilient, since the attackers can dynamically update the list of C2 servers and easily switch across different instances, in case the original one is taken down.

In the latest example, discovered by researchers at Secureworks, the dead drop resolver technique has been leveraged by the Iranian government-sponsored COBALT MIRAGE threat group for a campaign carried out via its Drokbk malware. In particular, this version of the malware uses the GitHub API to search for a specific repository and a specific account, and retrieves the list of servers inside the README.md file.

As the researchers note: “This approach gives the threat actors a degree of resiliency against shuttering of their GitHub account, as they can create a new account with a matching repository name. It also allows the malware to dynamically update its C2 server by repeating this process.”

This is yet another way to leverage GitHub, one of the most exploited cloud apps to deliver malware, for malicious purposes. Its flexibility is undoubtedly an advantage for threat actors to host the command and control infrastructure directly and, as we have seen in this example, the information that points to additional command and control infrastructure, in a chain of redirections that makes the latter more resilient.

​​How Netskope mitigates the dead drop resolver risk

GitHub is one of the thousands of services where the Netskope Next Gen SWG can provide granular access control, and one of the dozens for which instance detection is also available. To defend against attacks where a legitimate cloud service is exploited to host the information that points to command and control infrastructure, or directly hosts the command and control infrastructure itself, it is possible to configure a policy that prevents potentially dangerous activities (such as upload and download) in non-corporate instances. It is also possible to make the scope of the policy broader, blocking these activities for any unneeded cloud storage service that can potentially be exploited as a dead drop resolver, as a command and control, or even to distribute malware.

Netskope customers are also protected against malware distributed from a legitimate cloud service by Netskope Threat Protection. Netskope Threat Protection scans web and cloud traffic to detect known and unknown threats with a comprehensive set of engines including signature-based AV, machine learning-based detectors for executables and Office documents, and sandboxing. Netskope Cloud Exchange provides powerful integration tools to leverage investments across their security posture through the integration with third-party tools such as threat intelligence feeds and endpoint detection technologies.

Finally, Netskope Advanced Analytics provides specific dashboards to assess the risk of rogue cloud instances being exploited to deliver malware or becoming the target of anomalous communications, with rich details and insights, supporting security teams in the analysis and mitigation/remediation process.

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Stay safe!