Email Injection in Buffalo with Basic Auth

Email Injection in a Buffalo application becomes more dangerous when Basic Authentication is used because the authentication exchange is not inherently protected against injection if the application mishandles user-controlled input in email headers or message construction. Buffalo does not provide built-in email header sanitization; it relies on developers to validate and escape data before it reaches the mailer layer.

In a typical scenario, a developer might accept a user-supplied name or address field and directly interpolate it into an email header, such as To, Cc, or Subject. When Basic Auth is used—often via middleware that adds an Authorization header to requests—attackers can combine HTTP smuggling or header manipulation techniques with email injection to deliver malicious headers or crafted email content. For example, newline characters (\r\n) supplied through form fields can break out of the intended header context and inject additional headers, potentially enabling header injection or open relay abuse.

The presence of Basic Auth can create a false sense of security, leading developers to assume that because authentication is required, email inputs are safe. However, authentication and input validation are separate concerns. An authenticated session does not prevent an attacker from supplying newline characters in an email field if the application does not enforce strict allow-lists or sanitization. This becomes a critical issue in APIs or web handlers where email messages are constructed from user input, such as contact forms or notification endpoints, and the server uses Basic Auth for endpoint protection.

During a middleBrick scan, such an endpoint would be flagged under the Input Validation and Data Exposure checks. The scanner tests whether newline or carriage return characters can be injected into email headers and whether those characters propagate into the final message or SMTP transaction. If the mailer uses functions like mail() or a library such as gomail without proper sanitization, the scan can demonstrate how injected headers or content can bypass intended routing or expose sensitive information in error messages.

Because Buffalo applications often compose emails in handlers or background jobs, the risk is compounded when logging or error reporting includes user-supplied email fields. Sensitive data could be inadvertently included in logs or exposed through verbose error pages, which middleBrick identifies under Data Exposure. The scanner correlates these findings with the presence of authentication mechanisms, including Basic Auth, to highlight scenarios where authenticated endpoints still process untrusted input insecurely.

Remediation centers on strict input validation, safe header construction, and avoiding direct interpolation of user data into email headers. Use allow-list validation for any field that influences email routing or headers, and encode data according to the context (header, body, address).

Example of unsafe code that is vulnerable to email injection:

// Unsafe: directly using user input in email headers
func sendContactEmail(c buffalo.Context) error {
    name := c.Param("name")
    email := c.Param("email")
    subject := c.Param("subject")

    // Vulnerable to newline injection in name or subject
    mail.To = email
    mail.Subject = subject
    mail.Body = fmt.Sprintf("From: %s <%s>\r\n\r\n%s", name, email, c.Param("message"))
    return mail.Send()
}

Secure version with validation and sanitization:

import (
    "regexp"
    "strings"
)

var nameRegex = regexp.MustCompile(`^[A-Za-z0-9\-\s]{1,100}$`)
var emailRegex = regexp.MustCompile(`^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[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 sendContactEmail(c buffalo.Context) error {
    name := c.Param("name")
    email := c.Param("email")
    subject := c.Param("subject")

    if !nameRegex.MatchString(name) {
        return c.Render(400, r.JSON(H{"error": "invalid name"}))
    }
    if !emailRegex.MatchString(email) {
        return c.Render(400, r.JSON(H{"error": "invalid email"}))
    }
    // Remove any CR or LF to prevent header injection
    safeSubject := strings.ReplaceAll(subject, "\r", "")
    safeSubject = strings.ReplaceAll(safeSubject, "\n", "")

    // Use a mail library that builds headers safely
    m := gomail.NewMessage()
    m.SetHeader("From", fmt.Sprintf("%s <%s>", name, email))
    m.SetHeader("To", email)
    m.SetHeader("Subject", safeSubject)
    m.SetBody("text/plain", c.Param("message"))

    return smtpSend(m)
}

When using Basic Auth middleware, ensure that the authentication layer does not inadvertently expose or log raw headers that could include injected content. Combine this with content security policies for email clients and restrict the use of user input in any header-related context. middleBrick can verify that these mitigations are effective by testing for newline injection and header manipulation during a scan.