How to remediate – Zimbra Collaboration Server Aspell Spell Check Service Detection

1. Introduction

The Zimbra Collaboration Server Aspell Spell Check Service Detection indicates that a spell check service is running on your Zimbra server. This service supports the Zimbra mailbox functionality and is commonly found in open source messaging environments. A successful exploit could allow remote attackers to gain access to the service, potentially impacting confidentiality, integrity, and availability of email communications.

2. Technical Explanation

The spell check service within Zimbra Collaboration Server provides spell checking capabilities for users’ mailboxes. This service is accessible remotely. An attacker could exploit vulnerabilities in this service to execute commands or gain unauthorized access. While a specific CVE isn’t directly associated with the presence of the service itself, weaknesses within Aspell or its integration with Zimbra may be exploited.

• Root cause: The spell check service is enabled and accessible from outside the local host.
• Exploit mechanism: An attacker could send crafted requests to the spell check service, potentially leading to command execution or information disclosure. For example, a specially formed input string might trigger a buffer overflow or code injection vulnerability within Aspell.
• Scope: Zimbra Collaboration Server installations with the Aspell spell check service enabled are affected. Specific versions may be more vulnerable than others depending on the version of Aspell used.

3. Detection and Assessment

You can confirm if the service is running by checking its status, or scanning for open ports associated with it. A thorough assessment involves reviewing Zimbra logs for suspicious activity.

• Quick checks: Use the ps command to check for Aspell processes. For example: ps aux | grep aspell
• Scanning: Nessus plugin ID 16879 may identify this service, but results should be verified manually.
• Logs and evidence: Check Zimbra mail logs (typically located in /opt/zimbra/log) for any unusual activity related to the spell check service. Look for errors or unexpected connections.

ps aux | grep aspell

4. Solution / Remediation Steps

The recommended solution is to disable the Aspell spell check service if it’s not required, or ensure it’s running with appropriate security configurations.

4.1 Preparation

• Take a full backup of your Zimbra server before making any changes. Stop the Zimbra Collaboration Server service (zimbra-server) to prevent disruption during configuration updates.
• Ensure you have access to the Zimbra command line interface and appropriate administrative privileges. A roll back plan involves restoring the backup if issues occur.
• A change window may be needed depending on your organisation’s policies, with approval from IT management.

4.2 Implementation

1. Step 1: Log in to the Zimbra server as an administrator.
2. Step 2: Use the zmcli command to disable the Aspell spell check service: zmcli -E zimbraSpellCheckEnabled=false
3. Step 3: Restart the Zimbra Collaboration Server service: systemctl restart zimbra-server.

4.3 Config or Code Example

Before

zmcli -E zimbraSpellCheckEnabled=true

After

zmcli -E zimbraSpellCheckEnabled=false

4.4 Security Practices Relevant to This Vulnerability

List only practices that directly address this vulnerability type. Use neutral wording and examples instead of fixed advice. For example: least privilege, input validation, safe defaults, secure headers, patch cadence. If a practice does not apply, do not include it.

• Practice 1: Least privilege – limit the permissions granted to the spell check service account to reduce potential damage from exploitation.
• Practice 2: Safe defaults – disable unnecessary services like Aspell if they are not actively used by your organisation.

4.5 Automation (Optional)

#!/bin/bash
# Script to disable Zimbra Aspell spell check service
zimcli -E zimbraSpellCheckEnabled=false
systemctl restart zimbra-server
echo "Zimbra Aspell spell check service disabled."

5. Verification / Validation

Confirm the fix by checking the service status again and verifying that it is no longer running. Perform a basic smoke test to ensure email functionality remains intact.

• Post-fix check: Run ps aux | grep aspell. The output should not show any Aspell processes running.
• Re-test: Re-run the initial detection method (ps aux | grep aspell) to confirm that the service is disabled.
• Smoke test: Send and receive a test email through Zimbra to verify basic functionality remains operational.
• Monitoring: Monitor Zimbra logs for any errors related to spell check, which could indicate an issue with the configuration change.

ps aux | grep aspell

6. Preventive Measures and Monitoring

Regularly review your Zimbra server configuration to identify and disable unnecessary services. Implement a patch management process for Zimbra and its components.

• Baselines: Update your security baseline or policy to include the disabling of unused services like Aspell as a standard practice.
• Pipelines: Consider adding checks in your CI/CD pipeline to ensure that unnecessary services are not enabled during deployment.
• Asset and patch process: Implement a regular patch review cycle for Zimbra Collaboration Server, including updates to all associated components.

7. Risks, Side Effects, and Roll Back

Disabling the spell check service may affect user experience if users rely on it. If issues occur, you can re-enable the service.

• Roll back: Step 1: Log in to the Zimbra server as an administrator. Step 2: Use the zmcli command to re-enable the Aspell spell check service: zmcli -E zimbraSpellCheckEnabled=true. Step 3: Restart the Zimbra Collaboration Server service: systemctl restart zimbra-server.

8. References and Resources

• Vendor advisory or bulletin: https://www.zimbra.com/email-server-software/