Email Injection in Buffalo with Bearer Tokens

Email Injection in the Buffalo web framework occurs when user-controlled data is improperly sanitized before being used in email-related operations such as headers or the message body. When combined with Bearer Tokens used for API authentication, the risk pattern shifts from simple mail header injection to potential authorization bypass or sensitive data leakage across security boundaries.

Buffalo does not inherently sanitize email headers; if a developer builds an invitation or notification endpoint that directly interpolates user input (e.g., From:, Reply-To:, or Subject) into an email without validation, an attacker can inject additional headers like Cc:, Bcc:, or even split headers with newline characters (%0a or %0d%0a). This classic Email Injection flaw becomes more dangerous when the request also carries a Bearer Token intended for backend API calls. An attacker who can control the email flow might leverage the injected headers to manipulate referral or callback URLs that include the Bearer Token in query parameters or fragments, inadvertently exposing the token in logs or to unintended parties.

Consider a scenario where a Buffalo app accepts a JSON payload with email and token fields, then sends a confirmation email using the provided email address and forwards the token to an external identity provider. If the email field is unsanitized, an attacker submits [email protected]%0aIn-Reply-To:%20, causing the email client or server to treat the injected header as part of the message routing. Because the request included a Bearer Token in the Authorization header, the developer might assume the token is safely confined to the API call; however, if the app embeds the same token in a URL within the email body (e.g., a verification link), the injected headers can alter routing, enabling token leakage through open redirects or referrer headers.

The cross-context nature of this issue is critical: the Bearer Token is meant for outbound service-to-service communication, while Email Injection affects outbound mail delivery. An insecure composition of the two mechanisms can expose the token in mail server logs, browser history, or network traces. Moreover, Buffalo applications that parse Authorization headers manually might inadvertently treat modified requests as authenticated if the injected email flow triggers secondary actions that reuse the token without revalidating the original context.

To detect this pattern using a scanner like middleBrick, which runs 12 security checks in parallel including Input Validation and Unsafe Consumption, you can submit your Buffalo endpoint without credentials. The scan will test whether user input in email fields can manipulate headers and whether Bearer Tokens are improperly reflected in responses or logs. middleBrick also checks for SSRF and Data Exposure, which are relevant when injected email headers lead to internal service calls or token exfiltration.

Remediation focuses on strict input validation, avoiding concatenation of user data into email headers, and ensuring Bearer Tokens remain scoped to their intended API context. Below are concrete, idiomatic examples for Buffalo in Go that demonstrate secure handling.

1. Validate and sanitize email inputs

Use a allowlist regex for email addresses and avoid inserting user input directly into headers. Instead, map known safe values to predefined header fields.

import (
    "net/mail"
    "net/smtp"
    "github.com/gobuffalo/buffalo"
)

func safeInvite(c buffalo.Context) error {
    type InvitePayload struct {
        Email string `json:"email" validate:"required,email"`
    }
    var payload InvitePayload
    if err := c.Bind(&payload); err != nil {
        return err
    }
    // Use mail.Address to properly format a sender header
    from := mail.Address{Name: "App", Address: "[email protected]"}
    to := mail.Address{Name: "Invitee", Address: payload.Email}
    msg := &mail.Message{
        Header: mail.Header{
            "From":    {from.String()},
            "To":      {to.Address},
            "Subject": {"Invitation"},
        },
    }
    // Send via SMTP using the message; do NOT interpolate user email into raw string headers
    return smtp.SendMail("smtp.example.com:587", nil, from.Address, []string{payload.Email}, msg.Bytes())
}

2. Keep Bearer Tokens out of email flows

Ensure that any Bearer Token used for backend API calls is not embedded in email content, URLs, or headers. If a confirmation link is required, use a short-lived, single-use token stored server-side, not the same token sent to external APIs.

import (
    "net/http"
    "github.com/gobuffalo/buffalo"
)

func sendConfirmation(c buffalo.Context) error {
    authHeader := c.Request().Header.Get("Authorization")
    if authHeader == "" {
        return c.Error(401, "missing authorization")
    }
    // Bearer Token should only be used for outbound HTTP calls to trusted services
    client := &http.Client{}
    req, _ := http.NewRequest("GET", "https://api.example.com/verify", nil)
    req.Header.Set("Authorization", authHeader) // forward the original token, don’t reconstruct it from user input
    resp, err := client.Do(req)
    if err != nil || resp.StatusCode >= 400 {
        return c.Error(502, "backend verification failed")
    }
    // Do NOT write authHeader into email body or URL parameters
    return c.Render(200, r.HTML("emails/confirmation.html", nil))
}

3. Use middleware to enforce secure header handling

Add a Buffalo middleware that rejects requests containing newline characters in headers and logs suspicious patterns without altering the Bearer Token usage in downstream clients.

func secureHeaders(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        req := c.Request()
        for name, values := range req.Header {
            for _, v := range values {
                if containsNewline(v) {
                    return c.Error(400, "invalid header format")
                }
            }
        }
        return next(c)
    }
}

func containsNewline(s string) bool {
    return len(s) > 0 && (s[0] == '\n' || s[0] == '\r')
}

4. Prefer framework-provided URL generation

When building links for emails, use Buffalo’s url.For to avoid manual string concatenation that could expose tokens.

func buildInviteEmail(c buffalo.Context, email string) (string, error) {
    // Generate a signed token stored in your database; do not embed Bearer Tokens here
    token, err := generateVerificationToken(email)
    if err != nil {
        return "", err
    }
    link := url.For(c, "VerifyInviteRoute", "token", token, nil)
    body := fmt.Sprintf("Click %s to confirm", link)
    // Use a proper email library to set headers safely
    return body, nil
}