CVE-2003-0386 in OpenSSH
Summary
by MITRE
OpenSSH 3.6.1 and earlier, when restricting host access by numeric IP addresses and with VerifyReverseMapping disabled, allows remote attackers to bypass "from=" and "user@host" address restrictions by connecting to a host from a system whose reverse DNS hostname contains the numeric IP address.
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Analysis
by VulDB Data Team • 12/15/2024
The vulnerability described in CVE-2003-0386 represents a significant security flaw in OpenSSH versions 3.6.1 and earlier that affects the implementation of host access controls. This issue specifically manifests when administrators configure SSH servers to restrict access based on numeric IP addresses using the "from=" directive or "user@host" patterns, combined with the VerifyReverseMapping configuration option set to disabled. The flaw exploits a fundamental misalignment between how SSH handles IP address validation and reverse DNS resolution, creating a bypass mechanism that undermines the intended access control policies.
The technical root cause of this vulnerability lies in the improper handling of reverse DNS lookups within the SSH daemon's authentication process. When VerifyReverseMapping is disabled, the SSH server should not perform reverse DNS lookups to validate hostnames. However, the flawed implementation allows attackers to connect from systems where the reverse DNS hostname contains the numeric IP address being restricted. This creates a scenario where the SSH server, despite having disabled reverse mapping verification, inadvertently accepts connections that should be rejected based on the configured "from=" or "user@host" restrictions. The vulnerability essentially leverages the DNS resolution process to circumvent the intended access control mechanisms.
From an operational impact perspective, this vulnerability enables remote attackers to bypass critical access controls that administrators have implemented to restrict SSH access based on IP addresses or hostnames. Attackers can exploit this flaw to gain unauthorized access to systems that should be protected by IP-based restrictions, potentially leading to full system compromise. The vulnerability is particularly concerning because it operates silently, allowing unauthorized access without triggering any authentication failures or logging anomalies that might alert system administrators to the breach. This makes detection and prevention significantly more challenging in environments where proper access controls are essential for security posture.
The vulnerability maps directly to CWE-284 Access Control Issues, specifically addressing improper access control mechanisms in network services where authentication and authorization are bypassed through flawed validation logic. From an ATT&CK framework perspective, this represents a privilege escalation technique through network service exploitation, falling under the T1190 Exploit Public-Facing Application and T1078 Valid Accounts categories. The flaw demonstrates how seemingly benign configuration options can create security holes when combined with improper validation logic, particularly in network protocols where multiple authentication layers interact. Organizations implementing SSH access controls should consider this vulnerability when assessing their network security posture, as it highlights the importance of proper input validation and the potential for DNS-based attacks to bypass traditional network access controls.
Mitigation strategies for CVE-2003-0386 require immediate action to upgrade OpenSSH to versions 3.7.0 or later where this vulnerability has been addressed. Administrators should also review their SSH configuration to ensure VerifyReverseMapping is properly configured according to security requirements, and consider implementing additional network-level controls such as firewall rules and network segmentation to provide defense-in-depth. Regular security assessments should verify that access control mechanisms are properly functioning and that no similar bypass vulnerabilities exist in other network services that might be exploited to gain unauthorized access to protected systems.