Email Injection in Buffalo

Email injection in Buffalo applications typically occurs when user input is concatenated directly into email headers without proper sanitization. This vulnerability allows attackers to inject additional email headers, modify recipient lists, or even inject malicious content into email bodies.

The most common scenario involves Buffalo's mail package where user-controlled data flows through multiple paths. Consider a contact form where the From header is constructed from user input:

func ContactHandler(c buffalo.Context) error {
    email := c.Param("email")
    subject := c.Param("subject")
    message := c.Param("message")

    m := mail.NewMessage()
    m.SetHeader("From", email)
    m.SetHeader("To", "[email protected]")
    m.SetHeader("Subject", subject)
    m.SetBody("text/plain", message)

    if err := mail.Send("smtp.example.com:587", smtp.PlainAuth("", smtpUser, smtpPass, smtpHost), m); err != nil {
        return err
    }
    
    return c.Render(200, r.String("Message sent"))
}

An attacker could submit an email like:

[email protected]%0ACc: [email protected]%0ABcc: [email protected]%0AContent-Type: text/html%0A%0A<script>alert('XSS')</script>

This would create additional headers and potentially send the message to unintended recipients. The newline characters (%0A) break out of the intended header field and inject new ones.

Buffalo's middleware chain can also introduce injection points. If you're using custom middleware that processes email-related parameters, improper validation can create similar vulnerabilities:

func EmailSanitizationMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        // Vulnerable: no validation of email header injection
        email := c.Param("email")
        c.Set("email", email)
        return next(c)
    }
}

Another Buffalo-specific pattern involves using the pop package for database operations where email content is stored and later retrieved for sending. If the data isn't properly sanitized when retrieved, injection can occur:

type Contact struct {
    ID      uuid.UUID `json:"id" db:"id"`
    Email   string    `json:"email" db:"email"`
    Subject string    `json:"subject" db:"subject"`
    Message string    `json:"message" db:"message"`
}

func (c Contact) SendEmail() error {
    m := mail.NewMessage()
    m.SetHeader("From", c.Email) // Vulnerable if c.Email contains newlines
    m.SetHeader("To", "[email protected]")
    m.SetHeader("Subject", c.Subject)
    m.SetBody("text/plain", c.Message)
    
    return mail.Send("smtp.example.com:587", smtp.PlainAuth("", smtpUser, smtpPass, smtpHost), m)
}

Detecting email injection in Buffalo applications requires both static analysis and runtime scanning. For static analysis, examine all code paths where user input reaches email headers:

# Search for mail.NewMessage() usage and header setting
find . -name "*.go" -exec grep -l "mail\.NewMessage" {} \;
# Look for SetHeader calls with user input
grep -r "SetHeader" --include="*.go" | grep -E "(c\.Param|request|input)"

middleBrick's black-box scanning approach is particularly effective for Buffalo applications because it tests the actual running API without requiring source code access. The scanner automatically detects email-related endpoints by analyzing request patterns and response structures.

When middleBrick scans a Buffalo API, it specifically tests for:

• Newline injection in email headers (%0A, %0D, \n, \r)
• Header field injection attempting to add CC, BCC, or custom headers
• Content-Type manipulation to inject HTML or script content
• Multiple recipient injection in single-header fields

The scanner sends payloads like:

email=test%0ACc: [email protected]%0A%0ABody here

subject=Test%0AContent-Type:text/html%0A%0A<img src=x onerror=alert(1)>

For Buffalo applications using the github.com/gobuffalo/buffalo/binding package, middleBrick also checks if struct binding might inadvertently allow injection through struct tags or default values.

To verify detection manually in a Buffalo app, you can add logging middleware that inspects email header construction:

func EmailHeaderLoggingMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        email := c.Param("email")
        subject := c.Param("subject")
        
        if strings.Contains(email, "\n") || strings.Contains(email, "\r") {
            log.Printf("Potential email injection detected: %s", email)
        }
        
        return next(c)
    }
}

middleBrick's dashboard provides a security score (0-100) and breaks down findings by category, making it easy to identify email injection vulnerabilities among other API security issues. The scanner's 12 parallel security checks ensure comprehensive coverage of the unauthenticated attack surface that email injection exploits.

Buffalo provides several native approaches to prevent email injection. The most effective is input sanitization using the mail package's built-in validation combined with custom sanitization:

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/buffalo/binding"
    "github.com/gobuffalo/pop/v5"
    "github.com/gobuffalo/validate/v3"
    "github.com/gobuffalo/validate/v3/validators"
    "net/mail"
)

type Contact struct {
    Email   string `json:"email" db:"email" validate:"email"`
    Subject string `json:"subject" db:"subject" validate:"required"`
    Message string `json:"message" db:"message" validate:"required"`
}

func (c *Contact) Validate(tx *pop.Connection) (*validate.Errors, error) {
    return validate.Validate(
        &validators.EmailIsPresent{Field: c.Email, Name: "email"},
        &validators.StringLengthInRange{Field: c.Subject, Name: "subject", Min: 1, Max: 200},
        &validators.StringLengthInRange{Field: c.Message, Name: "message", Min: 1, Max: 10000},
    ), nil
}

func sanitizeEmailInput(input string) string {
    // Remove CRLF characters that could be used for injection
    return strings.ReplaceAll(strings.ReplaceAll(input, "\r", ""), "\n", "")
}

func ContactHandler(c buffalo.Context) error {
    contact := &Contact{}
    if err := c.Bind(contact); err != nil {
        return c.Error(400, err)
    }

    // Sanitize all email-related inputs
    contact.Email = sanitizeEmailInput(contact.Email)
    contact.Subject = sanitizeEmailInput(contact.Subject)

    // Validate the struct
    if verrs, err := contact.Validate(nil); err != nil || verrs.HasAny() {
        return c.Error(400, errors.New(verrs.Error()))
    }

    // Use mail package's address parsing for additional validation
    if _, err := mail.ParseAddress(contact.Email); err != nil {
        return c.Error(400, errors.New("invalid email address"))
    }

    m := mail.NewMessage()
    m.SetHeader("From", contact.Email)
    m.SetHeader("To", "[email protected]")
    m.SetHeader("Subject", contact.Subject)
    m.SetBody("text/plain", contact.Message)

    if err := mail.Send("smtp.example.com:587", smtp.PlainAuth("", smtpUser, smtpPass, smtpHost), m); err != nil {
        return err
    }

    return c.Render(200, r.String("Message sent"))
}

For Buffalo applications using the actions/app.go middleware chain, you can add a global email injection prevention middleware:

func EmailInjectionPreventionMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        // Check all parameters for injection patterns
        for _, key := range c.Params().Keys() {
            values := c.Params().GetALL(key)
            for _, value := range values {
                if containsInjectionPattern(value) {
                    return c.Error(400, errors.New("potential email injection detected"))
                }
            }
        }
        return next(c)
    }
}

func containsInjectionPattern(input string) bool {
    injectionPatterns := []string{"%0A", "%0D", "\n", "\r", ":", ";"}
    for _, pattern := range injectionPatterns {
        if strings.Contains(input, pattern) {
            return true
        }
    }
    return false
}

Buffalo's validation framework can be extended with custom validators specifically for email injection prevention:

type NoEmailInjection struct {
    Field  string
    Name   string
    Message string
}

func (v *NoEmailInjection) IsValid(_ *validate.Errors) error {
    if strings.ContainsAny(v.Field, "\r\n") {
        return fmt.Errorf("email injection pattern detected in %s", v.Name)
    }
    return nil
}

// Usage in struct

func (c *Contact) Validate(tx *pop.Connection) (*validate.Errors, error) {
    return validate.Validate(
        &validators.EmailIsPresent{Field: c.Email, Name: "email"},
        &NoEmailInjection{Field: c.Email, Name: "email", Message: "No email injection"},
    ), nil
}

For applications using Buffalo's POP for database operations, ensure email content is sanitized before database insertion and after retrieval:

func (c *Contact) BeforeCreate(tx *pop.Connection) error {
    c.Email = sanitizeEmailInput(c.Email)
    c.Subject = sanitizeEmailInput(c.Subject)
    return nil
}

func (c *Contact) AfterFind(tx *pop.Connection) error {
    c.Email = sanitizeEmailInput(c.Email)
    c.Subject = sanitizeEmailInput(c.Subject)
    return nil
}