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Weak CAPTCHA

Need​

Implementation of a strong and reliable CAPTCHA system

Context​

  • Usage of Scala 2.12 for developing scalable and high-performance applications
  • Usage of play.api.mvc for handling HTTP requests in Play Framework
  • Usage of play.api.data for handling form data in Play Framework
  • Usage of play.api.data.Forms for handling form data in Play Framework
  • Usage of javax.inject.Inject for dependency injection in Java

Description​

Non compliant code​

import play.api.mvc._
import play.api.data._
import play.api.data.Forms._

class HomeController @Inject()(val controllerComponents: ControllerComponents) extends BaseController {

  val captchaForm: Form[String] = Form(
    "captcha" -> text
  )

  def submitCaptcha = Action { implicit request =>
    val captcha = captchaForm.bindFromRequest.get
    // Insecure CAPTCHA validation
    if (captcha == "1234") {
      Ok("CAPTCHA passed")
    } else {
      BadRequest("Invalid CAPTCHA")
    }
  }

}

The above Scala code snippet is a simple Play Framework controller which includes a form that takes a CAPTCHA input from a user. The submitCaptcha function is an action that handles the form submission.

The vulnerability lies in the submitCaptcha function where the CAPTCHA validation is performed. The function simply checks if the submitted CAPTCHA is equal to "1234". This is a weak CAPTCHA implementation because it uses a static CAPTCHA value that can be easily guessed or brute forced. Moreover, it does not provide any protection against optical recognition tools as the CAPTCHA value is a simple, static string.

This implementation does not follow the best practices for CAPTCHA implementation which should include random CAPTCHA generation and validation to prevent automated bots from bypassing the CAPTCHA.

The vulnerability can be exploited by an attacker to bypass the CAPTCHA verification step, allowing them to perform actions that should be restricted to human users only.

Steps​

  • Replace the insecure CAPTCHA implementation with a more secure one.
  • Use a CAPTCHA service that provides stronger protection against automated recognition tools.
  • Implement additional CAPTCHA validation measures to ensure the integrity of the CAPTCHA response.
  • Consider implementing rate limiting or throttling mechanisms to prevent brute-force attacks on the CAPTCHA.
  • Regularly update and monitor the CAPTCHA implementation to stay ahead of emerging vulnerabilities and attack techniques.

Compliant code​

import play.api.mvc._
import play.api.data._
import play.api.data.Forms._
import play.filters.csrf._

class HomeController @Inject()(val controllerComponents: ControllerComponents, captchaService: CaptchaService) extends BaseController {

  val captchaForm: Form[String] = Form(
    "captcha" -> text
  )

  def submitCaptcha = CSRFCheck {
    Action { implicit request =>
      val captcha = captchaForm.bindFromRequest.get
      // Secure CAPTCHA validation
      if (captchaService.validate(captcha)) {
        Ok("CAPTCHA passed")
      } else {
        BadRequest("Invalid CAPTCHA")
      }
    }
  }

}

The updated code now uses a secure CAPTCHA service for validation. The captchaService.validate(captcha) method should implement a secure CAPTCHA validation mechanism. This could be a service like Google's reCAPTCHA or any other service that provides strong protection against automated recognition tools.

The CSRFCheck is added to the submitCaptcha action to prevent Cross-Site Request Forgery attacks. This ensures that the CAPTCHA response is not tampered with during the submission process.

The captchaService is injected into the HomeController class. This is a placeholder for the actual CAPTCHA service that you will use. You will need to implement this service and its validate method according to the documentation of the CAPTCHA service you choose.

Remember to implement rate limiting or throttling mechanisms to prevent brute-force attacks on the CAPTCHA. This could be done at the application level or by using a service like Cloudflare.

Regularly update and monitor the CAPTCHA implementation to stay ahead of emerging vulnerabilities and attack techniques. This includes keeping up-to-date with the latest versions of the CAPTCHA service you are using and regularly reviewing your application logs for any suspicious activity.

References​

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