CVE-2006-0883 in FreeBSD
Summary
by MITRE
OpenSSH on FreeBSD 5.3 and 5.4, when used with OpenPAM, does not properly handle when a forked child process terminates during PAM authentication, which allows remote attackers to cause a denial of service (client connection refusal) by connecting multiple times to the SSH server, waiting for the password prompt, then disconnecting.
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Analysis
by VulDB Data Team • 09/02/2019
The vulnerability identified as CVE-2006-0883 represents a critical denial of service weakness in OpenSSH implementations running on FreeBSD systems version 5.3 and 5.4 when integrated with OpenPAM authentication framework. This flaw specifically targets the process management mechanism during authentication cycles where the parent SSH daemon fails to properly account for child processes that terminate unexpectedly during PAM authentication sequences. The vulnerability operates through a sophisticated race condition that exploits the interaction between the SSH server's forking behavior and OpenPAM's authentication handling, creating a scenario where legitimate authentication attempts can be disrupted through carefully orchestrated connection patterns.
The technical root cause of this vulnerability stems from improper process state management within the OpenSSH daemon when utilizing OpenPAM for authentication services. When a client establishes an SSH connection and the authentication process begins, the parent SSH daemon forks a child process to handle the PAM authentication. If this child process terminates during authentication before completing its task, the parent daemon does not properly clean up or reset its internal state, leading to a condition where subsequent authentication attempts fail. This process management failure creates a cascading effect where the daemon becomes unresponsive to new connection attempts, effectively rendering the SSH service unavailable to legitimate users. The vulnerability operates under CWE-404 which specifically addresses improper resource management and process handling issues in software systems.
The operational impact of this vulnerability extends beyond simple service disruption to create a sophisticated denial of service vector that can be exploited by remote attackers with minimal privileges. Attackers can systematically exploit this weakness by establishing multiple SSH connections, waiting for the password prompt to appear, and then immediately disconnecting before authentication completion. This pattern causes the SSH daemon to accumulate zombie processes or corrupted state information that eventually leads to complete service exhaustion. The attack requires no authentication credentials and can be executed from any network location with access to the SSH service, making it particularly dangerous for publicly accessible systems. According to ATT&CK framework tactic T1499, this vulnerability represents a service disruption technique that can be classified under "Network Denial of Service" and "Resource Exhaustion" methods.
Mitigation strategies for this vulnerability require immediate system updates and configuration changes to address the underlying process management flaw. The most effective solution involves upgrading to FreeBSD versions that contain patches for this specific issue, along with implementing proper process monitoring and cleanup mechanisms within the SSH daemon configuration. System administrators should also consider implementing connection rate limiting and authentication attempt throttling to prevent exploitation through automated attack patterns. The vulnerability demonstrates the critical importance of proper process state management in server applications and highlights the need for robust error handling in authentication frameworks. Additionally, implementing intrusion detection systems that can monitor for unusual connection patterns and authentication failure sequences provides an additional layer of defense against exploitation attempts. Organizations should also review their SSH daemon configurations to ensure that proper resource cleanup occurs even when authentication processes fail or terminate unexpectedly, preventing the accumulation of corrupted states that lead to service unavailability.