Email Injection in Buffalo with Api Keys

Email injection in the Buffalo web framework typically occurs when user-controlled data is interpolated directly into email headers or the message body without validation or sanitization. When an application also uses API keys for external service authentication, a poorly designed email-sending flow can expose those keys or enable header manipulation that leads to unintended delivery paths.

Consider a scenario where an endpoint accepts user input for recipient, subject, or message body and constructs an email using Buffalo’s mail.Action. If the input is concatenated into headers such as Cc, Bcc, or Reply-To, an attacker can inject additional headers by supplying newline sequences (e.g., %0D%0A or \r\n). This can redirect email delivery, spoof senders, or exfiltrate information through injected recipients. When API keys are embedded in the email content or logged as part of request processing, they risk being exposed in outbound messages or server logs.

For instance, a handler that includes an API key in a debug or transaction email may inadvertently disclose it if user input modifies the email template structure. Attackers can use crafted newline injections to append extra content, turning a benign message into a vector for credential leakage. In Buffalo, the standard Action email methods do not inherently sanitize these fields, so developers must treat all external data as untrusted and apply strict validation before inclusion.

Because Buffalo does not automatically escape or validate header fields, developers must explicitly manage allowed characters and reject sequences like carriage returns and line feeds. This is especially important when API keys are involved, as their exposure can lead to unauthorized access to third-party services. Secure handling requires separating user data from control headers and validating email components independently of any templating convenience.

To address email injection when using API keys in Buffalo, ensure that user input is never directly placed into email headers or bodies. Use explicit allow-lists for known-safe values and avoid concatenating raw input into any part of the email construction process.

First, validate and sanitize all user-supplied fields. For recipient addresses, use a strict format check and reject any input containing control characters. For example, you can use a regular expression to allow only permitted characters and return an error if the input is invalid.

import (
	"regexp"
	"strings"
)

var validEmail = regexp.MustCompile(`^[a-zA-Z0-9.!#$%&'*+/=?^_` + "`" + `{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$`)

func isValidAddress(addr string) bool {
	// Reject newline characters which can enable header injection
	if strings.ContainsAny(addr, "\r\n") {
		return false
	}
	return validEmail.MatchString(addr)
}

Second, construct email messages using Buffalo’s mail actions with explicit field assignment rather than template interpolation for sensitive metadata. Keep API keys outside of any user-influenced context and store them in server-side configuration or environment variables accessed at runtime, not in request payloads.

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

func SendTransactionEmail(c buffalo.Context) error {
	// API key retrieved securely from environment, not user input
	apiKey := c.Value("env").(map[string]string)["TRANSACTION_API_KEY"]

	// Validate user-supplied address
	to := c.Param("to")
	if !isValidAddress(to) {
		return c.Render(400, r.JSON(map[string]string{"error": "invalid recipient"}))
	}

	// Build email with controlled fields only
	m := mail.NewMessage()
	m.SetHeader("From", "[email protected]")
	m.SetHeader("To", to)
	m.SetHeader("Subject", "Your transaction confirmation")
	// Never set header fields from user input

	// Use plain text or safe HTML template for body; do not inject API keys
	m.SetBody("text/plain", "Thank you for your transaction. Reference ID: "+c.Param("ref"))

	// Configure SMTP via app settings, not user data
	if err := mail.Send(m); err != nil {
		return c.Render(500, r.JSON(map[string]string{"error": "failed to send"}))
	}

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

Third, if your application must include dynamic content in emails, use templating with context isolation. Ensure that API keys are never part of the data passed to email templates. Configure logging to avoid capturing sensitive values, and review dependency usage to confirm that no middleware inadvertently appends user data to outgoing mail headers.