How to remediate – Enumerate the Network Routing configuration via SSH

1. Introduction

Nessus was able to retrieve network routing information from the remote host, meaning that an attacker with SSH access could enumerate the network configuration. This can aid reconnaissance and potentially identify internal networks or sensitive systems. Affected systems are typically those running a standard SSH server allowing privileged access. A successful exploit could lead to information disclosure.

2. Technical Explanation

The vulnerability occurs because the SSH service allows retrieval of network routing information without sufficient restriction. An attacker with valid credentials can use standard SSH commands to view this data. There is no known CVE associated with this specific enumeration, but it represents a general configuration weakness. For example, an attacker could connect via SSH and run commands to list routing tables and identify internal IP address ranges.

• Root cause: Insufficient access controls on network routing information accessible through the SSH service.
• Exploit mechanism: An attacker connects to the remote host using SSH with valid credentials and executes commands to retrieve network routing data.
• Scope: Systems running an SSH server that allows retrieval of network routing information, typically Linux or Unix-based systems.

3. Detection and Assessment

You can confirm vulnerability by attempting to retrieve the routing table via SSH. A thorough method involves reviewing SSH access logs for commands used to enumerate network configuration.

• Quick checks: Connect to the remote host using SSH and run the command `netstat -rn` or `route -n`. If output is displayed, the system is likely vulnerable.
• Scanning: Nessus plugin ID 10429 can identify this issue. Other vulnerability scanners may have similar plugins.
• Logs and evidence: Examine SSH logs (typically located in `/var/log/auth.log` or `/var/log/secure`) for commands like `netstat`, `route`, or other network configuration utilities executed by users with elevated privileges.

ssh user@host netstat -rn

4. Solution / Remediation Steps

Restrict SSH access to limit the ability to enumerate network routing information. Only allow necessary commands and consider using a more secure configuration.

4.1 Preparation

• Ensure you have valid credentials to access the system via SSH in case of rollback. A roll back plan is to revert configuration changes.
• Changes should be made during a scheduled maintenance window with appropriate approval from IT security.

4.2 Implementation

1. Step 1: Edit the `/etc/ssh/sshd_config` file and add or modify the `PermitUserEnvironment` directive to restrict allowed commands.
2. Step 2: Restart the SSH service using the command `sudo systemctl restart sshd`.

4.3 Config or Code Example

Before

# /etc/ssh/sshd_config
PermitUserEnvironment yes

After

# /etc/ssh/sshd_config
PermitUserEnvironment no

4.4 Security Practices Relevant to This Vulnerability

Least privilege and secure defaults are relevant practices for this vulnerability. Least privilege reduces the impact if an attacker gains access, while secure defaults minimize the attack surface. Input validation can also prevent malicious commands from being executed.

• Practice 1: Implement least privilege by granting users only the necessary permissions to perform their tasks.
• Practice 2: Configure SSH with secure defaults, such as disabling password authentication and using key-based authentication.

4.5 Automation (Optional)

#!/bin/bash
# This script modifies sshd_config to disable PermitUserEnvironment
sed -i 's/^PermitUserEnvironment yes$/PermitUserEnvironment no/' /etc/ssh/sshd_config
systemctl restart sshd

5. Verification / Validation

• Post-fix check: Connect to the remote host using SSH and run the command `netstat -rn` or `route -n`. No output should be displayed, indicating the system is no longer vulnerable.
• Re-test: Re-run the Nessus scan (plugin ID 10429) to confirm that the vulnerability is no longer detected.
• Smoke test: Verify that users can still connect to the SSH server and perform authorized tasks, such as file transfer or remote command execution within permitted limits.
• Monitoring: Monitor SSH logs for failed attempts to execute unauthorized commands.

ssh user@host netstat -rn

6. Preventive Measures and Monitoring

• Baselines: Update your security baseline to require restricted SSH access, including disabling unnecessary features and limiting allowed commands.
• Pipelines: Add static analysis checks in your CI/CD pipeline to detect insecure configurations in SSH configuration files.
• Asset and patch process: Establish a regular patch or config review cycle to ensure that systems are up-to-date with the latest security fixes and best practices.

7. Risks, Side Effects, and Roll Back

Restricting SSH access may impact users who rely on specific commands. A roll back involves reverting the changes made to `/etc/ssh/sshd_config` and restarting the SSH service.

• Risk or side effect 2: Incorrect configuration of `PermitUserEnvironment` could prevent all SSH access. Mitigation is to have a backup of the original configuration file.
• Roll back: 1) Restore the original `/etc/ssh/sshd_config` file from your backup. 2) Restart the SSH service using the command `sudo systemctl restart sshd`.

8. References and Resources

• Vendor advisory or bulletin: N/A – This is a general configuration issue, not a specific vendor vulnerability.
• NVD or CVE entry: N/A – No specific CVE associated with this enumeration.
• Product or platform documentation relevant to the fix: OpenSSH Configuration.