Source: cve@mitre.org
SSH 2.0.11 and earlier allows local users to request remote forwarding from privileged ports without being root.
Local users can exploit a vulnerability in SSH 2.0.11 and earlier to request remote port forwarding from privileged ports without requiring root privileges, potentially leading to unauthorized access and data exfiltration. This allows attackers to bypass security controls and establish covert channels for malicious activities. This vulnerability poses a significant risk to systems running vulnerable SSH versions.
Step 1: Local User Login: An unprivileged local user establishes a valid SSH connection to the vulnerable SSH server.
Step 2: Forwarding Request: The local user crafts an SSH request to forward traffic from a privileged port (e.g., port 80, 443) on the server to a destination of their choosing, potentially on a remote host they control.
Step 3: Inadequate Privilege Check: The SSH server, due to the vulnerability, fails to adequately verify the user's privileges before processing the forwarding request.
Step 4: Port Forwarding Establishment: The SSH server establishes the requested port forwarding, allowing traffic destined for the privileged port on the server to be redirected to the attacker-controlled destination.
Step 5: Traffic Redirection/Interception: The attacker can now intercept or redirect traffic intended for the privileged port on the server, potentially gaining access to sensitive information or control of the system.
The vulnerability stems from a flaw in how SSH 2.0.11 and earlier handles requests for remote port forwarding. Specifically, the software fails to properly validate the source of these requests, allowing a local user to specify a privileged port (ports below 1024) as the destination for forwarded traffic. The root cause is a missing or inadequate check on the user's authorization to request forwarding to privileged ports. The SSH daemon, upon receiving a forwarding request, should verify the user's privileges before establishing the connection. However, in this case, the check is either absent or flawed, permitting the forwarding request to proceed without the necessary authorization. This allows a local, unprivileged user to effectively 'hijack' privileged ports and potentially intercept or redirect network traffic.
This vulnerability is not directly tied to specific APT groups or malware campaigns due to its age. However, it represents a fundamental security flaw that could be exploited by any attacker with local access. The risk is compounded if the vulnerable SSH server is used in a network with other vulnerable systems. CISA KEV status: Not Listed due to the age of the vulnerability.
Monitor SSH server logs for suspicious port forwarding requests, especially those involving privileged ports.
Analyze network traffic for unusual patterns, such as traffic redirection to unexpected destinations from privileged ports.
Implement host-based intrusion detection systems (HIDS) to monitor for unauthorized SSH configuration changes.
Use file integrity monitoring (FIM) to detect any modifications to SSH configuration files.
Regularly scan systems for vulnerable SSH versions using vulnerability scanners.
Upgrade to a patched version of SSH (e.g., OpenSSH) that addresses the vulnerability. This is the primary and most effective remediation step.
If upgrading is not immediately possible, restrict access to the SSH server to only trusted users and networks.
Disable remote port forwarding if it is not required.
Implement strong authentication mechanisms, such as multi-factor authentication (MFA), to mitigate the impact of a successful exploit.
Regularly review and audit SSH configuration files to ensure they are secure and compliant with security best practices.
Implement a host-based firewall to restrict network access to the SSH server.