Value Shadowing

The program accesses a variable in an ambiguous way, which can leave it open to attack.

The HttpRequest class provides programmatic access to variables from the QueryString, Form, Cookies or ServerVariables collections in the form of an array access (e.g. Request["myParam"]). When more than one variable exists with the same name, the .NET framework returns the value of the variable that appears first when the collections are searched in the following order: QueryString, Form, Cookies then ServerVariables. Since QueryString comes first in the search order, it is possible for QueryString parameters to supersede values from forms, cookies, and server variables. Similarly, form values can supersede variables in the Cookies and ServerVariables collections and variables from the Cookies collection can supersede those from ServerVariables.
Example 1: Imagine a banking application temporarily stores a user's email address in a cookie and reads this value when it wants to contact the user. The following code reads the cookie value and sends an account balance to the specified email address.

...
    String toAddress = Request["email"];        //Expects cookie value
    Double balance = GetBalance(userID);
    SendAccountBalance(toAddress, balance);
...

Assume the code in Example 1 is executed when visiting http://www.example.com/GetBalance.aspx. If an attacker can cause an authenticated user to click a link that requests http://www.example.com/GetBalance.aspx?email=evil%40evil.com, an email with the user's account balance will be sent to evil@evil.com.