CVE-2006-0393 in Mac OS X
Summary
by MITRE
OpenSSH in Apple Mac OS X 10.4.7 allows remote attackers to cause a denial of service or determine account existence by attempting to log in using an invalid user, which causes the server to hang.
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Analysis
by VulDB Data Team • 06/22/2025
The vulnerability described in CVE-2006-0393 represents a significant security flaw in the OpenSSH implementation running on Apple Mac OS X 10.4.7 systems. This issue manifests as a denial of service condition that can be triggered remotely by attackers who attempt to establish SSH connections using invalid user accounts. The vulnerability specifically affects the SSH server component of the operating system's OpenSSH package, creating a scenario where legitimate system resources become consumed or locked in an indefinite wait state when processing malformed authentication requests.
The technical root cause of this vulnerability stems from improper input validation within the SSH authentication handling mechanism. When an attacker submits an authentication request with a non-existent username, the OpenSSH server fails to properly handle this condition and instead enters a state where it hangs or becomes unresponsive to further authentication attempts. This behavior violates the fundamental principle of robust error handling in network services, where invalid inputs should be processed gracefully without causing system instability or resource exhaustion. The vulnerability is categorized under CWE-20 as "Improper Input Validation" and represents a classic example of how malformed input can lead to denial of service conditions in network services.
The operational impact of this vulnerability extends beyond simple service disruption to potentially enable account enumeration attacks. Attackers can exploit this weakness to determine which user accounts exist on the target system by observing the server's behavior when attempting to authenticate with various usernames. When a valid account is targeted, the server may respond differently than when an invalid account is specified, allowing for systematic discovery of legitimate user accounts. This capability significantly weakens the security posture of the affected system and can serve as a precursor to more sophisticated attacks. The vulnerability aligns with ATT&CK technique T1078.004 for "Valid Accounts: SSH Keys" and T1562.001 for "Impair Defenses: Disable or Modify Tools" as it can be leveraged to establish a foothold for further reconnaissance and exploitation activities.
The implications of this vulnerability are particularly concerning given that it affects a widely deployed operating system version and impacts the core authentication service that many organizations rely upon for secure remote access. The denial of service component can be exploited to disrupt legitimate SSH services, potentially affecting system administrators who depend on remote access capabilities for maintenance and management tasks. Organizations running affected versions of Mac OS X should implement immediate mitigations including applying the relevant security patches provided by Apple, implementing network-level restrictions on SSH access, and monitoring for suspicious authentication patterns. The vulnerability demonstrates the critical importance of proper error handling and input validation in network services, as even seemingly benign conditions can be exploited to compromise system availability and security.