CAUDIT: Continuous Auditing of SSH Servers To Mitigate Brute-Force Attacks

This paper describes CAUDIT, an operational system deployed at the National Center for Supercomputing Applications (NCSA) at the University of Illinois. CAUDIT is a fully automated system that enables the identification and exclusion of hosts that are vulnerable to SSH brute-force attacks. Its key features include: 1) a honeypot for attracting SSH-based attacks over a /16 IP address range and extracting key metadata (e.g., source IP, password, SSH-client version, or key fingerprint) from these attacks; 2) executing audits on the live production network by replaying of attack attempts recorded by the honeypot; 3) using the IP addresses recorded by the honeypot to block SSH attack attempts at the network border by using a Black Hole Router (BHR) while significantly reducing the load on NCSA's security monitoring system; and 4) the ability to inform peer sites of attack attempts in real-time to ensure containment of coordinated attacks. The system is composed of existing techniques with custom-built components, and its novelty is its ability to execute at a scale that has not been validated earlier (with thousands of nodes and tens of millions of attack attempts per day). Experience over 463 days shows that CAUDIT successfully blocks an average of 57 million attack attempts on a daily basis using the proposed BHR. This represents a 66 times reduction in the number of SSH attempts compared to the daily average and has reduced the traffic to the NCSA's internal network-security-monitoring infrastructure by 78%.