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Threat Model Analysis for postalserver/postal

Threat: SMTP AUTH Bypass via Crafted Client

  • Description: An attacker crafts a malicious SMTP client that bypasses Postal's authentication mechanisms (e.g., exploiting vulnerabilities in the SMTP protocol handling or authentication logic within Postal's code). The attacker could send emails without valid credentials. This is a direct attack on Postal's SMTP implementation.
  • Impact: Unauthorized email sending, potential for spam and phishing campaigns originating from the server, reputational damage.
  • Postal Component Affected: smtp_server component, specifically the authentication handling logic within the postal/app/smtp_server.rb (and related files).
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Robust Input Validation: Thoroughly validate all input received from SMTP clients, including commands and parameters, to prevent unexpected behavior specific to Postal's parsing.
    • Protocol Compliance: Strictly adhere to the SMTP protocol specifications (RFC 5321 and related RFCs) and ensure that Postal's implementation is robust against common SMTP attacks, focusing on Postal's specific code paths.
    • Regular Security Audits of SMTP Code: Conduct regular code reviews and security audits of the smtp_server component, focusing on authentication and authorization logic within Postal.
    • Fuzz Testing: Use fuzz testing techniques to test the smtp_server component with a wide range of unexpected and malformed inputs, targeting Postal's implementation.

Threat: Mail Relay Abuse (Open Relay) due to Postal Misconfiguration

  • Description: An attacker discovers that Postal is misconfigured, specifically due to settings within Postal itself, and acting as an open relay, allowing them to send emails through the server without authentication. This focuses on Postal's configuration options and their potential for misuse.
  • Impact: The server becomes a source of spam, leading to IP address blacklisting, reputational damage, and potential legal issues.
  • Postal Component Affected: smtp_server component, specifically the configuration related to relaying and access control within Postal (postal/config/postal.yml and related settings, and how Postal interprets these settings).
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Disable Open Relaying by Default: Ensure that Postal is configured by default to not act as an open relay. This is a crucial Postal-specific setting.
    • Require Authentication for All Outbound Emails: Enforce authentication for all emails sent through Postal, except for those originating from trusted internal networks (if applicable and very carefully configured within Postal).
    • Clear Documentation: Provide clear and concise documentation on how to configure Postal securely and avoid creating an open relay using Postal's configuration options.
    • Regular Configuration Audits: Periodically review Postal's configuration file and its interpretation by Postal to ensure that open relaying is disabled.

Threat: Denial of Service via Message Queue Overload (Postal-Specific Handling)

  • Description: An attacker sends a large volume of emails or specially crafted messages designed to exploit weaknesses in Postal's specific handling of the message queue (e.g., RabbitMQ), leading to a denial of service. This focuses on how Postal interacts with the queue, not just the queue itself.
  • Impact: Postal becomes unavailable, preventing users from sending or receiving emails.
  • Postal Component Affected: Message queue integration (postal/app/workers and related files), how Postal interacts with RabbitMQ, and potentially the smtp_server if it's not handling large volumes of incoming messages gracefully according to Postal's logic.
  • Risk Severity: High
  • Mitigation Strategies:
    • Rate Limiting (per sender/IP, Postal-Enforced): Implement rate limiting within Postal's code at multiple levels (SMTP server, message queue interaction) to restrict the number of messages.
    • Message Size Limits (Postal-Enforced): Enforce limits on the size of individual email messages within Postal.
    • Robust Error Handling (Postal Workers): Implement robust error handling in Postal's message queue workers (postal/app/workers) to prevent them from crashing or becoming unresponsive due to malformed messages or queue overload as handled by Postal.
    • Postal's RabbitMQ Interaction Tuning: Optimize how Postal itself interacts with RabbitMQ for performance and resilience, including prefetch limits and how Postal handles queue connections.

Threat: DKIM Key Compromise (Postal Key Management)

  • Description: An attacker gains access to the private DKIM key used by Postal to sign outgoing emails, specifically targeting how Postal stores and manages this key.
  • Impact: The attacker can forge emails that appear to be legitimately signed by the domain, bypassing DKIM verification.
  • Postal Component Affected: DKIM signing functionality (postal/lib/postal/dkim_key.rb and related files), and specifically how Postal stores and accesses the key.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Secure Key Storage (Postal Implementation): Ensure Postal uses a secure method for storing the private DKIM key. Investigate and improve Postal's current key storage mechanism.
    • Key Rotation (Postal-Managed): Implement a mechanism within Postal to regularly rotate the DKIM key.
    • Access Control (Postal's Access to Key): Restrict access to the DKIM key as managed by Postal's code to only authorized users and processes.

Threat: Email Content Injection via Postal's Web Interface

  • Description: An attacker exploits a cross-site scripting (XSS) or other injection vulnerability in Postal's web interface to inject malicious content into emails composed through Postal's interface. This focuses on vulnerabilities within Postal's web interface code.
  • Impact:: Compromised email content, potential for phishing, reputational damage.
  • Postal Component Affected:: Web interface components, specifically the message composition form (postal/app/views/messages/new.html.erb and related JavaScript) and any other areas where user input is used to generate email content within Postal.
  • Risk Severity: High
  • Mitigation Strategies: * Strict Output Encoding (Postal's Templates): Encode all user-supplied data before displaying it in Postal's web interface or including it in email content, using context-specific encoding within Postal's templating system. * Input Sanitization (Postal's Input Handling): Sanitize all user input within Postal's code to remove or neutralize potentially harmful characters and code. * Framework Security Features (Rails, as used by Postal): Utilize the built-in security features of the Ruby on Rails framework (e.g., sanitize helper), ensuring they are correctly applied within Postal's context.