Detection of Malicious and Low Throughput Data Exfiltration Over the DNS Protocol

In the presence of security countermeasures, a malware designed for data exfiltration must do so using a covert channel to achieve its goal. Among existing covert channels stands the domain name system (DNS) protocol. Although the detection of covert channels over the DNS has been thoroughly studied in the last decade, previous research dealt with a specific subclass of covert channels, namely DNS tunneling. While the importance of tunneling detection is not undermined, an entire class of low throughput DNS exfiltration malware remained overlooked. The goal of this study is to propose a method for detecting both tunneling and low-throughput data exfiltration over the DNS. Towards this end, we propose a solution composed of a supervised feature selection method, and an interchangeable, and adjustable anomaly detection model trained on legitimate traffic. In the first step, a one-class classifier is applied for detecting domain-specific traffic that does not conform with the normal behavior. Then, in the second step, in order to reduce the false positive rate resulting from the attempt to detect the low-throughput data exfiltration we apply a rule-based filter that filters data exchange over DNS used by legitimate services. Our solution was evaluated on a medium-scale recursive DNS server logs, and involved more than 75,000 legitimate uses and almost 2,000 attacks. Evaluation results shows that while DNS tunneling is covered with at least 99% recall rate and less than 0.01% false positive rate, the detection of low throughput exfiltration is more difficult. While not preventing it completely, our solution limits a malware attempting to avoid detection with at most a 1kb/h of payload under the limitations of the DNS syntax (equivalent to five credit cards details, or ten user credentials per hour) which reduces the effectiveness of the attack.