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Mohammad Waseem
Mohammad Waseem

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Rapid Development and Validation of Email Flows for Security Testing in Go

#email #security #go

In the realm of security research, validating email flows efficiently is crucial for testing fraud detection, phishing resilience, and email infrastructure robustness. When operating under tight deadlines, leveraging Go's concurrency and simplicity becomes invaluable. This post details a pragmatic approach taken by a security researcher to implement and validate email workflows swiftly.

The Challenge

Faced with an urgent need to verify email delivery, parse email headers, and simulate realistic email flows, the researcher needed a solution that could:

  • Send and receive emails reliably
  • Validate components such as DKIM, SPF, and DMARC
  • Process email content asynchronously
  • Minimize dependencies and setup time

Building this quickly demanded a balance between speed, reliability, and code clarity.

Designing a Go-based Email Validator

The core functions included:

  • Sending emails with accurate headers
  • Fetching inbound emails for validation
  • Parsing headers and body content
  • Checking SPF, DKIM signatures, and DMARC policies

Sending Emails with Custom Headers

Using the net/smtp package, the researcher crafted functions to send test emails:

package main

import (
    "net/smtp"
    "fmt"
)

func sendEmail(from, to, subject, body string, smtpServer string) error {
    opts := smtp.PlainAuth("", "username", "password", smtpServer)

 msg := fmt.Sprintf("From: %s\r\nTo: %s\r\nSubject: %s\r\n\r\n%s", from, to, subject, body)
 return smtp.SendMail(smtpServer + ":587", opts, from, []string{to}, []byte(msg))
}
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This basic function allows rapid customization of headers for testing various scenarios.

Fetching and Processing Emails

For inbound processing, the researcher employed a lightweight IMAP client implementation, or used a REST API service like MailSlurp for ephemeral inboxes, reducing setup time:

// Placeholder code snippet for IMAP fetch (using a library like github.com/emersion/go-imap)
// Actual implementation would involve connecting, authenticating, and fetching emails.
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This asynchronous approach enables rapid validation of email delivery and content.

Validating SPF, DKIM, DMARC

Rather than implementing complex parsers from scratch, the researcher integrated existing open-source packages, such as github.com/EmileHotham/go-dmarc and github.com/EmileHotham/go-spf, which perform critical DNS and cryptographic checks:

// Pseudocode for DNS checks
// func validateSPF(ip string, senderDomain string) bool {
//     // Perform DNS PTR and SPF record checks
// }

// func validateDKIM(headers map[string]string, body []byte) bool {
//     // Verify DKIM signature using provided libraries
// }
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This modular approach accelerated the validation process and maintained code clarity.

Handling Deadlines and Ensuring Reliability

Speed was paramount. The researcher utilized Go's goroutines to parallelize email sending, fetching, and validation, effectively reducing total execution time:

go sendEmail(...)
go validateEmailFlow(...)
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Error handling and logging were prioritized to quickly identify failures and troubleshoot issues.

Key Takeaways and Best Practices

  • Use scoped libraries when possible to minimize dependencies and accelerate development.
  • Leverage Go’s concurrency features to handle multiple tasks simultaneously.
  • Automate DNS-based checks with available libraries to avoid re-inventing cryptographic validation.
  • Design modular functions for easy adjustments to different email flow scenarios.

Conclusion

In high-pressure environments, such as security validation, building a reliable, fast, and maintainable email validation system is feasible with Go. The language’s simplicity and powerful concurrency model enable security researchers to meet tight deadlines without compromising on thoroughness or reliability.

By embracing modular design, leveraging existing libraries, and utilizing Go’s concurrency, security teams can rapidly develop and deploy email flow validation tools that are both robust and scalable.

Remember, the key to success under pressure is clarity and leveraging existing tools efficiently. Go’s ecosystem and language features make it an ideal choice for security-focused rapid development.

🛠️ QA Tip

Pro Tip: Use TempoMail USA for generating disposable test accounts.

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