CVE-2015-6565 in OpenSSH
Summary
by MITRE
sshd in OpenSSH 6.8 and 6.9 uses world-writable permissions for TTY devices, which allows local users to cause a denial of service (terminal disruption) or possibly have unspecified other impact by writing to a device, as demonstrated by writing an escape sequence.
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Analysis
by VulDB Data Team • 11/18/2024
The vulnerability identified as CVE-2015-6565 affects the OpenSSH implementation in versions 6.8 and 6.9, specifically within the sshd daemon component. This issue represents a significant security flaw that stems from improper permission handling for terminal devices, creating a potential vector for local privilege escalation and system disruption. The vulnerability manifests when the sshd service creates or manages terminal devices with world-writable permissions, which violates fundamental security principles of least privilege and proper access control.
The technical flaw occurs at the system-level permission management within the OpenSSH daemon where TTY devices are created or accessed with overly permissive settings. When a local user can write to a world-writable TTY device, they gain the ability to inject escape sequences directly into the terminal session. This capability enables attackers to manipulate the terminal environment in ways that can disrupt normal operations, potentially leading to complete terminal session corruption or denial of service conditions. The vulnerability specifically exploits the fact that the sshd process does not properly enforce restrictive permissions on terminal devices, allowing unauthorized write access that can be leveraged for malicious purposes.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as it provides attackers with the means to potentially execute more sophisticated attacks. Local users can exploit this weakness to disrupt terminal sessions, which may affect system administrators conducting remote management tasks through SSH connections. The unspecified other impacts mentioned in the description suggest that this vulnerability could potentially enable privilege escalation or other security breaches, depending on the specific system configuration and access patterns. The attack vector is particularly concerning because it requires only local access to exploit, making it difficult to detect and prevent through network-based security measures.
Security professionals should note that this vulnerability aligns with CWE-732, which describes improper permission assignment for critical resources, and represents a clear violation of the principle of least privilege. The ATT&CK framework categorizes this type of vulnerability under privilege escalation techniques, specifically targeting local system access and terminal manipulation. Organizations should prioritize immediate patching of affected OpenSSH versions, as the vulnerability provides a straightforward path for local attackers to disrupt system operations. System administrators should also implement monitoring for unauthorized terminal device modifications and consider implementing additional access controls to limit local user privileges. The remediation process involves updating to OpenSSH versions that properly enforce restrictive permissions on TTY devices, ensuring that terminal sessions cannot be manipulated by unauthorized local users through simple write operations.