CVE-2007-0844 in pam_ssh
Summary
by MITRE
The auth_via_key function in pam_ssh.c in pam_ssh before 1.92, when the allow_blank_passphrase option is disabled, allows remote attackers to bypass authentication restrictions and use private encryption keys requiring a blank passphrase by entering a non-blank passphrase.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 07/30/2019
The vulnerability described in CVE-2007-0844 affects the pam_ssh module, which is part of the Pluggable Authentication Modules framework used for authentication in Unix-like operating systems. This flaw exists in versions of pam_ssh prior to 1.92 and specifically impacts the auth_via_key function within the pam_ssh.c source file. The issue manifests when the allow_blank_passphrase option is disabled, creating a security bypass condition that undermines the intended authentication controls. The vulnerability represents a classic case of improper input validation and authentication flow control that can be exploited by remote attackers to gain unauthorized access to systems.
The technical implementation flaw occurs in how pam_ssh handles passphrase validation when the allow_blank_passphrase setting is disabled. Normally, when this option is turned off, the system should reject any authentication attempts that use private keys requiring blank passphrases. However, the auth_via_key function contains a logic error where it fails to properly validate the passphrase entered by the attacker. Even though a non-blank passphrase is provided by the remote attacker, the function incorrectly processes this input and allows the authentication to proceed as if a blank passphrase were used. This represents a violation of proper authentication flow control and demonstrates a failure in input sanitization and validation mechanisms.
The operational impact of this vulnerability is significant as it allows remote attackers to bypass authentication restrictions that were specifically designed to prevent unauthorized access to systems using SSH keys. Attackers can exploit this weakness to authenticate using private encryption keys that require blank passphrases, effectively circumventing the security controls that were put in place to prevent such access. This vulnerability particularly affects systems where administrators have disabled blank passphrases for SSH key authentication as a security measure, creating a false sense of security that is subsequently undermined by this flaw. The vulnerability can be exploited from remote locations without requiring local system access, making it particularly dangerous in networked environments.
The flaw aligns with CWE-284, which addresses improper access control issues in authentication systems, and represents a specific case of inadequate privilege control where authentication bypass occurs due to improper validation of user inputs. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation techniques where an attacker can bypass authentication controls to gain access to protected resources. The vulnerability also relates to credential access patterns where attackers can exploit authentication bypass mechanisms to obtain unauthorized system access. Organizations using pam_ssh versions prior to 1.92 should immediately implement the available patch to address this issue. The recommended mitigation involves upgrading to pam_ssh version 1.92 or later, which contains the necessary code fixes to properly validate passphrase inputs and maintain the intended authentication restrictions. Additionally, system administrators should review their SSH key management policies and ensure that blank passphrases are not permitted for authentication keys, as this vulnerability demonstrates that even when such restrictions are in place, improper validation can still allow bypass.