Email Injection in Buffalo with Firestore

Email Injection in a Buffalo application that uses Firestore as a backend can occur when user-controlled input is reflected into email headers or bodies without proper validation or encoding. Buffalo provides convenient helpers for building forms and sending mail, but if developer code directly interpolates request parameters (e.g., name or reply-to fields) into headers like To, Cc, Reply-To, or Subject, an attacker can inject additional email headers or crafted content.

Because Firestore often stores user profiles and communication metadata, an attacker who can influence what is stored or retrieved might supply malicious payloads intended for email fields. When the application later uses that Firestore data to construct emails via Buffalo’s mailer, the injected content can alter routing, add recipients, or inject MIME parts, leading to spam relay, information disclosure, or phishing via your domain.

The risk is compounded when combined with insecure data handling practices in Firestore (for example, overly permissive rules) and insufficient output encoding in Buffalo templates or mailer functions. Although Firestore itself does not process email, the application layer that reads from Firestore and writes emails through Buffalo becomes the vector. The scanner’s checks for Input Validation and Data Exposure highlight these weaknesses by detecting unvalidated inputs and insecure data flows.

In a black-box scan, middleBrick tests such scenarios by probing endpoints that accept and store user data, then observing whether manipulated email fields result in header splitting or unexpected recipients. This demonstrates how an insecure integration between Buffalo, Firestore, and email sending can be abused without requiring authentication.

Remediation focuses on strict input validation, output encoding, and least-privilege Firestore rules. In Buffalo, validate and sanitize all user-supplied fields before they are stored in Firestore or used in email construction. Use allowlists for expected formats (e.g., email addresses) and reject or neutralize control characters and header metacharacters such as newline (\n and \r) that enable header injection.

When reading data from Firestore to build emails, treat every field as untrusted. Encode addresses and display names according to RFC 5322 and RFC 2047. Prefer using Buffalo’s mailer helpers that accept structured recipients rather than concatenating raw strings for headers.

Here is a secure example in Go using the Buffalo framework, Firestore Go SDK, and net/mail for validation:

import (
	"context"
	"fmt"
	"net/mail"

	"cloud.google.com/go/firestore"
	"github.com/gobuffalo/buffalo"
	"github.com/gobuffalo/mailer"
)

func SendContactEmail(c buffalo.Context) error {
	ctx := c.Request().Context()
	client, err := firestore.NewClient(ctx, "your-project-id")
	if err != nil {
		return c.Render(500, r.JSON(map[string]string{"error": "internal server error"}))
	}
	defer client.Close()

	// Assume Firestore document path is known and contains user email metadata
	docSnap, err := client.Collection("users").Doc("user123").Get(ctx)
	if err != nil {
		return c.Render(500, r.JSON(map[string]string{"error": "unable to fetch user"}))
	}

	var user struct {
		Email string `firestore:"email"`
		Name  string `firestore:"name"`
	}
	if err := docSnap.DataTo(&user); err != nil {
		return c.Render(500, r.JSON(map[string]string{"error": "unable to parse user data"}))
	}

	// Validate and encode sender and recipient
	from, err := mail.ParseAddress("[email protected]")
	if err != nil {
		return c.Render(500, r.JSON(map[string]string{"error": "invalid sender address"}))
	}
	to, err := mail.ParseAddress(fmt.Sprintf("%s <%s>", user.Name, user.Email))
	if err != nil {
		return c.Render(400, r.JSON(map[string]string{"error": "invalid recipient address"}))
	}

	// Compose email using Buffalo mailer, avoiding direct header concatenation
	m := mailer.New("mailers", "contact")
	m.From = fmt.Sprintf("[email protected]")
	m.To = []string{to.Address}
	m.Subject = "Contact form submission"
	m.HTML().Set("Name", user.Name)

	if err := m.Deliver(); err != nil {
		return c.Render(500, r.JSON(map[string]string{"error": "unable to send email"}))
	}

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

Additionally, enforce tight Firestore security rules to prevent unauthorized writes to email-related fields and to restrict reads to the owner or service accounts only. Combine these measures with Buffalo’s form validation and middleware to ensure that data flowing between Firestore and mail routines remains safe and predictable.