Reverse SSH: Secure Remote Access or Tool to Bypass Firewall

Tools like reverse SSH serve dual roles—powerful for legitimate system administration but potentially risky in the hands of adversaries. The reverse_ssh tool by NHAS facilitates secure, reliable remote access to systems behind firewalls or Network Address Translation (NAT), and while it can streamline remote support, it also highlights critical security concerns for defenders.

This blog dives into the operational mechanics of reverse SSH, its legitimate uses, potential risks if misused, and defensive recommendations to mitigate unauthorized access attempts.

Overview: What is reverse_ssh?

reverse_ssh is designed to bypass firewall and NAT restrictions by establishing a reverse connection from a target machine back to a remote server. Instead of opening a port on the target and waiting for an inbound SSH connection, reverse_ssh initiates the SSH connection from within the target environment. This approach effectively negates the need to configure firewall rules or manage port forwarding, providing a convenient option for accessing systems without direct IP accessibility.

How Reverse SSH Works

Reverse SSH allows an SSH client (typically on the target machine) to initiate a connection to an external SSH server controlled by the user. Once the connection is established:

• The target machine acts as a client, which is often less scrutinized by firewalls than incoming traffic.
• The external server accepts this connection, allowing bidirectional communication as if it were a standard SSH session.
• The result is persistent access to the target system, regardless of network configuration, which is particularly useful in restricted environments.

Real-world Example: Imagine an IT administrator needing to troubleshoot devices at a remote location with strict firewall policies. Using reverse SSH, they can configure the remote device to initiate an outbound SSH connection to a trusted server, from which they can manage the device as needed.

Key Functional Capabilities

Bypass Firewall and NAT Restrictions:

Reverse SSH leverages outbound connections, sidestepping inbound firewall configurations. As most firewalls permit outbound SSH traffic, this can be a quick workaround to gain access to systems in restrictive network settings.

Secure Remote Access:

Built on SSH, reverse SSH is encrypted, which is critical for transferring data securely across unsecured networks. The encryption protects the session from eavesdropping, providing administrators with a secure communication channel.

Persistent Access:

Once the reverse connection is established, administrators can perform remote tasks continuously without needing to reconnect, which is essential for tasks requiring real-time monitoring or extended troubleshooting.

Cybersecurity Risks and Challenges

While reverse SSH has legitimate uses, it presents significant risks when misused. By establishing a persistent, encrypted outbound connection, reverse SSH bypasses conventional firewall and monitoring solutions, posing a critical challenge for defenders.

Potential for Unauthorized Remote Access:

Reverse SSH, if implemented without secure configurations, can be used by attackers to establish a stealthy backdoor into target systems. This could lead to unauthorized remote control, data exfiltration, or further lateral movement within the network.

Example: Attackers who gain limited access to a compromised endpoint could deploy reverse SSH to establish a persistent connection, effectively bypassing firewall restrictions and connecting back to a command-and-control (C2) server.

Difficulty in Monitoring:

Reverse SSH initiates an outbound connection, which is typically allowed by firewalls. Unlike inbound connections, outbound SSH traffic may not be monitored as closely, allowing attackers to bypass security monitoring and avoid detection.

Example: Security operations teams relying on traditional monitoring tools might miss this reverse connection if they aren’t actively tracking all outbound traffic or monitoring unusual patterns.

Potential for Data Exfiltration:

Reverse SSH connections can serve as a channel for data exfiltration. Since the communication is encrypted, it is challenging to inspect or control the data flowing through the reverse SSH tunnel, making it an ideal method for transferring sensitive information out of a network undetected.

Defensive Measures for Protecting Against Reverse SSH Exploits

To mitigate risks associated with tools like reverse_ssh, organizations need to implement proactive measures that focus on monitoring, access control, and policy enforcement:

Implement Outbound Traffic Monitoring:

Monitoring outbound traffic is essential for identifying unauthorized reverse connections. Network monitoring tools should be configured to flag unusual patterns, such as persistent SSH connections from non-administrative endpoints or connections to unknown external IP addresses.

Recommendation: Use Intrusion Detection Systems (IDS) or Intrusion Prevention Systems (IPS) configured to detect and alert on unusual outbound SSH activity, especially from endpoints that typically don’t require SSH access.

Restrict SSH Access to Authorized Users:

Limit SSH access to only authorized personnel and enforce multi-factor authentication (MFA) for all SSH sessions. Ensure that only legitimate users can initiate SSH connections and that they have explicit permissions for reverse SSH if necessary.

Recommendation: Implement role-based access controls (RBAC) to restrict who can establish outbound SSH connections. Regularly review and audit user access to ensure compliance with security policies.

Log and Monitor SSH Session Activity:

Continuous monitoring of SSH session logs, including reverse SSH activity, can help detect and prevent unauthorized access. Logging outbound connections to SSH servers provides visibility into who is initiating connections, to which servers, and when.

Recommendation: Use Sysmon or other logging solutions to track SSH session activity, enabling faster detection of unauthorized SSH connections and aiding in incident investigation.

Regularly Audit Endpoints for Unauthorized Tools:

Reverse SSH can be initiated through legitimate or unauthorized tools, so regular endpoint audits are critical. Conduct audits to identify unauthorized software, scripts, or scheduled tasks that might be leveraging reverse SSH for malicious purposes.

Recommendation: Use endpoint detection and response (EDR) solutions to monitor for unauthorized installations and detect potentially malicious configurations that could establish reverse connections.

Educate and Train Security Teams:

Educating security teams on reverse SSH, its legitimate uses, and potential abuse cases helps improve response times and accuracy. By understanding the dual-use nature of tools like reverse_ssh, security personnel are better equipped to differentiate between legitimate administrative use and malicious activity.

Lessons from Past Incidents

Reverse SSH and similar techniques have been used in real-world attacks to establish hidden backdoors. For example, in APT29 campaigns, attackers leveraged legitimate remote access tools to evade detection and maintain persistence in networks, bypassing firewalls with outbound connections. These incidents highlight the importance of monitoring outbound connections, enforcing strict access controls, and identifying dual-use tools early on.

Final Thoughts

reverse_ssh by NHAS provides a useful mechanism for overcoming firewall restrictions and securing remote access, but it also illustrates the fine line between functionality and risk. While reverse SSH offers valuable capabilities for administrators, it requires careful oversight to prevent misuse. By enforcing robust monitoring, logging, and access controls, organizations can leverage tools like reverse_ssh securely while minimizing the risk of abuse.

At Terraeagle, we help organizations implement secure remote access solutions and develop robust monitoring practices to detect unauthorized connections. Contact us today to learn how we can help protect your environment from evolving remote access threats.