attack-surface.md

Attack Surface Analysis for mikel/mail

Attack Surface: Header Injection (Email Injection)

• Description: Attackers manipulate email headers by injecting malicious data into user-supplied fields that are used to construct email headers. This remains the most significant and direct attack vector.
  • How mail Contributes: The mail gem provides the core functionality for setting email headers. Improper use of these methods, without sufficient input validation and sanitization, directly enables header injection.
  • Example:
    • An attacker enters [email protected]\r\nBcc: [email protected] into a "Contact Us" form's email field. If the application uses this input directly in mail.to =, the attacker receives a BCC.
    • [email protected]\r\nFrom: [email protected] to spoof the sender.
  • Impact:
    • Email spoofing (impersonation).
    • Unauthorized disclosure of email content (BCC injection).
    • Reputation damage.
    • Phishing attacks.
    • Potential legal and compliance issues.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Strict Input Validation: Validate all user-supplied data used in any email header. Reject input containing CR (\r), LF (\n), or other unexpected characters. Use allow-lists.
    • Use mail Gem's Methods: Always use mail.to =, mail.subject =, mail.bcc =, etc. Never manually construct header strings.
    • Explicit Encoding: Consider using Mail::Encodings.q_value_encode even with the gem's methods.
    • Avoid Direct User Input: Use pre-defined, application-controlled values in sensitive headers like From: whenever possible.
    • Sanitize User Input: Sanitize all user input before using it in headers.

Attack Surface: Body Content Injection (XSS & Phishing)

• Description: Attackers inject malicious content (JavaScript, phishing links) into the email body, primarily targeting HTML emails.
  • How mail Contributes: The mail gem is directly responsible for constructing the email body (both plain text and HTML). Lack of sanitization when using user input in the body creation process directly enables this attack.
  • Example:
    • <script>alert('XSS');</script> in a comment field included in an HTML email.
    • A disguised phishing link in the email body.
  • Impact:
    • Cross-Site Scripting (XSS) in webmail clients.
    • Successful phishing attacks.
    • Malware distribution (via links).
    • Reputation damage.
  • Risk Severity: High
  • Mitigation Strategies:
    • HTML Sanitization: Always use a robust HTML sanitizer (e.g., sanitize gem) for HTML emails. Never trust user-supplied HTML.
    • Prefer Plain Text: Send plain text emails whenever possible to eliminate XSS.
    • Content Security Policy (CSP): Consider CSP within the HTML body (advanced, limited support).
    • Encode User Input: Encode user input before including it, even in plain text.
    • URL Validation and Rewriting: Validate and potentially rewrite user-supplied URLs.

Attack Surface: Attachment-Based Attacks

• Description: Attackers use attachments to deliver malware or exploit vulnerabilities.
  • How mail Contributes: The mail gem provides the direct functionality for adding attachments to emails.
  • Example:
    • A malicious .exe disguised as a .pdf.
    • An oversized file to cause a DoS.
  • Impact:
    • Malware infection.
    • Denial-of-service (DoS).
    • Data breaches.
  • Risk Severity: High
  • Mitigation Strategies:
    • Strict File Type Validation: Validate using file extension and MIME type detection.
    • File Size Limits: Enforce strict limits.
    • Malware Scanning: Scan all attachments before sending.
    • Secure Storage: Store attachments securely.
    • Avoid Executables: Prohibit executable attachments.
    • Content Disarm and Reconstruction: If possible, use CDR to process attachments.

Attack Surface: SMTP Relay and Sendmail Exploitation (Focus on sendmail Command Injection)

• Description: This entry is narrowed to focus on the direct and most critical risk associated with mail's interaction with delivery methods: sendmail command injection. While SMTP relay abuse is a concern, it's less directly tied to the mail gem's code itself.
  • How mail Contributes: If the application is configured to use the sendmail delivery method and user-supplied data is passed to the sendmail command without proper sanitization, the mail gem's interaction with sendmail becomes the direct attack vector.
  • Example:
    • User input containing shell metacharacters (e.g., ;, |, `) is passed to the sendmail command, allowing the attacker to execute arbitrary commands on the server.
  • Impact:
    • Complete server compromise (Remote Code Execution).
    • Data breaches.
    • System destruction.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Avoid sendmail: Strongly prefer SMTP over sendmail.
    • sendmail Sanitization (if unavoidable): If sendmail is absolutely required, ensure no user-supplied data reaches the command without extreme sanitization and escaping. Use a dedicated library for secure sendmail interaction if possible. This is a very high-risk configuration and should be avoided if at all possible. Consider this a last resort.
    • Use API based delivery method: If possible, use API based delivery method (like SendGrid, Mailgun, etc.).