CVE-2001-0816 in OpenSSH
Summary
by MITRE
OpenSSH before 2.9.9, when running sftp using sftp-server and using restricted keypairs, allows remote authenticated users to bypass authorized_keys2 command= restrictions using sftp commands.
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Analysis
by VulDB Data Team • 05/12/2019
The vulnerability identified as CVE-2001-0816 represents a significant security flaw in OpenSSH implementations prior to version 2.9.9 that affects the Secure File Transfer Protocol functionality. This issue specifically impacts systems utilizing sftp-server with restricted keypairs, creating a pathway for authenticated attackers to circumvent access controls that should limit user capabilities through authorized_keys2 command= directives. The flaw exists within the sftp subsystem's handling of user permissions and command execution restrictions, fundamentally undermining the security model designed to contain user activities within predefined boundaries.
The technical implementation of this vulnerability stems from insufficient validation of sftp commands when operating under restricted keypair configurations. When users establish connections through sftp-server using keypairs that contain command= directives in authorized_keys2 files, the system should enforce these restrictions to limit what commands or operations users can perform. However, the flaw allows authenticated users to execute sftp commands that bypass these restrictions, effectively granting them access to operations that should be prohibited. This occurs because the sftp-server fails to properly validate or enforce the command restrictions defined in the authorized_keys2 file during sftp session operations, creating an access control bypass that can be exploited by any authenticated user with appropriate credentials.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass potential data exfiltration, unauthorized system modifications, and complete compromise of file access controls within affected systems. An attacker exploiting this vulnerability can bypass restrictions that should limit users to specific directories, file operations, or command execution capabilities, potentially gaining access to sensitive files, directories, or system resources that should remain protected. This vulnerability particularly affects environments where sftp is used for secure file transfers with restricted user access, as it undermines the fundamental security principle of least privilege that administrators implement through command= restrictions in authorized_keys2 files. The flaw can be exploited by any authenticated user who has access to the sftp service, making it a critical concern for organizations relying on sftp-based access controls.
Organizations affected by this vulnerability should immediately implement mitigations including upgrading to OpenSSH version 2.9.9 or later, which contains the necessary patches to address the command restriction bypass. System administrators should also review existing authorized_keys2 files to ensure that command= restrictions are properly configured and that users are not granted unnecessary privileges. Additionally, implementing monitoring and logging of sftp activities can help detect potential exploitation attempts, while network segmentation and access control lists can provide additional defense-in-depth measures. This vulnerability aligns with CWE-284, which addresses improper access control, and maps to ATT&CK technique T1078 for valid accounts and privilege escalation through command execution bypass. The security implications of this flaw underscore the importance of maintaining up-to-date cryptographic software and proper access control configuration in secure file transfer environments.