CVE-2012-0920 in Communications Session Border Controller
Summary
by MITRE
Use-after-free vulnerability in Dropbear SSH Server 0.52 through 2012.54, when command restriction and public key authentication are enabled, allows remote authenticated users to execute arbitrary code and bypass command restrictions via multiple crafted command requests, related to "channels concurrency."
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Analysis
by VulDB Data Team • 01/02/2025
The CVE-2012-0920 vulnerability represents a critical use-after-free flaw in the Dropbear SSH server implementation that emerged from the intersection of command restriction mechanisms and concurrent channel handling. This vulnerability specifically affects versions ranging from 0.52 through 2012.54, creating a window of exposure where legitimate authenticated users can exploit the software's memory management practices to gain unauthorized code execution capabilities. The flaw manifests when the server employs both command restriction features and public key authentication, creating a complex attack surface that leverages the concurrency model of SSH channels to deliver malicious payloads.
The technical root cause of this vulnerability stems from improper memory deallocation handling within the channel management subsystem of Dropbear. When multiple concurrent command requests are processed through the SSH server, the software fails to properly synchronize access to shared memory resources, leading to situations where freed memory blocks are still referenced by subsequent operations. This use-after-free condition occurs during the handling of channel closures and reinitializations, particularly when command restrictions are enforced alongside public key authentication. The vulnerability is classified under CWE-416 as a use-after-free error, which represents a common but dangerous class of memory safety issues that can be exploited to execute arbitrary code through memory corruption attacks.
The operational impact of this vulnerability extends beyond simple privilege escalation to include complete system compromise when exploited successfully. An authenticated attacker can leverage this flaw to bypass command restrictions that are typically enforced by the SSH server, effectively allowing them to execute any command on the target system regardless of the configured restrictions. The concurrency aspect of the vulnerability means that the attacker does not need to be a privileged user to exploit this weakness, as the flaw exists in the server's handling of legitimate authenticated sessions. This creates a particularly dangerous scenario where even restricted users can gain full system access, making the vulnerability especially impactful in environments where command restrictions are used as security controls.
The exploitation of this vulnerability requires the attacker to be authenticated to the SSH server and to have access to the specific configuration where command restrictions and public key authentication are both enabled. The attack vector involves sending multiple crafted command requests that trigger the concurrent channel handling logic, causing the memory management system to enter an inconsistent state. The ATT&CK framework categorizes this as a privilege escalation technique through memory corruption, specifically leveraging the server's resource management weaknesses. Organizations using Dropbear SSH servers in production environments face significant risk from this vulnerability, as it can be exploited by users who have legitimate access to the system but should be restricted from executing certain commands.
Mitigation strategies for CVE-2012-0920 require immediate patching of affected Dropbear server installations to versions that address the memory management issues in channel handling. System administrators should also implement additional monitoring for unusual command execution patterns and concurrent channel activity that might indicate exploitation attempts. The vulnerability highlights the importance of proper synchronization mechanisms in multi-threaded or concurrent systems, particularly in security-critical applications like SSH servers. Organizations should consider implementing network-level controls and access restrictions to limit the impact of potential exploitation, while also ensuring that command restriction policies are properly configured and regularly audited to prevent unauthorized access to sensitive system functions.