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HTTP parameter pollution

Need​

Prevention of HTTP parameter pollution

Context​

  • Usage of Scala for building scalable and high-performance applications
  • Usage of play.api.mvc for handling HTTP requests and responses in Play Framework

Description​

Non compliant code​

import play.api.mvc._

def vulnerableAction = Action { request =>
  val parameter = request.getQueryString("param")
  parameter match {
    case Some(x) => Ok("Received parameter: " + x)
    case None => BadRequest("Missing parameter")
  }
}

The above Scala code is a simple Play Framework action that fetches a query string parameter named "param" from the HTTP request. The getQueryString method is used to retrieve the parameter value.

The vulnerability lies in the fact that the getQueryString method will only return the first occurrence of the parameter in the query string, ignoring any subsequent occurrences. This means that if an attacker injects extra "param" parameters into the HTTP request, these extra parameters will be ignored by the server, potentially causing unexpected behavior.

For instance, if an attacker sends a request like http://example.com/vulnerableAction?param=legitimate&param=malicious, the server will only see the "legitimate" value and will completely ignore the "malicious" value. This is a classic example of HTTP Parameter Pollution (HPP).

An attacker could exploit this vulnerability to bypass input validation mechanisms, interfere with the application logic, or even conduct more serious attacks like Cross-Site Scripting (XSS) or SQL Injection.

Steps​

  • Validate and sanitize all incoming parameters.
  • Implement a whitelist approach to only allow expected parameters.
  • Use a framework or library that automatically handles parameter validation and sanitization.
  • Avoid using user-provided parameters directly in the application logic.
  • Implement input validation to ensure that parameters meet the expected format and constraints.
  • Consider using a secure coding guideline or best practices for handling user input.

Compliant code​

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

// Define a form mapping that specifies the expected parameters
val parameterForm: Form[String] = Form(
  "param" -> nonEmptyText
)

def secureAction = Action { implicit request =>
  parameterForm.bindFromRequest.fold(
    formWithErrors => {
      // Handle the case where the form data does not match the expected format
      BadRequest("Invalid parameter")
    },
    parameter => {
      // The form data is valid, so we can safely use the parameter
      Ok("Received parameter: " + parameter)
    }
  )
}

The above code fixes the HTTP Parameter Pollution vulnerability by implementing a whitelist approach to only allow expected parameters.

The parameterForm is a form mapping that specifies the expected parameters. It uses the nonEmptyText constraint to ensure that the "param" parameter is a non-empty string.

The secureAction method uses the bindFromRequest method of the form to bind the request parameters to the form. If the form data does not match the expected format (i.e., if the "param" parameter is missing or is not a non-empty string), the fold method will execute the first function, returning a BadRequest response with a message of "Invalid parameter".

If the form data is valid, the fold method will execute the second function, which uses the parameter in the application logic. Because the form ensures that the parameter meets the expected format and constraints, we can safely use the parameter without worrying about HTTP Parameter Pollution.

This approach effectively prevents the injection of extra parameters into the HTTP communication protocol, thereby preventing unexpected behavior on the server.

References​

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