How to remediate – Multiple Anti-Virus SMTP Message Long Line Parsing DoS

1. Introduction

The Multiple Anti-Virus SMTP Message Long Line Parsing DoS vulnerability affects email servers running antivirus software. A specially crafted email message, containing a very long line of text without breaks, can cause the antivirus scanner to crash, leading to denial of service. This impacts availability as the server cannot process emails during the outage. Confidentiality and integrity are not directly affected.

2. Technical Explanation

Some antivirus products have a limitation in how they handle long lines within email messages received via SMTP. When processing an email with a line exceeding the scanner’s buffer size, it can cause a fatal error and crash the service. An attacker could send a malicious email to trigger this condition.

• Root cause: Insufficient input validation when parsing long lines in SMTP messages.
• Exploit mechanism: An attacker sends an email with a single, very long line of text in the message body or headers. For example, sending an email with a subject containing a string longer than 2048 characters without any newline characters could trigger the vulnerability.
• Scope: Email servers using vulnerable antivirus scanners integrated into their Mail Transfer Agent (MTA). Specific versions depend on the antivirus vendor and product.

3. Detection and Assessment

Confirming a system is vulnerable involves checking the status of the antivirus service and reviewing logs for crash events.

• Quick checks: Check the running status of your email server’s antivirus process using task manager or similar tools. Look for any recent crashes or restarts of the antivirus service.
• Scanning: Nessus plugin ID 16879 can identify this vulnerability, but results should be verified manually.
• Logs and evidence: Examine the antivirus logs (location varies by vendor) for errors related to parsing long lines or buffer overflows. Look for event IDs indicating a crash or unexpected termination of the antivirus process. Common log paths include /var/log/syslog or Windows Event Viewer under Application and Services Logs.

tasklist | findstr /i "antivirus_process_name"

4. Solution / Remediation Steps

Fixing this issue requires upgrading, reconfiguring, or replacing the email antivirus solution.

4.1 Preparation

• Ensure you have access to the latest version of your antivirus software or an alternative solution. A roll back plan involves restoring from the snapshot taken earlier.
• A change window may be required depending on service impact and business needs; approval from IT management is recommended.

4.2 Implementation

1. Step 1: Back up your current antivirus configuration file.
2. Step 2: Download the latest version of your antivirus software from the vendor’s website.
3. Step 3: Install the new version of the antivirus software, following the vendor’s instructions.
4. Step 4: Restart the email server or the MTA service to load the updated antivirus scanner.

4.3 Config or Code Example

Before

#Example configuration showing no line length limit (this is illustrative)
max_line_length = 0

After

#Example configuration with a reasonable line length limit set.
max_line_length = 2048

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: Patch management to ensure timely updates for all software components, including antivirus solutions.
• Practice 2: Input validation on incoming email data to reject messages with excessively long lines or invalid formatting.

4.5 Automation (Optional)

#Example PowerShell script to check antivirus version (replace with your specific product details)
Get-WmiObject Win32_Product | Where-Object {$_.Name -like "*Antivirus*"} | Select-Object Name, Version

5. Verification / Validation

Confirm the fix by checking the antivirus version and attempting to send a test email with a long line.

• Post-fix check: Verify the installed antivirus version is updated using the command from section 3, expecting an output showing the new version number.
• Re-test: Send a test email containing a subject or body with a string longer than 2048 characters without newline characters. Confirm that the antivirus scanner does not crash and the email is processed (or blocked).
• Monitoring: Monitor antivirus logs for any errors related to long line parsing or crashes, as an example alert: “Antivirus service crashed with error code X”.

tasklist | findstr /i "antivirus_process_name"

6. Preventive Measures and Monitoring

Suggest only measures that are relevant to the vulnerability type. Use “for example” to keep advice conditional, not prescriptive.

• Baselines: Update your security baseline or policy to include a requirement for regular antivirus software updates.
• Pipelines: Implement automated testing in CI/CD pipelines to validate email processing with various input lengths and formats.
• Asset and patch process: Establish a regular patch review cycle (e.g., weekly) to identify and apply security updates for all critical systems, including email servers and antivirus software.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 1: Upgrading antivirus software may introduce compatibility issues with other systems; test thoroughly in a staging environment first.
• Risk or side effect 2: Incorrect configuration of the new antivirus software could lead to false positives or missed detections; review documentation carefully.

8. References and Resources

• Vendor advisory or bulletin: [Replace with link to vendor’s specific advisory if available]
• NVD or CVE entry: [If a CVE exists, add the link here]
• Product or platform documentation relevant to the fix: [Link to antivirus product documentation on line length limits]