Web Development

Unraveling HTTP Parameter Pollution

October 28th, 2022 | By Jscrambler | 6 min read

Unraveling HTTP Parameter Pollution, or HPP, allows us to understand how this vulnerability affects multiple modern applications. Learn how to prevent it.

Did you know it can hide an attack?

Today, we dive into this not-so-known vulnerability, clarify the reasons behind this bug, share a real-world experience, and give possible mitigation strategies.

What is HTTP Parameter Pollution, or HPP?

HTTP Parameter Pollution is a vulnerability that occurs when query parameters in an HTTP request are supplied to an application in such a crafted manner that it processes the request unnaturally, leading to disastrous outcomes.

But here's the catch: most applications crash or spew out error messages when such unexpected inputs are encountered, but in the case of applications vulnerable to HPP, malicious inputs are processed as though nothing unusual has happened. But the results are the ones that cause the damage.

There are no strict standards regarding HTTP parameters and how to interpret multiple inputs.

RFC 3986, Uniform Resource Identifier (URI): Generic Syntax, mentions that a query is just a URI component used to fetch or point to a resource, whatever is located at that URI.

When an application's intended query format is twisted, the developers might not have considered it, resulting in twisted outputs. All of this depends on the parsing of HTTP requests and the related query parameters, about which the developers might be unaware and unintentionally leave their products vulnerable to HPP.

Understanding HPP

Let us take an example of a real-life HPP bug we encountered in a popular app. It used OTP-based authentication, where a user supplied an email, and an OTP was sent to the specified email.

Upon entering the OTP on the next screen, the login was granted.

Say the login endpoint was "/API/v0/send-otp". Following is the POST request body for generating the OTP:

	"email" : "[email protected]",
	"gen_otp" : "yes",
	"key" : "some_base64_string"

Pretty straightforward:

  • Email is submitted to the backend logic.

  • A corresponding OTP is sent to that email.

  • The submission screen is shown.

But from an attacker's perspective, there could be multiple ways to fiddle with this request in hopes of generating unusual outputs. We tried several crafted requests, but none of these worked. Two of which are shown below:

	"email" : "[email protected]",
	"gen_otp" : "yes",
	"key" : "some_base64_string"
	"email" : "[email protected]",
	"gen_otp" : "yes",
	"key" : "some_base64_string",
} // attempt to make the application parse two emails in a single request
	"email" : "[email protected]\[email protected]",
	"gen_otp" : "yes",
	"key" : "some_base64_string"
} // delimiting the email parameters with `\n` so that backend sends same OTP to both.

We had almost given up but tried to use one last delimiter (and felt dumb that we hadn't thought of this before). Instead of \n, we used a, (a comma). The request looked like this:

	"email" : "[email protected],[email protected]",
	"gen_otp" : "yes",
	"key" : "some_base64_string"

Lo and behold, we received the same OTP on two different emails! Taking over accounts after this was trivial as there was no 2FA facility.

We didn't have access to the application source code, but we can guess that the comma somehow made the application think there are two emails to which the OTP has to be sent.

I hope this example gives some clarity on how HPP works.

Categories of HTPP Parameter Pollution

Now HPP can be split into categories:

  • Server-side HPP

    When the attacker directly targets the application and sends a request with polluted parameters, it is called server-side HPP. Our real-life example in the previous section was an example of server-side HPP.

  • Client-side HPP

    While server-side HPP is a direct attack on the vulnerable application, client-side HPP also involves a middle stage: the victim user.

    An attacker crafts a malicious URL, which leads to the resulting webpage having polluted parameters.

    Let's take an example.

    Assume that in a banking application, the password reset page is at /resetPass?username=test.

    When visited, the webpage shows your username and a hyperlink to send a reset code to your email.

    This hyperlink is in the form: <a href = "/sendCode?username=test&[email protected]"> Send reset code to your email </a>.

    Since the application is vulnerable to HPP, the attacker forges the following URL: /resetPass?username=test&[email protected].

    This will lead to the webpage that has the link with injected payload: <a href = "/sendCode?username=test&[email protected]&[email protected]"> Send reset code to your email </a>.

    When the user clicks this hyperlink, the backend application will take [email protected] as the valid email, and the reset code will be sent to the attacker.

    This is just one scenario to provide better clarity towards client-side HPP (where victim interaction is involved).

Types of HTPP Parameter Pollution

There are three types of client-side HPP:

  1. Reflected HPP

    The example we gave above is a case of reflected HPP, where the victim needs to interact with the malicious URL to reach the webpage with polluted hyperlinks.

    Reflected attacks fail if the adversary can't make the victim click these URLs. This is usually achieved by phishing or social engineering. The victim believes they are interacting with legitimate resources, clicking on them.

  2. Stored HPP

    This happens when the malicious inputs are stored inside an application database (unlike reflected HPP, where things happen on the fly).

    If somehow the attacker makes [email protected] stored in the web application so that whenever /resetPass is accessed, the reset password hyperlinks always have attacker mail in the polluted parameter, anyone using the password reset functionality would be exploited.

    The attacker wouldn't have to resort to phishing, social engineering, or other less reliable routes.

  3. DOM-based HPP

    When the polluted parameters are injected inside Javascript (instead of the response of a request), it is known as DOM-based HPP.

    It is called DOM-based because when a webpage loads, a DOM object for the relevant JavaScript is created.

    If the injection point is inside the DOM object, the webpage will create the DOM object with the malicious payload, and the resulting javascript will be polluted.

Mitigation Strategies

Now we know what HTTP parameter pollution is and what kinds of varieties exist in this vulnerability.

HPP is the problem. What is the solution?

The developer needs to know how the platform they are working on parses multiple HTTP parameters. That's the key to handling unexpected/polluted parameters.

The below image lists a few of the popular web servers with their parsing functionalities:

Popular Webservers and how they parse parameters

Even if the developers use custom-made APIs, they should be aware of unexpected combinations in which parameters can be input.

Further, to prevent any client-side HPP (reflected, stored, or DOM-based), proper URL encoding and sanitization of input are necessary to perform uniform parsing at the backend.

Remember, treat every input as malicious!


Although HTTP parameter pollution is a less popular category of vulnerability compared to other more significant and prevailing bugs like XSS, CSRF, etc., that does not diminish the negative impact a successful HPP exploitation can have.

HPP is the first step of an attack that can lead to sensitive information exposure (data leakage), account takeovers, and many other devastating effects on an organization.

This article is an introduction to this bug class, which is relatively less known, and we advise digging through more research to get an even better understanding.


The leader in client-side Web security. With Jscrambler, JavaScript applications become self-defensive and capable of detecting and blocking client-side attacks like Magecart.

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