Source: cve@mitre.org
SSH Secure Shell for Servers 3.0.0 to 3.1.1 allows remote attackers to override the AllowedAuthentications configuration and use less secure authentication schemes (e.g. password) than configured for the server.
SSH Secure Shell for Servers versions 3.0.0 to 3.1.1 are vulnerable to a critical security flaw allowing remote attackers to bypass configured authentication restrictions. This vulnerability enables attackers to force the server to accept less secure authentication methods, such as password-based logins, potentially leading to complete system compromise and data breaches.
Step 1: Connection Initiation: The attacker initiates an SSH connection to the vulnerable server.
Step 2: Authentication Request: The attacker crafts an SSH authentication request, specifically requesting a less secure authentication method (e.g., password) regardless of the server's configuration.
Step 3: Bypassing Configuration: The server, due to the vulnerability, fails to properly enforce the AllowedAuthentications configuration. It does not validate the client's requested authentication method against the server's allowed methods.
Step 4: Authentication Success: The server, accepting the less secure method, proceeds with the authentication process (e.g., prompting for a password).
Step 5: Access Granted: If the attacker provides valid credentials for the chosen authentication method (e.g., a correct password), they gain unauthorized access to the system.
The vulnerability stems from a flaw in how SSH Secure Shell for Servers handles the AllowedAuthentications configuration. The server fails to properly validate or enforce the configured authentication methods. Specifically, the server's authentication logic doesn't adequately check the client's requested authentication methods against the server's configured allowed methods. This allows a malicious client to request a less secure method (e.g., password) even if the server is configured to only allow stronger methods (e.g., public key authentication). The root cause is likely a missing or flawed check within the authentication processing code, allowing the client's authentication request to override the server's intended security policy. This is not a buffer overflow or race condition, but a logic flaw in the authentication process.
While no specific APTs are directly linked to this specific CVE, any actor seeking initial access or privilege escalation would find this vulnerability valuable. This vulnerability is not listed on the CISA KEV at this time, but its potential impact warrants consideration in threat modeling.
Monitor SSH server logs for connections using less secure authentication methods (e.g., password) when the server is configured to disallow them.
Analyze SSH server configuration files to verify the AllowedAuthentications setting and ensure it aligns with security policies.
Network Intrusion Detection Systems (IDS) can be configured to detect SSH connection attempts using weak authentication methods.
Examine SSH server logs for unusual connection patterns or failed login attempts that might indicate exploitation attempts.
Upgrade to a patched version of SSH Secure Shell for Servers (versions 3.2 or later).
If upgrading is not immediately possible, disable password authentication and other weak authentication methods in the SSH server configuration. Enforce the use of strong authentication methods like public key authentication.
Regularly review and audit SSH server configurations to ensure they align with security best practices.
Implement a robust patching and vulnerability management program to address known vulnerabilities promptly.
Consider using a Web Application Firewall (WAF) to filter malicious requests.