Detecting Domain-Generation Algorithm (DGA) Based Fully-Qualified Domain Names (FQDNs) with Shannon Entropy
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Domain Name System (DNS) is the backbone of the Internet. However, threat actors have abused the antiquated protocol to facilitate command-and-control (C2) actions, to tunnel, or to exfiltrate sensitive information in novel ways. The FireEye breach and Solarwinds intrusions of late 2020 demonstrated the sophistication of hacker groups. Researchers were eager to reverse-engineer the malware and eager to decode the encrypted traffic. Noticeably, organizations were keen on being first to "solve the puzzle". Dr. Eric Cole of SANS Institute routinely expressed "prevention is ideal, but detection is a must". Detection analytics may not always provide the underlying context in encrypted traffic, but will at least give a fighting chance for defenders to detect the anomaly. SUNBURST is an open-source moniker for the backdoor that affected Solarwinds Orion. While analyzing the malware with security vendor research, there is a possible single-point-of-failure in the C2 phase of the Cyber Kill Chain provides an avenue for defenders to exploit and detect the activity itself. One small chance is better than none. The assumption is that encryption increases entropy in strings. SUNBURST relied on encryption to exfiltrate data through DNS queries of which the adversary prepended to registered Fully-Qualified Domain Names (FQDNs). These FQDNs were typo-squatted to mimic Amazon Web Services (AWS) domains. SUNBURST detection is possible through a simple 1-variable t-test across all DNS logs for a given day. The detection code is located on GitHub (https://github.com/MalwareMorghulis/SUNBURST).
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