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Table Of Contents
The Web Application Hacker's Handbook
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The other principal way in which applications use client-side controls to restrict data submitted by clients occurs with data that was not originally specified by the server but that was gathered on the client computer itself.
HTML forms are the simplest and most common way to capture input from the user and submit it to the server. With the most basic uses of this method, users type data into named text fields, which are submitted to the server as name/value pairs. However, forms can be used in other ways; they can impose restrictions or perform validation checks on the user-supplied data. When an application employs these client-side controls as a security mechanism to defend itself against malicious input, the controls can usually be easily circumvented, leaving the application potentially vulnerable to attack.
Consider the following variation on the original HTML form, which imposes a maximum length of 1 on the quantity field:
<form...
Change the font size
Change margin width
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