Detection of Unusual SSH Client Banners

Adversaries frequently employ specialized offensive frameworks, custom-built malware, and tunneling utilities to facilitate command-and-control (C2) or lateral movement across enterprise networks. These tools often initiate SSH connections that present unique or non-standard client banners during the initial handshake, differing significantly from common administrative clients such as OpenSSH, PuTTY, or WinSCP. By analyzing Zeek SSH logs, security teams can baseline known-good client banners and identify outliers. This approach allows defenders to detect unauthorized infrastructure or covert communication channels that might otherwise evade traditional signature-based detection. Organizations should monitor for these anomalies to investigate potential unauthorized remote access or data exfiltration attempts.

Impact

Successful exploitation of SSH for covert communication or lateral movement can allow attackers to bypass standard network egress filters, maintain persistent access, and exfiltrate sensitive data. Detecting these anomalies provides visibility into unauthorized use of the SSH protocol for malicious objectives, preventing long-term compromise of internal assets.

Recommendation

• Deploy the provided detection logic to analyze Zeek SSH telemetry and identify outliers that deviate from the established allowlist of known client software.
• Establish a baseline of legitimate SSH client usage within your environment to tune the filter and minimize false positives generated by proprietary or less common administrative tools.
• Investigate any flagged connections to confirm the identity of the client binary and the intent of the communication, especially when associated with sensitive internal systems.