HTTP Parameter Pollution (HPP)

HTTP Parameter Pollution exploits the ability to include multiple parameters with the same name in an HTTP request. Depending on the web application, these parameters will be parsed in different ways (i.e. only parsing the first/last occurrence of the parameter or concatenating all occurrences into one field or array). This can create unexpected behaviour on both the client and server side of the application.

Attack Vector

In an HPP attack, the attacker first needs to research how exactly the web application handles parsing HTTP parameter inputs. With this information, the attacker can modify an HTTP request typically expected by the web application by adding extra parameters with the same name as those in the original request, but with unexpected values.

In a client-side attack, the request is then placed so that another user will interact with it, such as by sending a url to the victim for them to click on.

In a server-side attack, the attacker injects the HTTP request in a place on the web application where they see a possible vulnerability to exploit, such as interacting with a database.


The impact of an HTTP Parameter Pollution attack depends on the functionality of the web application and which part is targeted. This attack accomplishes different outcomes such as overwriting the actions of hard coded HTTP parameters, accessing uncontrolled variables and therefore possibly exploiting them, bypassing web application firewalls (WAFs) or input validation checks, and changing behaviors of the application.

Attack Examples

Example 1 - Client-side

Eve was not invited to Bob's birthday party and she is not happy about it. She knows, however, that Bob invited Alice with an online invitation at the link

Eve knows that, when given multiple parameters of the same name, only takes the last parameter. So she decides to use an HPP attack and creates this new link to send to Alice (%26 is the encoding of &). When Alice clicks on the link, it is evaluated into resulting in the invitation being sent to Eve rather than Alice.

Example 2 - Server-side stores all of its past transactions in a database accessible at the url Eve wants to see Bob's past transactions because he owes her some money. She knows that will concatenate all HTTP parameter values of the same parameter name into one value separated by commas, so she uses an HPP attack to perform SQL injection. She inputs the url receiverpage=amount from transactions where sender=Bob. Since the sql statement is split into two inputs, no alarms are raised when validating the input by the system, however it is still evaluated as "select receiver, amount from transactions where sender = Bob". This then displays all of Bob's transactions to Eve.


To defend against HPP attacks, a thorough sanitization and validation of HTTP requests must be performed before executing the request. This requires specific knowledge of the application itself and how it handles parsing HTTP parameters.

Criteria for Demonstration

To demonstrate an HPP attack, provide the exact HTTP request with parameters you added and where this request is used on the flag website. Show how these added parameters led to unexpected behavior.

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