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<title>Dangerous Use of Eval</title>
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<p><b>Lesson Plan Title:</b> Dangerous Use of Eval)</p>
<p><b>Concept / Topic To Teach:</b><br/>
It is always a good practice to validate all input on the server side. XSS can occur when unvalidated user input is reflected directly into an HTTP response. In this lesson, unvalidated user-supplied data is used in conjunction with a Javascript eval() call. In a reflected XSS attack, an attacker can craft a URL with the attack script and store it on another website, email it, or otherwise trick a victim into clicking on it.
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<p><b>General Goal(s):</b><br/>
For this exercise, your mission is to come up with some input which, when run through eval, will execute a malicious script. In order to pass this lesson, you must 'alert()' document.cookie.
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<b>Solution:</b><br/>
The value of the digit access code field is placed in the Javascript eval() function. This is the reason why your attack will not require the "<script>" tags.<br/>
Enter: 123');alert(document.cookie);('<br/><br/>
The result on the server is:<br/><br/>
eval('<font color="#ff0000">123');<br/>
alert(document.cookie);<br/>
('</font>');
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