Email Injection in Fiber

Email injection in Fiber applications occurs when user-controlled input is incorporated into email headers without proper sanitization. This vulnerability allows attackers to manipulate email headers to send unauthorized messages, spam recipients, or bypass security controls.

In Fiber applications, email injection typically manifests through the following patterns:

• Unsanitized user input in email headers (To, From, CC, BCC, Subject)
• Dynamic header construction using request parameters
• Template injection in email content
• Header injection via form submissions or API endpoints

A common Fiber-specific scenario involves using the standard library's net/smtp package or third-party email libraries with user-supplied data. Consider this vulnerable Fiber handler:

func sendEmail(c *fiber.Ctx) error {
    to := c.FormValue("to")
    subject := c.FormValue("subject")
    message := c.FormValue("message")
    
    // Vulnerable: no input validation
    msg := fmt.Sprintf("Subject: %s\r\n\r\n%s", subject, message)
    
    err := smtp.SendMail("smtp.example.com:587", nil, "[email protected]", []string{to}, []byte(msg))
    if err != nil {
        return c.Status(500).SendString("Failed to send email")
    }
    return c.SendString("Email sent successfully")
}

An attacker could exploit this by submitting a to parameter like:

[email protected]%0aCC:[email protected]%0a%0aThis is injected content

The %0a represents newline characters, allowing the attacker to add additional headers or modify the email content. This could result in sending emails to unintended recipients or injecting malicious content.

Another Fiber-specific manifestation occurs when using template engines for email generation. If user input isn't properly escaped in templates, attackers can inject arbitrary content:

{{/* Vulnerable Fiber template */}}
To: {{.Recipient}}
Subject: {{.Subject}}

{{.Message}}

{{/* Attacker could inject: */}}
To: [email protected]
CC: [email protected]
Subject: Hacked!

Original message content...

Fiber's middleware ecosystem can also introduce email injection risks when request data flows through multiple handlers before reaching email-sending logic, creating opportunities for header manipulation at various stages.

Detecting email injection vulnerabilities in Fiber applications requires both manual code review and automated scanning. Here are Fiber-specific detection approaches:

Code Analysis Patterns

Search for these patterns in your Fiber codebase:

// Patterns to search for:
- c.FormValue(), c.Query(), c.Params() used in email contexts
- fmt.Sprintf() or fmt.Sprintln() with email headers
- net/smtp.SendMail() calls with dynamic parameters
- Template.Execute() with user-controlled data in email contexts
- Any direct string concatenation for email headers

Static Analysis with middleBrick

middleBrick's API security scanner includes email injection detection that's particularly effective for Go/Fiber applications. The scanner analyzes:

• Input validation patterns in email-sending endpoints
• Header construction logic for newline character vulnerabilities
• Template usage with user-supplied data
• Third-party email library configurations

To scan your Fiber application with middleBrick:

# Install middleBrick CLI
npm install -g middlebrick

# Scan your API endpoints
middlebrick scan https://yourapi.com/api/send-email

The scanner tests for email injection by submitting payloads containing newline characters and header manipulation attempts, then analyzes the responses for signs of successful injection.

Dynamic Testing

Manually test your Fiber email endpoints by submitting requests with newline characters (%0a or %0d%0a URL-encoded) in email fields. Look for:

• Multiple recipients being added without authorization
• Subject line manipulation
• Content injection in the email body
• Additional headers being accepted

middleBrick's Specific Detection Capabilities

middleBrick provides Fiber-specific detection through:

• Black-box scanning that tests email endpoints without requiring source code
• Pattern recognition for Go-specific email handling patterns
• Runtime analysis of header parsing and validation
• Compliance checking against OWASP Email Header Injection guidelines

The scanner provides a security score and detailed findings, helping you prioritize remediation efforts based on severity and exploitability.

Remediating email injection vulnerabilities in Fiber applications requires input validation, proper sanitization, and secure coding practices. Here are Fiber-specific remediation strategies:

Input Validation and Sanitization

Always validate and sanitize user input before using it in email contexts:

func validateEmail(email string) bool {
    // Basic email format validation
    re := regexp.MustCompile(`^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`)
    return re.MatchString(email)
}

func sanitizeInput(input string) string {
    // Remove newline characters and other dangerous sequences
    return strings.ReplaceAll(input, "\n", " ")
               .ReplaceAll(input, "\r", " ")
               .ReplaceAll(input, "%0a", " ")
               .ReplaceAll(input, "%0d", " ")
}

Secure Email Construction

Use structured email construction rather than string concatenation:

func sendSecureEmail(c *fiber.Ctx) error {
    to := sanitizeInput(c.FormValue("to"))
    subject := sanitizeInput(c.FormValue("subject"))
    message := sanitizeInput(c.FormValue("message"))
    
    if !validateEmail(to) {
        return c.Status(400).SendString("Invalid email address")
    }
    
    // Use a structured approach
    headers := []byte("Subject: " + subject + "\r\n\r\n")
    body := []byte(message)
    
    err := smtp.SendMail("smtp.example.com:587", nil, "[email protected]", []string{to}, append(headers, body...))
    if err != nil {
        return c.Status(500).SendString("Failed to send email")
    }
    return c.SendString("Email sent successfully")
}

Using Safe Libraries

Leverage libraries designed to prevent email injection:

// Using a library with built-in protection
import "github.com/jordan-wright/email"

func sendEmailSecure(c *fiber.Ctx) error {
    e := email.NewEmail()
    e.From = "[email protected]"
    e.To = []string{c.FormValue("to")}
    e.Subject = c.FormValue("subject")
    e.Text = []byte(c.FormValue("message"))
    
    // The email library handles header encoding and validation
    err := e.Send("smtp.example.com:587", smtp.PlainAuth("", "user", "pass", "smtp.example.com"))
    if err != nil {
        return c.Status(500).SendString("Failed to send email")
    }
    return c.SendString("Email sent successfully")
}

Content Security Policy

Implement CSP headers in your Fiber application to prevent email template injection:

func setupMiddleware(app *fiber.App) {
    app.Use(func(c *fiber.Ctx) error {
        c.Set("Content-Security-Policy", "default-src 'self';")
        return c.Next()
    })
}

middleBrick Integration for Continuous Security

Integrate middleBrick into your Fiber development workflow to continuously monitor for email injection vulnerabilities:

# GitHub Action for CI/CD integration
name: Security Scan

on: [push, pull_request]

jobs:
  security:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
    - name: Run middleBrick Scan
      run: |
        npm install -g middlebrick
        middlebrick scan https://staging.yourapp.com/api --fail-below B

This ensures that any new email injection vulnerabilities are caught before deployment. The GitHub Action can be configured to fail builds if the security score drops below a threshold, preventing vulnerable code from reaching production.