Need

Secure communication channel for SMTP

Context

• Usage of Go 1.16 for building high-performance and scalable applications
• Usage of net/smtp for sending emails via SMTP protocol
• Usage of gin-gonic/gin for building web applications in Go

Description

Non compliant code

        package main

import (
	"net/smtp"
	"github.com/gin-gonic/gin"
)

func main() {
	r := gin.Default()

	r.GET("/send-email", func(c *gin.Context) {
		auth := smtp.PlainAuth("", "user@example.com", "password", "smtp.example.com")
		err := smtp.SendMail("smtp.example.com:25", auth, "sender@example.org", []string{"recipient@example.net"}, []byte("This is the email body."))
		if err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		c.JSON(200, gin.H{
			"message": "Email sent successfully!",
		})
	})

	r.Run() // listen and serve on 0.0.0.0:8080
}
        
        

The above code is a simple Go application using the Gin framework to send an email using the SMTP protocol. However, the SMTP service is configured to not use SSL, which is a security vulnerability.

In the /send-email route, the smtp.SendMail function is used to send an email. The function takes the SMTP server address, authentication details, sender email address, recipient email addresses, and the email body as parameters.

The SMTP server address is set to smtp.example.com:25, which indicates that the application is connecting to the SMTP server on port 25. This is the standard port for unencrypted SMTP connections, and it does not use SSL.

The smtp.PlainAuth function is used to authenticate to the SMTP server with a username and password. Because the connection is not encrypted, these credentials and any data sent over the connection could be intercepted and read by anyone who can access the network traffic.

This is a serious security vulnerability because it exposes sensitive data to potential eavesdroppers. To fix this vulnerability, the application should be configured to use a secure connection (SSL/TLS) when communicating with the SMTP server.

Steps

• Configure the SMTP service to use SSL/TLS encryption.
• Update the code to use the secure channel (SMTPS) instead of the insecure channel (SMTP).
• Ensure that the SMTP server certificate is valid and trusted.
• Implement proper authentication mechanisms for the SMTP service.
• Encrypt the sensitive information (e.g., username, password) used for SMTP authentication.
• Consider using a secure email library or package that handles encryption and authentication automatically.

Compliant code

        package main

import (
	"crypto/tls"
	"net/smtp"
	"github.com/gin-gonic/gin"
)

func main() {
	r := gin.Default()

	r.GET("/send-email", func(c *gin.Context) {
		auth := smtp.PlainAuth("", "user@example.com", "password", "smtp.example.com")

		tlsconfig := &tls.Config{
			InsecureSkipVerify: false,
			ServerName:         "smtp.example.com",
		}

		conn, err := tls.Dial("tcp", "smtp.example.com:465", tlsconfig)
		if err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		client, err := smtp.NewClient(conn, "smtp.example.com")
		if err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		if err = client.Auth(auth); err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		if err = client.Mail("sender@example.org"); err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		if err = client.Rcpt("recipient@example.net"); err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		w, err := client.Data()
		if err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		_, err = w.Write([]byte("This is the email body."))
		if err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		err = w.Close()
		if err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		if err = client.Quit(); err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		c.JSON(200, gin.H{
			"message": "Email sent successfully!",
		})
	})

	r.Run() // listen and serve on 0.0.0.0:8080
}
        
        

The updated code now uses the secure SMTPS protocol instead of the insecure SMTP. This is achieved by using the tls.Dial function to establish a secure connection to the SMTP server. The tls.Config struct is used to configure the TLS settings. The InsecureSkipVerify field is set to false to ensure that the server's certificate is verified. The ServerName field is set to the hostname of the SMTP server.

The smtp.NewClient function is used to create a new SMTP client. This client is then used to authenticate with the server, send the email, and finally close the connection.

The client.Auth function is used to authenticate with the server using the provided authentication details. The client.Mail and client.Rcpt functions are used to set the sender and recipient of the email, respectively. The client.Data function is used to get a writer to which the email body can be written.

The w.Close function is used to indicate that the email body has been fully written. The client.Quit function is used to cleanly shut down the connection to the server.

If any of these operations fail, an error message is returned as a JSON response. If all operations succeed, a success message is returned as a JSON response.

References

• 149. Use of an insecure channel - SMTP