Email Injection in Buffalo with Jwt Tokens

Email Injection in the Buffalo web framework occurs when user-controlled data is directly interpolated into email headers without validation or sanitization. When JWT tokens are involved—such as when a token payload is used to set email headers or trigger email actions—the risk increases because attackers can craft tokens containing newline characters or header-like sequences. In Buffalo, if a handler reads a claim like user_email from a JWT and passes it directly to an email library (e.g., Go’s net/smtp or a templating function that builds headers), newline characters (\n, \r) can inject additional headers such as Cc:, Bcc:, or even Subject:. This is a classic email injection pattern, analogous to SMTP header injection. Because JWTs are often trusted (parsed and verified), developers may skip input validation, assuming the token is trustworthy. However, if the token is obtained from an unauthenticated source or contains attacker-controlled claims, the injection becomes viable in the unauthenticated attack surface that middleBrick tests. middleBrick’s checks for Unsafe Consumption and Input Validation can flag scenarios where JWT-derived data flows into email routines without sanitization. The scanner also tests for SSRF where injected headers might redirect mail servers, and checks Data Exposure for tokens that may leak sensitive email content. Real-world impact includes spoofed sender addresses, phishing emails sent from your domain, or internal information disclosure via BCC. For example, an attacker could supply a token with email: "attacker@example.com\r\nCc: victim@example.com", causing the email system to send a copy to an unintended recipient. Because Buffalo does not provide built-in email header sanitization, developers must explicitly validate and encode any user-influenced data, including JWT claims, before using it in email construction.

To remediate email injection when using JWT tokens in Buffalo, ensure strict validation and encoding of all claims used in email contexts. Do not trust any claim that originates from a token unless it has been explicitly verified and constrained. Below are concrete code examples for Buffalo (Go) that demonstrate safe handling.

Example 1: Safe Email Header Construction

Validate and sanitize the email claim before using it in headers. Use a strict allowlist for email format and remove or reject newlines and control characters.

// handlers/email.go
package handlers

import (
    "net/mail"
    "net/smtp"
    "strings"

    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/packr/v2"
    "github.com/dgrijalva/jwt-go"
)

func SendVerificationEmail(c buffalo.Context) error {
    tokenString := c.Params().Get("token")
    claims := jwt.MapClaims{}
    token, err := jwt.ParseWithClaims(tokenString, claims, func(token *jwt.Token) (interface{}, error) {
        // your key logic here
        return []byte("secret"), nil
    })
    if err != nil || !token.Valid {
        return c.Render(401, r.JSON(map[string]string{"error": "invalid token"}))
    }

    rawEmail, ok := claims["email"]
    if !ok {
        return c.Render(400, r.JSON(map[string]string{"error": "email claim missing"}))
    }

    emailStr, ok := rawEmail.(string)
    if !ok {
        return c.Render(400, r.JSON(map[string]string{"error": "email claim invalid"}))
    }

    // Validate and sanitize email
    addr, err := mail.ParseAddress(emailStr)
    if err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "invalid email format"}))
    }

    // Ensure no newlines remain
    cleanEmail := strings.ReplaceAll(addr.Address, "\r", "")
    cleanEmail = strings.ReplaceAll(cleanEmail, "\n", "")

    // Construct headers safely
    headers := map[string]string{
        "From":    "noreply@yourdomain.com",
        "To":      cleanEmail,
        "Subject": "Verify your account",
    }

    // Compose message
    message := "Subject: " + headers["Subject"] + "\r\n" +
        "From: " + headers["From"] + "\r\n" +
        "To: " + headers["To"] + "\r\n" + "\r\n" +
        "Please verify your account."

    auth := smtp.PlainAuth("", "smtp_user", "smtp_pass", "smtp.yourdomain.com")
    err = smtp.SendMail("smtp.yourdomain.com:587", auth, headers["From"], []string{headers["To"]}, []byte(message))
    if err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "failed to send email"}))
    }

    return c.Render(200, r.JSON(map[string]string{"status": "email sent"}))
}

In this example, mail.ParseAddress validates the email format and helps prevent header injection. We additionally strip carriage returns and newlines to eliminate injection vectors. The JWT is parsed and verified before any claims are used, aligning with middleBrick’s checks for Authentication and Unsafe Consumption. For continuous coverage, integrate the middleBrick CLI (middlebrick scan <url>) or add the GitHub Action to fail builds if risk scores drop below your threshold.

Example 2: Using Claims in Templates Safely

If you render email templates, ensure dynamic values are escaped and not interpreted as header delimiters.

// actions/email_actions.go
package actions

import (
    "html/template"
    "net/mail"
    "strings"

    "github.com/gobuffalo/buffalo"
    "github.com/dgrijalva/jwt-go"
)

func EmailTemplateAction(c buffalo.Context) error {
    tokenString := c.Params().Get("token")
    claims := jwt.MapClaims{}
    token, _, _ := new(jwt.Parser).ParseUnverified(tokenString, claims)

    rawEmail, ok := claims["email"]
    if !ok {
        return c.Render(400, r.JSON(map[string]string{"error": "email claim missing"}))
    }

    emailStr, ok := rawEmail.(string)
    if !ok {
        return c.Render(400, r.JSON(map[string]string{"error": "email claim invalid"}))
    }

    addr, err := mail.ParseAddress(emailStr)
    if err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "invalid email format"}))
    }

    // Escape for safe use in templates; treat as plain text, not headers
    safeEmail := template.HTMLEscapeString(strings.ReplaceAll(addr.Address, "\r", ""))
    safeEmail = template.HTMLEscapeString(strings.ReplaceAll(safeEmail, "\n", ""))

    tmpl := template.Must(template.New("email").Parse(`
Send confirmation to: {{.Email}}
`))
    data := struct {
        Email string
    }{Email: safeEmail}

    // Render to string or pass to mail body as needed
    // For actual email sending, use the header-safe method from Example 1
    return c.Render(200, r.HTML(200, "email_template", data))
}

These examples emphasize strict type assertions, format validation, and removal of control characters. They align with middleBrick’s findings for Input Validation, Unsafe Consumption, and Data Exposure. For ongoing protection, use the Web Dashboard to track scores over time and enable alerts via the Pro plan’s continuous monitoring or Slack/Teams notifications.