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Email flooding

Need​

Mitigation of email flooding attacks

Context​

  • Usage of Go 1.16 for developing high-performance and efficient applications
  • Usage of gin-gonic/gin for building web applications in Go
  • Usage of net/http for handling HTTP requests and responses in a Node.js server

Description​

Non compliant code​

package main

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

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

	r.POST("/sendEmail", func(c *gin.Context) {
		email := c.PostForm("email")
		message := c.PostForm("message")
		// Vulnerable code: No check for the number of emails sent to one address
		sendEmail(email, message)
		c.String(http.StatusOK, "Email sent successfully")
	})

	r.Run()
}

func sendEmail(email string, message string) {
	// Code to send email
}

The above code is a simple Go backend application using the Gin framework. It has a POST route "/sendEmail" that accepts an email address and a message from the request body and sends an email to the provided email address.

The vulnerability here is that there is no restriction or check on the number of emails that can be sent to a single email address. As a result, an attacker can exploit this to send a large number of emails to a single email address, causing email flooding. This can lead to the saturation of the victim's inbox and denial of service as the victim may not be able to receive any more emails. This could also potentially lead to the email service provider blocking the application's email sending capabilities due to the high volume of emails being sent.

Steps​

  • Implement rate limiting to restrict the number of emails that can be sent to one address within a certain time frame.
  • Use a reliable email service provider that has built-in protection against email flooding.
  • Implement email validation to ensure that the email address provided is valid before sending the email.
  • Consider implementing a CAPTCHA or other form of user verification to prevent automated email flooding attacks.
  • Monitor email sending activity and set up alerts for suspicious or excessive email sending patterns.

Compliant code​

package main

import (
	"github.com/gin-gonic/gin"
	"github.com/didip/tollbooth"
	"github.com/didip/tollbooth_gin"
	"net/http"
	"time"
)

var emailCount = make(map[string]int)

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

	// Set up rate limiter
	limiter := tollbooth.NewLimiter(1, nil) // 1 request per second
	limiter.SetIPLookups([]string{"RemoteAddr", "X-Forwarded-For", "X-Real-IP"})
	limiter.SetMethods([]string{"POST"})

	r.POST("/sendEmail", tollbooth_gin.LimitHandler(limiter), func(c *gin.Context) {
		email := c.PostForm("email")
		message := c.PostForm("message")

		// Check if email count for this address is above limit
		if emailCount[email] >= 10 { // limit of 10 emails per minute
			c.String(http.StatusTooManyRequests, "Too many requests")
			return
		}

		sendEmail(email, message)
		emailCount[email]++

		// Reset email count for this address after 1 minute
		time.AfterFunc(1*time.Minute, func() { emailCount[email] = 0 })

		c.String(http.StatusOK, "Email sent successfully")
	})

	r.Run()
}

func sendEmail(email string, message string) {
	// Code to send email
}

The above code fixes the email flooding vulnerability by implementing rate limiting using the tollbooth library. This library provides rate limiting functionality that can be easily integrated with the gin framework.

The rate limiter is set up to allow only 1 request per second. It uses the RemoteAddr, X-Forwarded-For, and X-Real-IP headers to identify the client making the request. It only applies to POST requests.

In the /sendEmail route handler, we first check if the email count for the provided email address is above the limit (10 emails per minute in this case). If it is, we return a 429 Too Many Requests response and do not send the email.

If the email count is below the limit, we send the email and increment the email count for the provided email address.

Finally, we use time.AfterFunc to reset the email count for the provided email address after 1 minute. This ensures that the email count is only limited to a certain time frame, and not for the entire lifetime of the application.

This solution effectively prevents email flooding by limiting the number of emails that can be sent to a single email address within a certain time frame.

References​

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