This is a theoretical post about types of SQL Injection attacks and the concepts behind SQL Injection. I have added this here since so far we had been dealing with URLs, and will continue to do so. For the attacker, there is no direct way to write complete queries and he/she may only make changes to the URL or input form. However, the knowledge of MySQL (or any other DBMS) part of the attack is necessary, since it will be required when you deal with more robust websites where the standard attacks won't work and you need to get creative. Before reading this post, I recommend these 3:-
- SQL injection + insufficient authentication
- SQL injection + DDoS attacks
- SQL injection + DNS hijacking
- SQL injection + XSS
Technical implementations
Incorrectly filtered escape characters
This form of SQL injection occurs when user input is not filtered for
escape characters and is then passed into a SQL statement. This results in the potential manipulation of the statements performed on the database by the end-user of the application.
The following line of code illustrates this vulnerability:
statement = "SELECT * FROM users WHERE name ='" + userName + "';"
This SQL code is designed to pull up the records of the specified username from its table of users. However, if the "userName" variable is crafted in a specific way by a malicious user, the SQL statement may do more than the code author intended. For example, setting the "userName" variable as:
' or '1'='1
or using comments to even block the rest of the query (there are three types of SQL comments). All three lines have a space at the end:
' or '1'='1' --
' or '1'='1' ({
' or '1'='1' /*
renders one of the following SQL statements by the parent language:
SELECT * FROM users WHERE name = '' OR '1'='1';
SELECT * FROM users WHERE name = '' OR '1'='1' -- ';
If this code were to be used in an authentication procedure then this example could be used to force the selection of a valid username because the evaluation of '1'='1' is always true.
The following value of "userName" in the statement below would cause the deletion of the "users" table as well as the selection of all data from the "userinfo" table (in essence revealing the information of every user), using an
API that allows multiple statements:
a';DROP TABLE users; SELECT * FROM userinfo WHERE 't' = 't
This input renders the final SQL statement as follows and specified:
SELECT * FROM users WHERE name = 'a';DROP TABLE users; SELECT * FROM userinfo WHERE 't' = 't';
While most SQL server implementations allow multiple statements to be executed with one call in this way, some SQL APIs such as
PHP's
mysql_query() function do not allow this for security reasons. This prevents attackers from injecting entirely separate queries, but doesn't stop them from modifying queries.
Incorrect type handling
This form of SQL injection occurs when a user-supplied field is not
strongly typed or is not checked for
type constraints. This could take place when a numeric field is to be used in a SQL statement, but the programmer makes no checks to validate that the user supplied input is numeric. For example:
statement := "SELECT * FROM userinfo WHERE id =" + a_variable + ";"
It is clear from this statement that the author intended a_variable to be a number correlating to the "id" field. However, if it is in fact a
string then the
end-user may manipulate the statement as they choose, thereby bypassing the need for escape characters. For example, setting a_variable to
1;DROP TABLE users
will drop (delete) the "users" table from the database, since the SQL becomes:
SELECT * FROM userinfo WHERE id=1;DROP TABLE users;
Blind SQL injection
Blind SQL Injection is used when a web application is vulnerable to an SQL injection but the results of the injection are not visible to the attacker. The page with the vulnerability may not be one that displays data but will display differently depending on the results of a logical statement injected into the legitimate SQL statement called for that page. This type of attack can become time-intensive because a new statement must be crafted for each bit recovered. There are several tools that can automate these attacks once the location of the vulnerability and the target information has been established.
Conditional responses
One type of blind SQL injection forces the database to evaluate a logical statement on an ordinary application screen. As an example, a book review website uses a
query string to determine which book review to display. So the
URL http://books.example.com/showReview.php?ID=5 would cause the server to run the query
SELECT * FROM bookreviews WHERE ID = 'Value(ID)';
from which it would populate the review page with data from the review with
ID 5, stored in the
table bookreviews. The query happens completely on the server; the user does not know the names of the database, table, or fields, nor does the user know the query string. The user only sees that the above URL returns a book review. A
hacker can load the URLs
http://books.example.com/showReview.php?ID=5 OR 1=1 and
http://books.example.com/showReview.php?ID=5 AND 1=2, which may result in queries
SELECT * FROM bookreviews WHERE ID = '5' OR '1'='1';
SELECT * FROM bookreviews WHERE ID = '5' AND '1'='2';
respectively. If the original review loads with the "1=1" URL and a blank or error page is returned from the "1=2" URL, and the returned page has not been created to alert the user the input is invalid, or in other words, has been caught by an input test script, the site is likely vulnerable to a SQL injection attack as the query will likely have passed through successfully in both cases. The hacker may proceed with this query string designed to reveal the version number of
MySQL running on the server:
http://books.example.com/showReview.php?ID=5 AND substring(@@version,1,1)=4
, which would show the book review on a server running MySQL 4 and a blank or error page otherwise. The hacker can continue to use code within query strings to glean more information from the server until another avenue of attack is discovered or his or her goals are achieved.
Second Order SQL Injection
Second order SQL injection occurs when submitted values contain malicious commands that are stored rather than executed immediately. In some cases, the application may correctly encode a SQL statement and store it as valid SQL. Then, another part of that application without controls to protect against SQL injection might execute that stored SQL statement. This attack requires more knowledge of how submitted values are later used. Automated web application security scanners would not easily detect this type of SQL injection and may need to be manually instructed where to check for evidence that it is being attempted.
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