SQL Injection is a technique used to take advantage of non-validated input vulnerabilities to pass SQL commands through a web application for execution by a backend database. Attackers take advantage of the fact that programmers often chin together SQL commands with user-provided parameters and can therefore embed SQL commands inside these parameters. The result is that attacker can execute arbitrary SQL queries and/or commands on the backend database server through the web application.
History of SQL Injection
On Christmas day, 1998 Phrack 54 was issued. Phrack is a hacker magazine written by the community for the community. It is an excellent source of technical security information in this particular edition, 54. There was an article entitled “NT Web Technology Vulnerabilities” written by rfp-or rain forest puppy. Amongst other things this article described a number of attacks that employed SQL injection, though at no point is this term used in the article. rfp discusses IDC and ASP application running.
On Microsoft’s Internet Information Server feeding into SQL Server 6.5. This article is the first real public outing of SQL injection. Next of note was security advisory published by Allaire on February 4th 1999, a little over a month after rfp’s article. the security bulletin discusses the thread posed by multiple SQL statements in dynamic Queries.
Three months later, there are another interesting note from rfp-coauthored by Matthew Astley. Entitled “NT ODBC Remote Compromise”, the advisory discusses injecting VBA code into Access SQL Queries. One day short of yea aftr the Allaire bulletin, on 3rd og February rfp posts an advisory entitled, “How I hacked Packetstorm – A look at hacking wwwthreads via SQL”. Though the term SQL injection had still yet to surface, rfp was making major inroads into the exploitation of such vulnerabilities. Packetstorm is a hacker site full of useful documents, exploits and scripts. By exploiting holes in the wwwthreads perl application rfp was able to inject arbitrary SQL gaining control of the database server – by making himself an administrator. In September, seven months after this, I submitted a talk for Blackhat Europe called “Application Assessments on IIS” and, as part of it, I discuss attacking database servers via ASP applications using “SQL insertion”. I presented the paper the day after Chip Andrews published the “SQL Injection FAQ” on the 23rd of October at SQLSecurity.com. To my knowledge Chip’s is the first usage of the term “SQL Injection” in a public document, though SANS were using the term in their weekly bulletins at around the same time; which came first is not certain. In April 2001, again at Blackhat, I presented a paper on “Remote Web Application Disassembly using ODBC error messages”. This paper introduced some new techniques that can be used to work out the exact structure of the database application using SQL injection. The next major leap forward came in January 2002 when Chris Anley published a paper entitled “Advanced SQL Injection in SQL Server Applications”. This was the first paper to discuss SQL injection in great depth. Two days before this Kevin Spett had released his paper “SQL Injection - Are your web applications vulnerable?” and in June, Chris follows up with another excellent paper that introduces time delays as a technique for accessing data, cunningly called, “(more) Advanced SQL Injection”. For a while, the pen-testers at NGSSoftware had been using xp_cmdshell ‘ping –n 10 127.0.0.1’ to determine whether we could access the stored procedure – if the application paused for approximately 10 seconds then we could access it.
Chris extended this to using time delays to drill for data and is the first example an inference attack. In August 2002, Cesar Cerrudo released a paper called “Manipulating Microsoft SQL Server Using SQL Injection” and provided a tool called DataThief with the paper to enable data querying via the openrowset function. In the first week of September 2003 Ofer Maor and Amichai Shulman release a paper “Blindfolded SQL injection” and at the end of September 2003 Sanctum release their take on “Blind SQL Injection”. At Blackhat 2004 0x90.org released SQueaL, now known as Absinthe, a tool used to automate the querying of data via SQL injection. This short history mentions the key advancements in SQL injection but it must be noted that there have been many other papers discussing injection techniques for various database servers and for a range of application environments. Readers are encouraged to read the papers listed in this section unless they have already done so.
Forms of SQL Injection Vulnerabilities
Incorrectly filtered escape character
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 a 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
a' or 't'='t
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.
On some SQL servers such as MS SQL Server any valid SQL command may be injected via this method, including the execution of multiple statements. 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 "data" table (in essence revealing the information of every user):
a';DROP TABLE users; SELECT * FROM data WHERE name LIKE '%
This input renders the final SQL statement as follows:
SELECT * FROM users WHERE name = 'a';DROP TABLE users; SELECT * FROM DATA WHERE name LIKE '%';
Other SQL implementations won't execute multiple commands in the same SQL query as a security measure. This prevents hackers 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 data 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 delete the "users" table from the database as the rendered SQL would be rendered as follows:
SELECT * FROM DATA WHERE id = 1;DROP TABLE users;
Blind SQL Injection
Blind SQL Injection is used when a web application is vulnerable to 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 byte recovered. A tool called Absinthe 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 will prove that a blind sql injection is possible, leaving the attacker to devise statements that evaluate to true or false depending on the contents of a field in another table.
Conditional Errors
This type of blind SQL injection causes a 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 are a type of blind SQL injection that cause 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, user input must be escaped, or parameterized statements must be used.
Using Parameterized Statements
Parameterized statements use 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 PASSWORD=?");
prep.setString(1, pwd);
Enforcing the Use of Parameterized Statements
There are two ways to ensure an application is not vulnerable to SQL injection: using code reviews (which is a manual process), and enforcing the use of parameterized statements. Enforcing the use of parameterized statements means that SQL statements with embedded user input are rejected at runtime. Currently only the H2 Database Engine supports this feature.
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