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Introduction: A Window into Covert Cyber Tactics
In the ever-evolving landscape of cybersecurity, understanding how attackers leverage common tools for malicious purposes is critical. This analysis dives into a real-world honeypot experiment conducted by a SANS.edu intern, Sihui Neo, that uncovered the widespread abuse of SSH tunneling to establish proxy networks. By monitoring thousands of unauthorized connection attempts, the study reveals how threat actors weaponize simple SSH features to obscure their tracks, access restricted services, and distribute malicious trafficâoften redirecting it to sensitive locations like Russia-based SMTP servers. These findings shed light on how overlooked vulnerabilities, such as reused SSH credentials, can transform a seemingly innocuous server into a silent accomplice in cybercrime.
Massive Abuse of SSH Tunnels Uncovered in Honeypot Logs
Honeypot Setup and Monitoring Infrastructure
As part of her academic program, the author deployed a Cowrie honeypot on an AWS EC2 instance in Japan to simulate a vulnerable SSH server. To centralize and analyze logs efficiently, she connected this honeypot to an ELK-based SIEM system running on a separate VPS, guided by instructions from a SANS mentor. This allowed her to gain deep visibility into connection attempts and unusual traffic patterns.
Detecting Anomalous direct-tcp Connection Requests
Within a month of deployment, over 1,000 unique IP addresses attempted to establish direct-tcp SSH connections through the honeypot. Surprisingly, about 75% of those connections targeted the same destination: IP 77.88.21.158 on port 25âan endpoint linked to Yandex Mailâs SMTP server in Russia. These connection attempts were not accidental; they were part of a deliberate and sophisticated strategy to use the honeypot as a proxy.
Tracing the Origin and Analyzing Traffic
Although the direct-tcp requests appeared to originate from localhost (127.0.0.1), closer inspection revealed the true source IPs were external. In one example, the attacker used IP 125.20.251.66, initiating a TCP stream on port 32069, which matched corresponding PCAP data and confirmed SSH-based traffic forwarding. The use of SSH-2.0-OpenSSH_7.4 further indicated typical Linux environments were used for the attack.
Brute Force Entry and Tunnel Creation
Attackers employed a stealthy brute-force approach, primarily targeting SSH port 2222. Attempts were carefully spaced (2+ hours apart) to avoid detection. Once access was obtained, the SSH tunnel was created instantlyâallowing traffic to be rerouted through the honeypot, effectively masking the original senderâs identity.
Exploiting Honeypot as a Proxy for Email Traffic
The attackers configured their local machines to route traffic (often SMTP email) through the compromised honeypot by pointing to 127.0.0.1:1080. This forwarded traffic through the SSH tunnel to the real targetâYandexâs SMTP server. Notably, because the honeypot didnât serve real SSH on port 2222, these connections were short-lived, but still meaningful enough to expose intent.
Risks of Server Exploitation via SSH Tunneling
Using SSH tunnels this way allows attackers to:
Evade geo-blocking or censorship
Obfuscate original IPs
Exploit clean IP addresses for delivering spam or launching attacks
Avoid traceability unlike commercial VPNs, which require registration and leave an audit trail
Once a server is turned into a proxy, it becomes vulnerable to:
Attribution of malicious activity (e.g., appearing responsible for a DDoS or data breach)
Bandwidth overuse and cloud billing issues
IP blacklisting, which may disrupt legitimate services
What Undercode Say:
Proxying Tactics Are Evolving Rapidly
The rise of SSH tunneling in threat actor playbooks underscores a dangerous shift. Attackers are no longer content with merely breaching a systemâthey now weaponize access to create anonymous infrastructure. Whatâs most alarming is the simplicity: no root access is needed, just an exposed SSH port and weak credentials. This makes countless servers, especially in cloud environments, soft targets.
The Anatomy of Brute Force Precision
Unlike aggressive brute-force attempts that trip alerts, this campaign showed methodical spacing. Spreading password guesses across long time intervals prevents traditional detection systems from flagging them. The use of a TTL below 50 suggests attacks originate from Unix-like systems, often running customized scripts that adapt to honeypot detection measures.
SMTP as an Attack Vector
The focus on Yandexâs SMTP server (port 25) reveals a likely motive: spam campaigns or phishing operations. SMTP traffic, when tunneled through innocent-looking hosts in regions like Japan or the US, can bypass regional firewalls and content filters. These are high-value behaviors for botnet operators and cybercriminal groups specializing in email-borne threats.
The Implications for Cloud Users
This case is a wake-up call for organizations relying on cloud-based instances without strict access controls. AWS free-tier EC2 instances, like the one used here, are widely deployed for development or testing but often poorly secured. One compromised instance could inadvertently turn into an attackerâs anonymous mail relay.
The Need for Centralized Visibility
Deploying an ELK SIEM stack was a smart move. Without this centralized logging and visual analysis via Kibana, these patterns may have remained hidden. Investing in observability tools is not optional anymoreâitâs the frontline of defense in catching stealthy, persistent threats.
Forwarding
Many system administrators still underestimate the power of SSH forwarding. While often used for legitimate reasons, itâs also a perfect tool for lateral movement, data exfiltration, or masked command-and-control (C2) traffic. Worse, since it doesnât raise immediate red flags, it bypasses many basic security tools.
Recommendations to Harden Infrastructure
Disable unused SSH ports and monitor 2222 especially
Enforce strong authentication (SSH keys + 2FA)
Limit user permissions to prevent tunneling
Monitor outbound SMTP connections
Use threat intel feeds to detect unusual destination IPs like 77.88.21.158
SSH
The ease with which attackers exploited SSH tunnels to proxy through the honeypot reveals a blind spot in many organizationsâ security strategies. Too often, SSH is seen as a utility rather than a threat vector. A cultural shift is neededâSSH must be treated with the same scrutiny as open RDP ports or exposed admin panels.
đ Fact Checker Results:
â
Over 1,000 unique IPs attempted SSH tunnel connections through the honeypot
â
75% of those routed traffic to a Russian SMTP server (77.88.21.158:25)
â No outbound packets were recorded beyond tunnel creation (due to honeypot design)
đ Prediction:
SSH tunneling abuse will continue to rise as attackers shift from centralized VPN services to decentralized, hacked infrastructure. Expect a surge in brute-force campaigns targeting non-standard SSH ports and a higher demand for proxy access to email servers like Yandex. Security teams will need to evolve detection techniques, focusing on behavioral anomalies rather than traditional port-scanning or volume-based alerts. SSH misuse is no longer a rare APT tacticâitâs becoming a commodity tool in cybercrime operations.
References:
Reported By: isc.sans.edu
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