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Threat Model Analysis for mikel/mail

Threat: SMTP Credential Leakage

  • Description: An attacker gains access to the application's SMTP credentials. This could happen through various means:

    • Hardcoded Credentials: The credentials are found directly in the application's source code (e.g., committed to a public repository).
    • Insecure Configuration: Credentials are stored in plain text in configuration files that are accessible to unauthorized users or processes.
    • Environment Variable Exposure: Credentials stored in environment variables are exposed due to misconfiguration or a compromised server.
    • Logging: Credentials are inadvertently logged to application logs or monitoring systems.
    • Debugging: Credentials are exposed during debugging sessions.

  • Impact: The attacker can send emails using the application's legitimate SMTP server. This allows for:

    • Spam/Phishing Campaigns: Sending unsolicited emails, potentially damaging the application's reputation and leading to blacklisting.
    • Malware Distribution: Sending emails containing malicious attachments or links.
    • Impersonation: Sending emails that appear to originate from the application or its users.
    • Data Exfiltration: Potentially sending sensitive data via email.

  • Affected Component:

    • Mail::SMTP (and related configuration handling within the application). Specifically, any code that handles the settings hash passed to Mail::SMTP.new or uses Mail.defaults.

  • Risk Severity: Critical

  • Mitigation Strategies:

    • Secure Configuration Management: Use a dedicated secrets management solution (e.g., HashiCorp Vault, AWS Secrets Manager, Azure Key Vault).
    • Environment Variables (Securely): If using environment variables, ensure they are set securely and only accessible to the application process.
    • No Hardcoding: Absolutely never hardcode credentials in the source code.
    • Least Privilege: The SMTP user should have minimal permissions (only send, not manage the server).
    • Credential Rotation: Regularly rotate SMTP credentials.
    • Logging Discipline: Never log credentials. Use redaction if necessary.
    • OAuth 2.0: If the mail provider supports it, use OAuth 2.0 for authentication, avoiding the need to store passwords.

Threat: Email Header Injection

  • Description: An attacker injects malicious data into email headers by exploiting insufficient validation of user-supplied input used to construct headers (e.g., To, From, Subject, CC, BCC, Reply-To). The attacker might:

    • Provide specially crafted input containing newline characters (\r, \n) and header field names.
    • Exploit vulnerabilities in how the application concatenates strings to form headers.

  • Impact:

    • Email Spoofing: Forge the From address, making emails appear to come from a trusted source.
    • BCC Hijacking: Add their own address to the BCC field, secretly receiving copies of all emails.
    • Reply-To Redirection: Redirect replies to a malicious address.
    • Spam Filter Bypass: Inject headers to evade spam filters.
    • Information Disclosure: Potentially reveal internal server information through injected headers.

  • Affected Component:

    • Any mail library methods that set headers: mail.to=, mail.from=, mail.subject=, mail.cc=, mail.bcc=, mail.reply_to=, mail.headers=. Also, any custom code that manually constructs header strings.

  • Risk Severity: High

  • Mitigation Strategies:

    • Strict Input Validation: Validate and sanitize all user-supplied input used in headers. Use a whitelist approach.
    • Library Methods:** Use the mail library's built-in methods for setting headers (e.g., mail.to = '[email protected]'). Do not construct headers manually by string concatenation.
    • Encoding: Ensure proper encoding of header values (e.g., quoted-printable, base64) where necessary, especially for non-ASCII characters. The library should handle this, but verify.
    • Length Limits: Enforce reasonable length limits on header values.

Threat: Email Body Injection (HTML/Plain Text)

  • Description: An attacker injects malicious content into the email body. This is similar to XSS in web applications. The attacker might:

    • Provide input containing HTML tags and JavaScript if the email is sent as HTML.
    • Inject content that could be misinterpreted by the recipient or downstream systems, even in plain text emails.

  • Impact:

    • HTML Injection (XSS): If the email is HTML, the attacker can inject JavaScript, leading to XSS attacks against recipients using vulnerable email clients.
    • Plain Text Injection: Inject content that could be misinterpreted (e.g., commands, URLs).
    • Phishing: Craft convincing phishing emails to steal credentials or other sensitive information.
    • Reputation Damage: Send offensive or inappropriate content.

  • Affected Component:

    • mail.body=, mail.html_part=, mail.text_part=. Any code that sets the email body content.

  • Risk Severity: High

  • Mitigation Strategies:

    • HTML Sanitization: If sending HTML emails, use a robust HTML sanitization library (e.g., OWASP Java HTML Sanitizer, Bleach in Python) to remove dangerous tags and attributes. Do not rely solely on the mail library.
    • Plain Text Preference: If HTML is not essential, send emails as plain text only.
    • Input Validation/Sanitization: Sanitize and encode all user-supplied input included in the body, even for plain text emails.
    • Content Security Policy (CSP): Consider using a CSP header in HTML emails (though email client support is limited).

Threat: Malicious Attachments

  • Description: An attacker uploads a malicious file (e.g., malware, virus, exploit document) that is then included as an attachment in an outgoing email.

  • Impact: Recipients who open the attachment could be infected with malware.

  • Affected Component:

    • mail.add_file, mail.attachments. Any code that handles file uploads and attachments.

  • Risk Severity: High

  • Mitigation Strategies:

    • File Type Validation: Strictly validate file types using a whitelist (not a blacklist). Do not rely solely on file extensions. Use MIME type detection and potentially magic number analysis.
    • Antivirus Scanning:** Scan all attachments with an up-to-date antivirus/anti-malware solution before including them in emails.
    • File Size Limits:** Enforce reasonable limits on attachment sizes.
    • Secure Storage: Store attachments securely, preventing unauthorized access.
    • Sandboxing: Consider processing attachments in a sandboxed environment.
    • Renaming: Rename uploaded files to prevent path traversal attacks.

Threat: IMAP/POP3 Credential Leakage

  • Description: Identical to SMTP credential leakage, but for credentials used to receive emails via IMAP or POP3.

  • Impact: The attacker gains full access to the mailbox, allowing them to read, delete, and potentially modify emails.

  • Affected Component:

    • Mail::IMAP (and related configuration), Mail::POP3 (and related configuration). Any code that handles retrieving emails.

  • Risk Severity: Critical

  • Mitigation Strategies: Same as for SMTP credential leakage.

Threat: Email Parsing Vulnerabilities

  • Description: The mail library (or any underlying parsing libraries it uses) contains vulnerabilities that can be exploited by a specially crafted email. This could lead to buffer overflows, remote code execution, or denial of service.

  • Impact: The attacker could gain control of the application or cause it to crash.

  • Affected Component:

    • The entire mail library, particularly the parsing components (e.g., MIME parsing, header parsing, body parsing). This includes underlying libraries like treetop (if used).

  • Risk Severity: High

  • Mitigation Strategies:

    • Keep Updated:** Keep the mail library and all its dependencies up to date. Regularly check for security updates.
    • Fuzz Testing:** Fuzz test the email parsing functionality with a variety of malformed email inputs.
    • Memory Safety:** If possible, use a memory-safe language or memory safety features.
    • Sandboxing:** Consider running email parsing in a separate, sandboxed process.
    • Input Validation:** Implement robust input validation before passing data to the parsing library.

Threat: Command Injection via Email Processing

  • Description: The application processes emails in a way that involves executing system commands, and unsanitized email content is passed to these commands. An attacker crafts a malicious email containing command injection payloads.
    • Impact: The attacker can execute arbitrary commands on the server, potentially gaining full control.
    • Affected Component:
      • Any code that uses system calls (e.g., system(), exec(), backticks) with data derived from email content. This is not a direct part of the mail library itself, but a potential misuse of it.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Avoid System Calls: Avoid using system commands to process email content whenever possible.
      • Parameterized Commands: If system commands are unavoidable, use parameterized commands or APIs that prevent injection.
      • Least Privilege: Run the email processing component with the least possible privileges.
      • Input Sanitization: Rigorously sanitize any email data used in system commands, even if using parameterized commands.