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Mitigation Strategies Analysis for postalserver/postal

Mitigation Strategy: Strict SPF, DKIM, and DMARC Configuration (within Postal)

  • Description:

    1. DKIM (Postal-Specific): Within Postal's configuration (postal.yml or the web interface), ensure DKIM signing is enabled for all outgoing emails. Generate and manage DKIM keys within Postal. Use strong key lengths (2048-bit or higher). Regularly rotate DKIM keys through Postal's interface or configuration management. Ensure the private keys are stored securely by Postal and are never exposed.
    2. SPF & DMARC (Coordination): While the records themselves are DNS entries, Postal's configuration should inform what goes into those records. Postal's IP address(es) must be accurately reflected in the SPF record. Postal's DKIM selector must be used in the DKIM DNS record.
    3. Postal's Sending Domains: Use Postal's interface to manage the list of allowed sending domains. This list should be kept up-to-date and consistent with your DNS records.

  • Threats Mitigated:

    • Email Spoofing (Outbound): Severity: High.
    • Phishing (Outbound): Severity: High.
    • Reputation Damage: Severity: Medium.

  • Impact:

    • Email Spoofing: Risk reduction: High.
    • Phishing: Risk reduction: High.
    • Reputation Damage: Risk reduction: Medium-High.

  • Currently Implemented:

    • DKIM: Fully implemented (2048-bit keys, rotated annually) within Postal.
    • SPF/DMARC Coordination: Partially implemented.

  • Missing Implementation:

    • Ensure Postal's configuration is the "source of truth" for SPF and DMARC settings, and that changes are reflected in DNS.

Mitigation Strategy: Sender Domain Verification (within Postal)

  • Description:

    1. Postal's Verification Mechanism: Utilize Postal's built-in domain verification features. This likely involves configuring Postal to require either email verification (sending a confirmation email to an address at the domain) or DNS verification (checking for a specific TXT record).
    2. Enforcement: Configure Postal to strictly enforce domain verification. Ensure there are no settings or workarounds that allow users to send from unverified domains. This should be a global setting within Postal, not a per-user option.
    3. Postal's Domain List: Regularly review the list of verified domains within Postal's interface to ensure they are still valid and authorized. Remove any domains that are no longer in use or are no longer authorized.

  • Threats Mitigated:

    • Email Spoofing (Outbound): Severity: High.
    • Phishing (Outbound): Severity: High.
    • Abuse by Malicious Users: Severity: Medium.

  • Impact:

    • Email Spoofing: Risk reduction: High.
    • Phishing: Risk reduction: High.
    • Abuse by Malicious Users: Risk reduction: Medium.

  • Currently Implemented:

    • Partially implemented. Email verification is used, but it's not strictly enforced for all users within Postal.

  • Missing Implementation:

    • Enforce domain verification for all users and sender domains within Postal's configuration, without exception.
    • Regularly audit the list of verified domains within Postal.

Mitigation Strategy: Outbound Rate Limiting (within Postal)

  • Description:

    1. Postal's Rate Limiting Settings: Use Postal's built-in rate limiting features (configured in postal.yml or through the web interface).
    2. Configure Limits: Set specific limits for:
      • Per User/Account: Limit emails per user per hour/day.
      • Per Sender Domain: Limit emails per domain per hour/day.
      • Per Sending IP (If Supported): If Postal supports it, limit per sending IP.
    3. Postal's Logging: Configure Postal to log rate limiting events.
    4. Alerting (within Postal): If Postal has built-in alerting for rate limit violations, configure it.

  • Threats Mitigated:

    • Spam Outbreaks: Severity: High.
    • Phishing Campaigns: Severity: High.
    • Reputation Damage: Severity: Medium.
    • Denial of Service (DoS): Severity: Low.

  • Impact:

    • Spam Outbreaks: Risk reduction: High.
    • Phishing Campaigns: Risk reduction: High.
    • Reputation Damage: Risk reduction: Medium.
    • Denial of Service (DoS): Risk reduction: Low-Medium.

  • Currently Implemented:

    • Basic rate limiting is implemented per user within Postal, but not per sender domain or IP address.

  • Missing Implementation:

    • Implement rate limiting per sender domain and, if supported by Postal, per IP address within Postal's configuration.
    • Refine per-user rate limits.
    • Configure Postal's built-in alerting (if available) for rate limit violations.

Mitigation Strategy: Disable Open Relay (within Postal)

  • Description:

    1. Postal Configuration Review: Thoroughly examine Postal's configuration files (postal.yml and any related files) to ensure there are absolutely no settings that enable open relay functionality. Specifically, look for and disable any options related to unauthenticated relaying, anonymous access, or relaying for external domains.
    2. Authentication Enforcement: Within Postal's configuration, ensure that authentication (username/password or API key) is mandatory for all SMTP connections originating from outside your trusted network (which should be defined within Postal, if possible).
    3. Testing (External, but informed by Postal): While the testing is external, the configuration to prevent open relay is entirely within Postal.

  • Threats Mitigated:

    • Spam Relay Abuse: Severity: Critical.
    • Reputation Damage: Severity: High.
    • Resource Exhaustion: Severity: Medium.

  • Impact:

    • Spam Relay Abuse: Risk reduction: Critical.
    • Reputation Damage: Risk reduction: High.
    • Resource Exhaustion: Risk reduction: Medium.

  • Currently Implemented:

    • Believed to be fully implemented within Postal's configuration, but needs regular verification.

  • Missing Implementation: * Regularly audit Postal's configuration to ensure open relay remains disabled.

Mitigation Strategy: API Key Management (within Postal)

  • Description:

    1. Postal's API Key Interface: Use Postal's built-in interface for managing API keys.
    2. Least Privilege (within Postal): When creating API keys within Postal, grant only the necessary permissions. Create separate keys for different applications, each with the minimum required access.
    3. Rotation (within Postal): Use Postal's interface to regularly rotate API keys. Revoke old keys after new keys are deployed.
    4. Monitoring (within Postal): If Postal provides API key usage logs or statistics, monitor them for suspicious activity.

  • Threats Mitigated:

    • Compromised API Key: Severity: High.
    • Unauthorized Access: Severity: High.
    • Privilege Escalation: Severity: Medium.

  • Impact:

    • Compromised API Key: Risk reduction: High.
    • Unauthorized Access: Risk reduction: High.
    • Privilege Escalation: Risk reduction: Medium-High.

  • Currently Implemented:

    • API keys are used, but not all applications have separate keys within Postal. Rotation is not consistently performed through Postal's interface.

  • Missing Implementation:

    • Create separate API keys for all applications within Postal, with least privilege.
    • Implement regular API key rotation using Postal's features.
    • Utilize Postal's API key monitoring capabilities (if available).

Mitigation Strategy: Regular Updates (of Postal)

  • Description:

    1. Update Mechanism: Use Postal's recommended update mechanism (e.g., postal upgrade, Docker image updates, package manager).
    2. Schedule: Establish a regular schedule for updating the Postal software itself.
    3. Testing: Before applying updates to the production Postal instance, test them in a staging environment that mirrors the production setup.

  • Threats Mitigated:

    • Exploitation of Known Vulnerabilities: Severity: High.
    • Zero-Day Exploits (Indirectly): Severity: Medium.

  • Impact:

    • Exploitation of Known Vulnerabilities: Risk reduction: High.
    • Zero-Day Exploits (Indirectly): Risk reduction: Low-Medium.

  • Currently Implemented:

    • Updates are performed, but not on a regular schedule.

  • Missing Implementation:

    • Establish a regular update schedule for the Postal software.
    • Improve the patching process, including testing in a staging environment before updating the production Postal instance.