How to remediate – EMail Security Virtual Appliance learn-msg.cgi Remote Code Exe…

1. Introduction

The EMail Security Virtual Appliance learn-msg.cgi script contains a remote code execution vulnerability. This allows an attacker on the network to run commands on your email security appliance, potentially gaining full control of it. Systems running affected versions of the Email Security Virtual Appliance are at risk. Successful exploitation could lead to complete compromise of confidentiality, integrity and availability of the system.

2. Technical Explanation

The vulnerability exists in the ‘id’ parameter of the ‘learn-msg.cgi’ script within the EMail Security Virtual Appliance web application. Insufficient input validation allows an attacker to inject arbitrary commands that are then executed by the web server process. An unauthenticated attacker can send a specially crafted HTTP request to trigger this flaw.

• Root cause: Missing or inadequate input sanitization of the ‘id’ parameter in the ‘learn-msg.cgi’ script allows command injection.
• Exploit mechanism: An attacker sends an HTTP request containing malicious commands within the ‘id’ parameter, which are then executed on the server. For example, a request like http://[target]/learn-msg.cgi?id=; whoami could execute the ‘whoami’ command.
• Scope: Affected versions of the Email Security Virtual Appliance.

3. Detection and Assessment

You can confirm vulnerability by checking the version of your EMail Security Virtual Appliance installation. Scanning tools may also identify this issue.

• Quick checks: Check the web interface for the installed version number, or use command line tools if available to determine the software version.
• Scanning: Nessus vulnerability ID 55050 can detect this vulnerability. Other scanners may have similar signatures.
• Logs and evidence: Review web server logs for requests targeting ‘learn-msg.cgi’ with suspicious parameters, particularly those containing shell metacharacters (`;`, `|`, `&`).

4. Solution / Remediation Steps

Currently there is no known solution for this vulnerability. Mitigation focuses on network segmentation and monitoring.

4.1 Preparation

• There are no known dependencies for this vulnerability. A roll back plan involves restoring from backup if necessary.
• Change windows should be planned during off-peak hours with appropriate approvals.

4.2 Implementation

1. Step 1: Implement network segmentation to limit access to the Email Security Virtual Appliance from untrusted networks.
2. Step 2: Enable detailed web server logging and monitoring for suspicious activity targeting ‘learn-msg.cgi’.

4.3 Config or Code Example

Before

After

4.4 Security Practices Relevant to This Vulnerability

Practices like least privilege and input validation can help prevent this issue.

• Practice 1: Least privilege – restrict access to the Email Security Virtual Appliance to only authorized users and networks, reducing the potential impact of a successful exploit.
• Practice 2: Input validation – implement strict input validation on all user-supplied data, including parameters passed to web applications like ‘learn-msg.cgi’, to prevent command injection attacks.

4.5 Automation (Optional)

5. Verification / Validation

Verify the fix by confirming network segmentation is in place and monitoring logs for suspicious activity. A negative test involves attempting to access ‘learn-msg.cgi’ from an untrusted network and verifying that requests are blocked.

• Post-fix check: Verify firewall rules block external access to port 80 or 443 on the Email Security Virtual Appliance.
• Re-test: Attempt to exploit the vulnerability from an untrusted network and confirm that the request is denied.
• Monitoring: Monitor web server logs for any requests targeting ‘learn-msg.cgi’ with suspicious parameters, particularly those containing shell metacharacters.

6. Preventive Measures and Monitoring

Update security baselines to include restrictions on access to sensitive web applications like the Email Security Virtual Appliance. Implement CI/CD pipeline checks for known vulnerabilities.

• Baselines: Update your security baseline or policy to restrict network access to critical systems, including the Email Security Virtual Appliance.
• Pipelines: Add vulnerability scanning in your CI/CD pipelines to identify and block deployments of vulnerable software.
• Asset and patch process: Establish a regular patch review cycle for all systems, including the Email Security Virtual Appliance.

7. Risks, Side Effects, and Roll Back

Network segmentation may require changes to firewall rules that could temporarily disrupt email flow. Roll back involves restoring the original firewall configuration.

• Risk or side effect 2: Changes to network segmentation may require coordination with other teams.
• Roll back: Restore the original firewall configuration from backup if necessary.

8. References and Resources

Links to official advisories and trusted documentation related to this vulnerability.

• Vendor advisory or bulletin: https://www.securityfocus.com/bid/55050
• NVD or CVE entry: No specific CVE available at this time.
• Product or platform documentation relevant to the fix: Refer to your Email Security Virtual Appliance vendor’s documentation for firewall configuration and security best practices.