In computing, SQL injection is a code injection technique used to attack data-driven applications, in which malicious SQL statements are inserted into an entry field for execution (e.g. to dump the database contents to the attacker). SQL injection must exploit a security vulnerability in an application's software, for example, when user input is either incorrectly filtered for string literal escape characters embedded in SQL statements or user input is not strongly typed and unexpectedly executed. SQL injection is mostly known as an attack vector for websites but can be used to attack any type of SQL database.
SQL injection attacks allow attackers to spoof identity, tamper with existing data, cause repudiation issues such as voiding transactions or changing balances, allow the complete disclosure of all data on the system, destroy the data or make it otherwise unavailable, and become administrators of the database server. Document-oriented NoSQL databases can also be affected by this security vulnerability.
SQL injection remains a widely recognized security risk due to its potential to compromise sensitive data. The Open Web Application Security Project (OWASP) describes it as a vulnerability that occurs when applications construct database queries using unvalidated user input. Exploiting this flaw, attackers can execute unintended database commands, potentially accessing, modifying, or deleting data. OWASP outlines several mitigation strategies, including prepared statements, stored procedures, and input validation, to prevent user input from being misinterpreted as executable SQL code.
History Discussions of SQL injection began in the late 1990's, including in a 1998 article in Phrack Magazine. SQL injection was ranked among the top 10 web application vulnerabilities of 2007 and 2010 by the Open Web Application Security Project (OWASP). In 2013, SQL injection was listed as the most critical web application vulnerability in the OWASP Top 10.
In 2017, the OWASP Top 10 Application Security Risks grouped SQL injection under the broader category of "Injection," ranking it as the third most critical security threat. This category included various types of injection attacks, such as SQL, NoSQL, OS command, and LDAP injection. These vulnerabilities arise when an application processes untrusted data as part of a command or query, potentially allowing attackers to execute unintended actions or gain unauthorized access to data.
By 2021, injection remained a widespread issue, detected in 94% of analyzed applications, with reported incidence rates reaching up to 19%. That year’s OWASP Top 10 further expanded the definition of injection vulnerabilities to include attacks targeting Object Relational Mapping (ORM) systems, Expression Language (EL), and Object Graph Navigation Library (OGNL). To address these risks, OWASP recommends strategies such as using secure APIs, parameterized queries, input validation, and escaping special characters to prevent malicious data from being executed as part of a query.
Root cause
SQL injection is a common security vulnerability that arises from letting attacker-supplied data become SQL code. This happens when programmers assemble SQL queries either by string interpolation or by concatenating SQL commands with user supplied data. Therefore, injection relies on the fact that SQL statements consist of both data used by the SQL statement and commands that control how the SQL statement is executed. For example, in the SQL statement select * from person where name = 'susan' and age = 2 the string 'susan' is data and the fragment and age = 2 is an example of a command (the value 2 is also data in this example).
SQL injection occurs when specially crafted user input is processed by the receiving program in a way that allows the input to exit a data context and enter a command context. This allows the attacker to alter the structure of the SQL statement which is executed.
As a simple example, imagine that the data 'susan' in the above statement was provided by user input. The user entered the string 'susan' (without the apostrophes) in a web form text entry field, and the program used string concatenation statements to form the above SQL statement from the three fragments select * from person where name=', the user input of 'susan', and ' and age = 2.
Now imagine that instead of entering 'susan' the attacker entered ' or 1=1; --.
The program will use the same string concatenation approach with the 3 fragments of select * from person where name=', the user input of ' or 1=1; --, and ' and age = 2 and construct the statement select * from person where name='' or 1=1; --' and age = 2. Many databases will ignore the text after the '--' string as this denotes a comment. The structure of the SQL command is now select * from person where name='' or 1=1; and this will select all person rows rather than just those named 'susan' whose age is 2. The attacker has managed to craft a data string which exits the data context and entered a command context.
Ways to exploit
Although the root cause of all SQL injections is the same, there are different techniques to exploit it. Some of them are discussed below:
Getting direct output or action
Imagine a program creates a SQL statement using the following string assignment command :
var 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 authentication procedure then this example could be used to force the selection of every data field (*) from all users rather than from one specific user name as the coder intended, 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.
Blind SQL injection Blind SQL injection is used when a web application is vulnerable to a 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 has traditionally been considered time-intensive because a new statement needed to be crafted for each bit recovered, and depending on its structure, the attack may consist of many unsuccessful requests. Recent advancements have allowed each request to recover multiple bits, with no unsuccessful requests, allowing for more consistent and efficient extraction. 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 https://books.example.com/review?id=5 would cause the server to run the query SELECT * FROM bookreviews WHERE ID = '5'; 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 https://books.example.com/review?id=5' OR '1'='1 and https://books.example.com/review?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 an 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: https://books.example.com/review?id=5 AND substring(@@version, 1, INSTR(@@version, '.') - 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 achieve their goal directly, or to glean more information from the server in hopes of discovering another avenue of attack.
Second-order SQL injection Second-order SQL injection occurs when an application only guards its SQL against immediate user input, but has a less strict policy when dealing with data already stored in the system. Therefore, although such application would manage to safely process the user input and store it without issue, it would store the malicious SQL statement as well. Then, when another part of that application would use that data in a query that isn't protected from SQL injection, this malicious statement might get executed. 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.
In order to protect from this kind of attack, all SQL processing must be uniformly secure, despite the data source.
Out-of-Band SQL injection Out-of-Band (OOB) SQL injection occurs when the results of the SQLi can only be retrieved by a separate outbound HTTP or DNS channel from the target to a listening server. For OOB SQLi to be exploited: the malicious SQL query must be executed, the server must be communicating on a public network, and a listening server must be used to capture the results. Burp Suite Collaborator can be used as a listening server.
SQL injection mitigation SQL injection is a well-known attack that can be mitigated with established security measures. However, a 2015 cyberattack on British telecommunications company TalkTalk exploited an SQL injection vulnerability, compromising the personal data of approximately 400,000 customers. The BBC reported that security experts expressed surprise that a major company remained vulnerable to such an exploit.