External Control of Assumed-Immutable Web Parameter

The web application does not sufficiently verify inputs that are assumed to be immutable but are actually externally controllable, such as hidden form fields.


If a web product does not properly protect assumed-immutable values from modification in hidden form fields, parameters, cookies, or URLs, this can lead to modification of critical data. Web applications often mistakenly make the assumption that data passed to the client in hidden fields or cookies is not susceptible to tampering. Improper validation of data that are user-controllable can lead to the application processing incorrect, and often malicious, input.

For example, custom cookies commonly store session data or persistent data across sessions. This kind of session data is normally involved in security related decisions on the server side, such as user authentication and access control. Thus, the cookies might contain sensitive data such as user credentials and privileges. This is a dangerous practice, as it can often lead to improper reliance on the value of the client-provided cookie by the server side application.


The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

In this example, a web application uses the value of a hidden form field (accountID) without having done any input validation because it was assumed to be immutable.

String accountID = request.getParameter("accountID");
User user = getUserFromID(Long.parseLong(accountID));

Example Two

Hidden fields should not be trusted as secure parameters.

An attacker can intercept and alter hidden fields in a post to the server as easily as user input fields. An attacker can simply parse the HTML for the substring:

<input type="hidden"

or even just "hidden". Hidden field values displayed later in the session, such as on the following page, can open a site up to cross-site scripting attacks.

See Also

Comprehensive Categorization: Exposed Resource

Weaknesses in this category are related to exposed resource.

OWASP Top Ten 2021 Category A04:2021 - Insecure Design

Weaknesses in this category are related to the A04 "Insecure Design" category in the OWASP Top Ten 2021.

Validate Inputs

Weaknesses in this category are related to the design and architecture of a system's input validation components. Frequently these deal with sanitizing, neutralizing a...

Comprehensive CWE Dictionary

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Weaknesses Introduced During Implementation

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