How to remediate – CVS pserver Detection

1. Introduction

A CVS pserver is listening on a remote port, indicating that Concurrent Versions System (CVS) is running with an unencrypted protocol. This poses a risk as communication can be intercepted, potentially exposing sensitive version control data like source code history. Systems running older software development tools are usually affected. A compromise could lead to loss of confidentiality, integrity and availability of the CVS repository.

2. Technical Explanation

The Concurrent Versions System (CVS) server uses the ‘pserver’ protocol for communication, which transmits data in plain text. This allows attackers on the network to intercept credentials and source code. The vulnerability occurs when CVS is configured to allow access via pserver instead of more secure methods like SSH or RSH. An attacker could use a tool like `cvs` to connect to the server and retrieve files without encryption, potentially gaining access to sensitive information.

• Root cause: Use of unencrypted ‘pserver’ protocol for CVS communication.
• Exploit mechanism: An attacker connects to the pserver port using a `cvs` client and retrieves data from the repository.
• Scope: Systems running CVS server with pserver enabled are affected.

3. Detection and Assessment

You can confirm if a system is vulnerable by checking for an open port associated with the CVS pserver protocol. A thorough method involves attempting to connect to the server using `cvs`.

• Quick checks: Use `netstat` or `ss` to check for listening ports on 2401/tcp, the default pserver port.
• Scanning: Nessus plugin ID 35687 can detect CVS server with pserver enabled (example only).
• Logs and evidence: Check system logs for connections related to the CVS pserver protocol.

netstat -tulnp | grep 2401

4. Solution / Remediation Steps

To fix this issue, use CVS over SSH or RSH instead of relying on the unencrypted pserver protocol. This will encrypt communication and protect sensitive data.

4.1 Preparation

• Ensure you have SSH or RSH configured correctly for secure access. A roll back plan is to restore from backup.
• A change window may be needed depending on service impact and approval requirements.

4.2 Implementation

1. Step 1: Configure CVS to use SSH for connections by modifying the `~/.cvs/clientrc` file or equivalent configuration file. Add or modify the line `ssh = `.
2. Step 2: Disable pserver access in the CVS server configuration file (e.g., `/etc/cvsd.conf`). Comment out or remove any lines related to pserver.
3. Step 3: Restart the CVS service to apply the changes.

4.3 Config or Code Example

Before

server = :pserver:user@hostname:/path/to/repository

After

server = :ssh:user@hostname:/path/to/repository

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege limits the impact if exploited, and secure defaults reduce the risk of misconfiguration.

• Practice 1: Least privilege – restrict access to the CVS repository to only authorized users.
• Practice 2: Secure Defaults – configure services with the most secure options enabled by default.

4.5 Automation (Optional)

# Example Bash script to disable pserver access in cvsd.conf
# WARNING: This requires root privileges and may disrupt service. Back up the file first!
# sed -i 's/^pserver.*/#&/' /etc/cvsd.conf
# systemctl restart cvsd

5. Verification / Validation

Confirm the fix by checking that pserver is no longer listening on port 2401 and that you can successfully connect to the CVS repository using SSH. A smoke test involves retrieving a file from the repository via SSH.

• Post-fix check: `netstat -tulnp | grep 2401` should return no results.
• Re-test: Run the initial detection method (e.g., Nessus scan) and confirm it no longer reports the vulnerability.
• Smoke test: Use `cvs checkout ` via SSH to verify you can retrieve files from the repository.
• Monitoring: Monitor system logs for failed connection attempts on port 2401 (example only).

netstat -tulnp | grep 2401

6. Preventive Measures and Monitoring

Update security baselines to include secure CVS configuration settings, such as disabling pserver access. Add checks in CI/CD pipelines to prevent insecure configurations from being deployed.

• Baselines: Update a security baseline or policy to require the use of SSH for CVS connections.
• Pipelines: Include static analysis tools in your CI pipeline to check for insecure CVS configuration settings.
• Asset and patch process: Review CVS configurations regularly as part of an asset management process.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 1: Disruption of existing workflows using pserver. Mitigation: Communicate changes and ensure SSH access is available for all users.
• Risk or side effect 2: Potential downtime during service restart. Mitigation: Perform the change during a maintenance window.
• Roll back: Restore the original CVS configuration file from backup, then restart the CVS service.

8. References and Resources

• Vendor advisory or bulletin: No specific vendor advisory available for this general issue.
• NVD or CVE entry: No specific CVE associated with this general issue.
• Product or platform documentation relevant to the fix: GNU CVS Documentation