How to remediate – FKey Arbitrary Remote File Disclosure

1. Introduction

The FKey Arbitrary Remote File Disclosure vulnerability allows an attacker to read sensitive files on a remote system running the finger daemon. This could allow attackers to gain access to credentials, configuration details, and other confidential information which they can use to escalate attacks or compromise systems further. Systems commonly affected are those running a vulnerable version of the finger daemon, often found on older Unix-like operating systems. Impact is likely high for confidentiality, medium for integrity, and low for availability.

2. Technical Explanation

The vulnerability occurs because the remote finger daemon (possibly ‘fkey’) does not properly validate user input when handling file requests. Specifically, it allows users to request files with names 10 characters or shorter. An attacker can exploit this by supplying a malicious filename that points to sensitive system files. The bugtraq advisory details how an attacker could read arbitrary files on the remote server.

• Root cause: Missing input validation in the file name parameter of the finger daemon.
• Exploit mechanism: An attacker sends a request to the finger daemon specifying a filename that is less than or equal to 10 characters long, allowing them to read the contents of that file. For example, requesting “/etc/passwd” could reveal user account information.
• Scope: Systems running vulnerable versions of the ‘fkey’ finger daemon are affected.

3. Detection and Assessment

To confirm if a system is vulnerable, first check if the finger daemon is running. Then verify its version. If you cannot determine the exact version, assume it is vulnerable and proceed with disabling the service.

• Quick checks: Use the command ps aux | grep finger to see if the finger daemon is running.
• Scanning: Nessus plugin ID 12321 can detect this vulnerability. This is an example only and may require updating.
• Logs and evidence: Check system logs for connections to port 79 (the default finger port). Look for unusual file access attempts.

ps aux | grep finger

4. Solution / Remediation Steps

The recommended solution is to disable the vulnerable service as there is no known fix currently available. Follow these steps carefully.

4.1 Preparation

• There are no dependencies to consider, but ensure you have alternative methods for user account management if relying on finger. A roll back plan is to re-enable the service.
• Change windows may be required depending on your environment and approval processes.

4.2 Implementation

1. Step 1: Stop the finger daemon service using the command sudo systemctl stop finger (or equivalent for your operating system).
2. Step 2: Disable the finger daemon service to prevent it from starting automatically on boot using the command sudo systemctl disable finger.

4.3 Config or Code Example

Before

# No specific configuration example available, as the vulnerability is in the daemon itself.  The service will be running by default on many systems.

After

sudo systemctl disable finger
sudo systemctl stop finger

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this type of vulnerability. Least privilege reduces the impact if exploited, and input validation prevents unsafe data from being processed. Patch cadence ensures systems are updated with the latest security fixes.

• Practice 1: Implement least privilege to limit access to sensitive files and resources.
• Practice 2: Enforce strict input validation on all user-supplied data to prevent malicious requests.

4.5 Automation (Optional)

#!/bin/bash
# This script stops and disables the finger daemon on Debian/Ubuntu systems.
sudo systemctl stop finger
sudo systemctl disable finger
echo "Finger daemon stopped and disabled."

5. Verification / Validation

Confirm that the fix worked by checking if the finger daemon is no longer running. Re-run the earlier detection method to verify it’s gone. Perform a simple service smoke test to ensure other services are unaffected.

• Post-fix check: Run ps aux | grep finger and confirm that no processes related to ‘finger’ are listed.
• Re-test: Re-run the command from section 3 (ps aux | grep finger) to ensure the daemon is not running.
• Smoke test: Verify other network services, such as SSH, remain operational.
• Monitoring: Monitor system logs for any unexpected errors related to the finger daemon.

ps aux | grep finger # Expected output: no results

6. Preventive Measures and Monitoring

Update security baselines to include disabling unnecessary services like the finger daemon. Implement checks in CI/CD pipelines to prevent vulnerable software from being deployed. Maintain a sensible patch or configuration review cycle that fits your risk profile.

• Baselines: Update your system baseline to explicitly disable the finger daemon.
• Pipelines: Add checks in your deployment pipeline to ensure unnecessary services are not enabled.
• Asset and patch process: Review and apply security patches regularly, especially for critical infrastructure components.

7. Risks, Side Effects, and Roll Back

Disabling the finger daemon may impact applications or users that rely on it for account information. The roll back steps are to re-enable the service.

• Risk or side effect 2: Users accustomed to using finger may experience reduced functionality.
• Roll back:
  1. Step 1: Re-enable the finger daemon service using the command sudo systemctl enable finger.
  2. Step 2: Start the finger daemon service using the command sudo systemctl start finger.

8. References and Resources

• Vendor advisory or bulletin: https://seclists.org/bugtraq/2005/Jan/229
• NVD or CVE entry: No specific CVE is listed in the provided context.
• Product or platform documentation relevant to the fix: Documentation for your operating system on managing services (e.g., systemd).