Database is one of the most important components of any given application. Through SQL injection the hackers tend to tamper and destroy the data in a business model. This article helps the readers understand the destructive aspect of SQL injection through examples of real time SQL codes.
Based on the hypothesis and simplification of the structure of a business, it is arguable that SQL (Structured Query Language) injection is a problem for both strategic planners and tactical implementers. The business communities are increasingly able to understand the problems the SQL injection. Yet, the problem of SQL injection still exists. Open source or closed source, Oracle or MySQL, databases are still being attacked through SQL problems. The economics of information with the likes of phishing and pharming motivate crime and increase the black market value of corporate information; keeping SQL injection exploitation alive.
In the coming sections of this article, we will explore the technicalities of SQL injection. In addition we will also traverse through the different forms of vulnerability with real time example of SQL code. As some of the sections explains the details of SQL injection with coding examples, this article is intended for SQL programmers with technical knowledge of SQL database.
The Term
SQL injection is a code injection technique that exploits a security vulnerability occurring in the database layer of an application. The vulnerability is present when user input is either incorrectly filtered for string literal escape characters embedded in SQL statements or user input is not strongly typed and thereby unexpectedly executed. It is an instance of a more general class of vulnerabilities that can occur whenever one programming or scripting language is embedded inside another. SQL injection attacks are also known as SQL insertion attacks.
Forms of Vulnerability
Having known what SQL injection is, let's understand the various forms of vulnerability along with an example of SQL code.
Incorrectly filtered escape characters
This form of SQL injection occurs when user input is not filtered for escape characters and is then passed into an 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
renders this SQL statement by the parent language:
SELECT * FROM users WHERE name = 'a' OR 't'='t'; |
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 't'='t' is always true.
While most SQL server implementations allow multiple statements to be executed with one call, some SQL APIs such as php's mysql_query() do not allow this for security reasons. This prevents attackers from injecting entirely separate queries, but doesn't stop them from modifying queries. 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:
SELECT * FROM users WHERE name = 'a';DROP TABLE users; SELECT * FROM userinfo WHERE 't' = 't'; |
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
will drop (delete) the "users" table from the database, since the SQL would be rendered as follows:
SELECT * FROM userinfo WHERE id=1;DROP TABLE users; |
Vulnerabilities inside the database server
Sometimes vulnerabilities can exist within the database server software itself, as was the case with the MySQL server's mysql_real_escape_string() function. This would allow an attacker to perform a successful SQL injection attack based on bad Unicode characters even if the user's input is being escaped. This bug was patched with the release of version 5.0.22 (released on 24th May 2006).
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.
SELECT booktitle FROM booklist WHERE bookId = 'OOk14cd' AND 1=1; |
will result in a normal page while
SELECT booktitle FROM booklist WHERE bookId = 'OOk14cd' AND 1=2; |
will likely give a different result if the page is vulnerable to a SQL injection. An injection like this may suggest to the attacker that a blind SQL injection is possible, leaving the attacker to devise statements that evaluate to true or false depending on the contents of another column or table outside of the SELECT statement's column list.
Conditional errors
This type of blind SQL injection causes an SQL error by forcing the database to evaluate a statement that causes an error if the WHERE statement is true. For example,
SELECT 1/0 FROM users WHERE username='Ralph'; |
the division by zero will only be evaluated and result in an error if user Ralph exists.
Time delays
Time Delays are a type of blind SQL injection that causes the SQL engine to execute a long running query or a time delay statement depending on the logic injected. The attacker can then measure the time the page takes to load to determine if the injected statement is true.
Preventing SQL injection
To protect against SQL injection, user input must not directly be embedded in SQL statements. Instead, parameterized statements must be used (preferred), or user input must be carefully escaped or filtered.
Parameterized statements
With most development platforms, parameterized statements can be used that work with parameters (sometimes called placeholders or bind variables) instead of embedding user input in the statement. In many cases, the SQL statement is fixed. The user input is then assigned (bound) to a parameter. This is an example using Java and the JDBC API:
PreparedStatement prep = conn.prepareStatement("SELECT * FROM USERS WHERE USERNAME=? AND PASSWORD=?");
prep.setString(1, username);
prep.setString(2, password);
prep.executeQuery(); |
Similarly, in C#:
using (SqlCommand myCommand = new SqlCommand("SELECT * FROM USERS WHERE USERNAME=@username AND PASSWORD=HASHBYTES('SHA1',
@password)", myConnection))
{
myCommand.Parameters.AddWithValue("@username", user);
myCommand.Parameters.AddWithValue("@password", pass);
myConnection.Open();
SqlDataReader myReader = myCommand.ExecuteReader())
...................
} |
In PHP version 5 and above, there are multiple choices for using parameterized statements. The PDO database layer is one of them:
$db = new PDO('pgsql:dbname=database');
$stmt = $db->prepare("SELECT priv FROM testUsers WHERE username=:username AND password=:password");
$stmt->bindParam(':username', $user);
$stmt->bindParam(':password', $pass);
$stmt->execute(); |
In ColdFusion, the CFQUERYPARAM statement is useful in conjunction with the CFQUERY statement to nullify the effect of SQL code passed within the CFQUERYPARAM value as part of the SQL clause. An example is below.
<cfquery name="Recordset1" datasource="cafetownsend">
SELECT *
FROM COMMENTS
WHERE COMMENT_ID =<cfqueryparam value="#URL.COMMENT_ID#" cfsqltype="cf_sql_numeric">
</cfquery> |
Enforcement at the database level
Currently only the H2 Database Engine supports the ability to enforce query parameterization. However, one drawback is that query-by-example may not be possible or practical because it's difficult to implement query-by-example using parametrized queries.
Enforcement at the coding level
Using object-relational mapping libraries avoids the need to write SQL code. The ORM library in effect will generate parameterized SQL statements from object-oriented code.
Escaping
A straight-forward, though error-prone way to prevent injections is to escape dangerous characters. One of the reasons for it being error prone is that it is a type of blacklist which is less robust than a whitelist. For instance, every occurrence of a single quote (') in a parameter must be replaced by two single quotes ('') to form a valid SQL string literal. In PHP, for example, it is usual to escape parameters using the function mysql_real_escape_string before sending the SQL query:
$query = sprintf("SELECT * FROM Users where UserName='%s' and Password='%s'",
mysql_real_escape_string($Username),
mysql_real_escape_string($Password));
mysql_query($query); |
Real Time Exploits
* On November 1, 2005, a high school student used a SQL injection to break into the site of a Taiwanese information security magazine from the Tech Target group and steal customers' information.
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On March 29, 2006, Susam Pal discovered a SQL injection flaw in an official Indian government tourism site.
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On March 2, 2007, Sebastian Bauer discovered a SQL injection flaw in the knorr.de login page.
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On June 29, 2007, a computer criminal defaced the Microsoft U.K. website using SQL injection. U.K. website The Register quoted a Microsoft spokesperson acknowledging the problem.
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In January 2008, tens of thousands of PCs were infected by an automated SQL injection attack that exploited a vulnerability in application code that uses Microsoft SQL Server as the database store.
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In May 2008, a server farm inside China used automated queries to Google's search engine to identify SQL server websites which were vulnerable to the attack of an automated SQL injection tool.
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In July 2008, Kaspersky's Malaysian site was broken into by a Turkish computer criminal going by the handle of "m0sted", who claimed to have used SQL injection.
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In 2008, at least April through August, a sweep of attacks began exploiting the SQL injection vulnerabilities of Microsoft's IIS web server and SQL Server database server. The attack doesn't require guessing the name of a table or column, and corrupts all text columns in all tables in a single request. A HTML string that references a malware JavaScript file is appended to each value. When that database value is later displayed to a website visitor, the script attempts several approaches at gaining control over a visitor's system. The number of exploited web pages is estimated at 500,000.
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On August 17, 2009, the United States Justice Department charged an American citizen Albert Gonzalez and two unnamed Russians with the theft of 130 million credit card numbers using an SQL injection attack. In reportedly "the biggest case of identity theft in American history", the man stole cards from a number of corporate victims after researching their payment processing systems. Among the companies hit were credit card processor Heartland Payment Systems, convenience store chain 7-Eleven, and supermarket chain Hannaford Brothers.
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In December 2009, an attacker breached a RockYou! plaintext database containing the unencrypted usernames and passwords of about 32 million users by using a SQL injection attack.
Conclusion
Oracle, Microsoft, MySQL and other software vendors have, working with the security community evolved their patching routines and business relationships improving security along the way. In spite of all of this mitigation, problems of SQL injection are still occurring, even in new technologies. Some answers may rest in software engineering and academic training; however, a great number of SQL web developers are not computer science graduates.
Database vendors continue to improve patching methodologies and the security of their code and appear to be helping to solve their side of the problem (not every security researcher agrees). However, SQL injection is mostly a problem from and for the consumer. SQL provides the database consumer with flexibility to manipulate data as they wish (i.e. SQL though PHP, Java and PL/SQL). This is something that would be difficult to change, especially as it is a good idea! For that reason, until database consumers are encouraged to mitigate the security threats of SQL injection, the problem will persist. This is why the educational relationships between the vendor, customer and security community are more important than ever in solving the short term as well as the long term problems of SQL injection.
—By: R. Manoj. The author is an Assistant Editor at Fanatic Media, Bangalore. He is also an Independent Researcher, specialising in Systems Security. He has an active interest in designing security algorithms for securing mission critical systems. He can reached at infosecurity@fanaticmedia.com |
