How to remediate – Anonymous SFTP Enabled

1. Introduction

The vulnerability “Anonymous SFTP Enabled” means that the SSH service on a server allows users to log in without needing valid credentials. This is a security risk because anyone could gain access to sensitive files and data. Systems running an SSH service, particularly those used for file transfer or remote administration, are usually affected. A successful exploit could lead to confidentiality, integrity, and availability compromise of the system’s files.

2. Technical Explanation

The root cause is typically a misconfiguration allowing anonymous access via SFTP. An attacker can connect to the SSH service and authenticate without providing a username or password. This often happens when the server is set up with default settings that permit guest logins, or when administrators unintentionally enable this feature. There isn’t a specific CVE associated with this general configuration issue; however, it falls under CWE-306: Insufficient Credential Check. An attacker could simply attempt to connect using an SFTP client without providing any credentials and gain access if the service is configured to allow it. Affected platforms include Linux, Unix, and Windows servers running OpenSSH or similar SSH implementations.

• Root cause: Misconfigured SSH server allowing anonymous SFTP logins.
• Exploit mechanism: An attacker connects via SFTP without credentials; if allowed, access is granted. For example, using a standard SFTP client and leaving the username/password fields blank.
• Scope: Servers running OpenSSH or similar SSH implementations on Linux, Unix, and Windows platforms.

3. Detection and Assessment

You can confirm if your system is vulnerable by attempting an anonymous login. A thorough method involves reviewing the SSH server configuration file for settings related to anonymous access.

• Quick checks: Attempt to connect via SFTP without providing a username or password.
• Scanning: Nessus plugin ID 35804 can detect this issue, but results should be verified manually.
• Logs and evidence: Check SSH server logs (typically /var/log/auth.log on Linux) for successful logins from unknown users or connections without authentication details.

sftp -o User=anonymous 

4. Solution / Remediation Steps

The following steps disable anonymous SFTP access and improve server security.

4.1 Preparation

• Ensure you have alternative methods for accessing the server in case of issues during the configuration change. A roll back plan involves restoring the original SSH configuration file.
• A standard change window may be needed, and approval from system owners is recommended.

4.2 Implementation

1. Step 1: Edit the SSH server configuration file (typically /etc/ssh/sshd_config on Linux).
2. Step 2: Find the line “AllowAnonymous yes” or similar and change it to “AllowAnonymous no”. If the line doesn’t exist, add it.
3. Step 3: Save the changes to the SSH configuration file.
4. Step 4: Restart the SSH service to apply the new configuration.

4.3 Config or Code Example

Before

# /etc/ssh/sshd_config
AllowAnonymous yes

After

# /etc/ssh/sshd_config
AllowAnonymous no

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege reduces the impact if an attacker gains access, and safe defaults ensure that services are configured securely out of the box. Regularly reviewing server configurations is also crucial for identifying and addressing potential vulnerabilities.

• Practice 1: Implement least privilege to limit user access rights.
• Practice 2: Enforce secure default settings for all services.

4.5 Automation (Optional)

# Example Ansible task to disable anonymous SFTP:
- name: Disable Anonymous SFTP
  lineinfile:
    path: /etc/ssh/sshd_config
    regexp: '^AllowAnonymous yes'
    line: 'AllowAnonymous no'
  notify: Restart SSH service
handlers:
  - name: Restart SSH service
    service:
      name: sshd
      state: restarted

5. Verification / Validation

Confirm the fix by attempting an anonymous login again and verifying that it is no longer possible. Check the SSH server configuration file to ensure the changes have been applied correctly.

• Post-fix check: Attempt to connect via SFTP without credentials; connection should be refused.
• Re-test: Run the earlier quick check (sftp -o User=anonymous ) and confirm it fails.
• Smoke test: Verify that legitimate users can still log in using their valid credentials.
• Monitoring: Monitor SSH logs for failed login attempts from unknown sources, which could indicate ongoing reconnaissance activity.

sftp -o User=anonymous  # Connection should be refused

6. Preventive Measures and Monitoring

Regularly update security baselines to include this check. Implement automated configuration checks in CI/CD pipelines to prevent misconfigurations from being deployed. A sensible patch or config review cycle of at least monthly is recommended.

• Baselines: Update your server security baseline to explicitly disable anonymous SFTP access.
• Pipelines: Add a check in your deployment pipeline to ensure the SSH configuration file does not allow anonymous logins.
• Asset and patch process: Review server configurations regularly, at least monthly, as part of your vulnerability management program.

7. Risks, Side Effects, and Roll Back

Disabling anonymous SFTP access may disrupt any existing services or scripts that rely on it (though this is rare). If issues occur, restore the original SSH configuration file to revert the changes.

• Risk or side effect 2: Temporary loss of access if the configuration is incorrect; mitigate by having a backup plan for accessing the server.
• Roll back:
  1. Stop the SSH service.
  2. Restore the original SSH configuration file from your backup.
  3. Restart the SSH service.

8. References and Resources

• Vendor advisory or bulletin: OpenSSH Security Page
• NVD or CVE entry: Not applicable for this general configuration issue.
• Product or platform documentation relevant to the fix: sshd_config man page