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Mitigating Open Mail Relaying on SMTP Server

Mitigating Open Mail Relaying on SMTP Server


Security Advisory: Preventing Unauthorized Mail Relaying on SMTP Server 

Risk Information:

  • CVSS v2: Critical, Base Score: 10, Temporal Score: 8.5, Vector: AV:N/AC:L/Au:N/C:C/I:C/A:C
  • CVSS v3: High, Base Score: 7.5, Temporal Score: 6.9, Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H


The SMTP server configured on the remote host is operating as an open relay, potentially allowing unauthorized individuals to send bulk emails through your infrastructure. This vulnerability could result in network bandwidth abuse, risk of being blacklisted, and reputational damage.


  1. Access Control Configuration:

    • Ensure the SMTP server requires user authentication to send emails, preventing anonymous relaying.
    • Configure the server to restrict relaying to trusted IP addresses or networks only.
  2. Implement Email Authentication Techniques:

    • SPF (Sender Policy Framework): Create SPF records to specify which mail servers are permitted to send email on behalf of your domain.
    • DKIM (DomainKeys Identified Mail): Set up DKIM to add a digital signature to outgoing emails, verifying the message was sent from an authorized mail server.
    • DMARC (Domain-based Message Authentication, Reporting, and Conformance): Implement DMARC policies to provide instructions on handling emails that fail SPF or DKIM checks.
  3. Monitoring and Auditing:

    • Regularly review SMTP server logs for patterns indicative of abuse and set up alerts for suspicious activities.
    • Perform routine audits of SMTP configurations to ensure compliance with the organization's email security policies.

Confirmation & Additional Information:

To confirm the mitigation is effective, attempt to relay an email from an unauthorized external network or account to verify that the server rejects the attempt. Additionally, regularly consult security resources and updates to stay informed about new threats and mitigation strategies.

Reference Information:

  • CVEs: CVE-1999-0512, CVE-2002-1278, CVE-2003-0285
  • Vulnerability Publication Date: 1/1/1990


Addressing the open mail relaying issue is critical to safeguarding your organization's email infrastructure against misuse. By implementing robust access controls, adopting email authentication standards, and maintaining vigilant monitoring practices, the risk associated with unauthorized mail relaying can be significantly mitigated, preserving the integrity and reliability of your email services.