CVE-2004-1653 in OpenSSH
Summary
by MITRE
The default configuration for OpenSSH enables AllowTcpForwarding, which could allow remote authenticated users to perform a port bounce, when configured with an anonymous access program such as AnonCVS.
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Analysis
by VulDB Data Team • 07/06/2025
The vulnerability described in CVE-2004-1653 represents a significant security flaw in the default OpenSSH configuration that enables TCP forwarding capabilities. This issue specifically impacts systems where OpenSSH is configured with AllowTcpForwarding set to yes, creating an avenue for authenticated remote attackers to exploit port bouncing techniques. The vulnerability becomes particularly dangerous when combined with anonymous access programs such as AnonCVS, which can amplify the attack surface and provide additional entry points for malicious actors.
The technical flaw resides in the default SSH server configuration that permits TCP port forwarding without adequate restrictions. When AllowTcpForwarding is enabled, it allows users to establish forwarded connections through the SSH tunnel, effectively creating a proxy mechanism that can be abused for port bouncing attacks. This functionality, while useful for legitimate purposes such as accessing internal network resources, becomes problematic when combined with anonymous access services that may not properly validate or restrict connection attempts. The vulnerability operates at the network protocol level, leveraging SSH's built-in forwarding capabilities to redirect traffic through the compromised server, potentially bypassing firewall restrictions and network segmentation controls.
The operational impact of this vulnerability extends beyond simple port bouncing, as it can enable attackers to perform various malicious activities including network reconnaissance, data exfiltration, and lateral movement within compromised networks. When combined with anonymous access programs like AnonCVS, the vulnerability creates a scenario where authenticated users can leverage the SSH server as a relay point to access services that would otherwise be restricted. This can lead to unauthorized access to sensitive data repositories, internal network services, and potentially escalate privileges within the affected environment. The attack vector is particularly concerning because it requires only authentication to the SSH service, making it accessible to anyone with valid credentials.
Mitigation strategies for CVE-2004-1653 should focus on disabling unnecessary TCP forwarding capabilities and implementing proper access controls. Organizations should configure OpenSSH servers with AllowTcpForwarding set to no unless explicitly required for legitimate business purposes, aligning with security best practices outlined in the Center for Internet Security (CIS) benchmarks and NIST guidelines. Additionally, implementing strict access controls, monitoring SSH connections, and regularly auditing SSH configurations can help detect and prevent exploitation attempts. The vulnerability demonstrates the importance of principle of least privilege and proper configuration management, as highlighted in CWE-276 which addresses improper privilege management. Security teams should also consider implementing network segmentation and firewall rules to limit access to sensitive services, following ATT&CK framework techniques that address command and control communications and lateral movement through network services. Regular security assessments and vulnerability scanning should include verification of SSH configurations to ensure compliance with security policies and prevent unauthorized use of forwarding capabilities.