SQL injection

A classification of SQL injection attacking vector as of 2010.

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.

In a 2012 study, security company Imperva observed that the average web application received 4 attack campaigns per month, and retailers received twice as many attacks as other industries.

History

The first public discussions of SQL injection started appearing around 1998. For example, a 1998 article in Phrack Magazine.

Form

SQL injection (SQLI) is considered one of the top 10 web application vulnerabilities of 2007 and 2010 by the Open Web Application Security Project. In 2013, SQLI was rated the number one attack on the OWASP top ten. There are four main sub-classes of SQL injection:

• Classic SQLI
• Blind or Inference SQL injection
• Database management system-specific SQLI
• Compounded SQLI

• SQL injection + insufficient authentication
• SQL injection + DDoS attacks
• SQL injection + DNS hijacking
• SQL injection + XSS

The Storm Worm is one representation of Compounded SQLI.

This classification represents the state of SQLI, respecting its evolution until 2010—further refinement is underway.

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 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:

' 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 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.

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, 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 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 an 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.

Mitigation

An SQL injection is a well known attack and easily prevented by simple measures. After an apparent SQL injection attack on Talktalk, security experts were stunned that such a large company would be vulnerable to it.

Parameterized statements

Main article: Prepared statement

With most development platforms, parameterized statements that work with parameters can be used (sometimes called placeholders or bind variables) instead of embedding user input in the statement. A placeholder can only store a value of the given type and not an arbitrary SQL fragment. Hence the SQL injection would simply be treated as a strange (and probably invalid) parameter value.

In many cases, the SQL statement is fixed, and each parameter is a scalar, not a table. The user input is then assigned (bound) to a parameter.

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 straightforward, though error-prone way to prevent injections is to escape characters that have a special meaning in SQL. The manual for an SQL DBMS explains which characters have a special meaning, which allows creating a comprehensive blacklist of characters that need translation. 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. For example, in PHP it is usual to escape parameters using the function mysqli_real_escape_string(); before sending the SQL query:

$mysqli = new mysqli('hostname', 'db_username', 'db_password', 'db_name');
$query = sprintf("SELECT * FROM `Users` WHERE UserName='%s' AND Password='%s'",
                  $mysqli->real_escape_string($username),
                  $mysqli->real_escape_string($password));
$mysqli->query($query);

This function prepends backslashes to the following characters: \x00, \n, \r, \, ', " and \x1a. This function is normally used to make data safe before sending a query to MySQL.
There are other functions for many database types in PHP such as pg_escape_string() for PostgreSQL. The function addslashes(string $str) works for escaping characters, and is used especially for querying on databases that do not have escaping functions in PHP. It returns a string with backslashes before characters that need to be quoted in database queries, etc. These characters are single quote ('), double quote ("), backslash (\) and NUL (the NULL byte).
Routinely passing escaped strings to SQL is error prone because it is easy to forget to escape a given string. Creating a transparent layer to secure the input can reduce this error-proneness, if not entirely eliminate it.

Pattern check

Integer, float or boolean,string parameters can be checked if their value is valid representation for the given type. Strings that must follow some strict pattern (date, UUID, alphanumeric only, etc.) can be checked if they match this pattern.

Database permissions

Limiting the permissions on the database logon used by the web application to only what is needed may help reduce the effectiveness of any SQL injection attacks that exploit any bugs in the web application.

For example, on Microsoft SQL Server, a database logon could be restricted from selecting on some of the system tables which would limit exploits that try to insert JavaScript into all the text columns in the database.

deny select on sys.sysobjects to webdatabaselogon;
deny select on sys.objects to webdatabaselogon;
deny select on sys.tables to webdatabaselogon;
deny select on sys.views to webdatabaselogon;
deny select on sys.packages to webdatabaselogon;

Hexadecimal conversion

Hexadecimal Conversion is the conversion of plain text into its hexadecimal representation for use in an SQL command. In PHP, the functions used are either the bin2hex() function or the dechex function. The bin2hex() function is the preferred method since it will convert any character and not just numbers. In this section we will only use the bin2hex() function.

Example of PHP's bin2hex() function:

echo bin2hex("test");

The output of the above would be:

74657374

For purposes of this discussion we will only talk about the MySQL database. In MySQL, the unhex() function is used to convert a hexadecimal string back to plain text.

Example of MySQL's unhex() function:

SELECT * FROM myTable WHERE id=unhex('32');

If we converted the unhex() string to plain text, it would become:

SELECT * FROM myTable WHERE id=2;

Hexadecimal conversion eliminates SQL injection attacks because the hexadecimal string sent to the unhex() function is returned as a string that is used and not interpreted.

Programming example

In the following short program we present both a PHP program as well as a PHP function. The program depicts an SQL injection attack on an plain SQL command. It then demonstrates how converting all incoming information into hexadecimal stops SQL injection attacks. The PHP function is a simple set of commands to handle the sending of SQL commands to a database as well as the retrieval of the output from the SQL database. As stated above, the database in use is a MySQL database.

Example code

File: test.php
<?php

    include_once "dosql.php";
#
#   Put your own database information here.  I'm using my log file's data.
#
    $hostname = "myhost";
    $username = "myUser";
    $password = "myPassword";
    $database = "myDatabase";

    $mysqli = new mysqli($hostname, $username, $password, $database);

    if ($mysqli->connect_errno) {
        echo "Failed to connect to MySQL: (", $mysqli->connect_errno, ") ", $mysqli->connect_error;
        exit;
    }
    echo "SQL INJECTION - Plain\n";
    $sql = "SELECT * FROM log WHERE log_id='2' OR 1=1; #'";
    $res = dosql($sql);
    foreach ($res[0] as $k => $v) {
        echo "RES[$k] = $v\n";
    }

    echo "\n\nSQL INJECTION = Hexadecimal\n";
    $sql = "SELECT * FROM log WHERE log_id=unhex('" . bin2hex("2' or 1=1; #'") . "')";
    $res = dosql($sql);
    foreach ($res[0] as $k => $v) {
        echo "RES[$k] = $v\n";
    }

    exit;
?>

File: dosql.php
<?php

################################################################################
#   dosql(). Do the SQL command.
################################################################################
function dosql($sql)
{
    global $mysqli;

    $cmd = "INSERT INTO log (date,entry) VALUES (NOW(), unhex('" . bin2hex($sql) . "'))";
    $res = $mysqli->query($cmd);

    $res = $mysqli->query($sql);
    if (!$res) {
        $array = debug_backtrace();
        if (isset($array[1])) { $a = $array[1]['line']; }
        else if (isset($array[0])) { $a = $array[0]['line']; }
        else { $a = "???"; }

        echo "ERROR @ ", $a, " : (", $mysqli->errno, ")\n", $mysqli->error, "\n\n";
        echo "SQL = $sql\n";
        exit;
    }

    if (preg_match("/INSERT/i", $sql)) { return $mysqli->insert_id; }
    if (preg_match("/DELETE/i", $sql)) { return null; }
    if (!is_object($res)) { return null; }

    $count = -1;
    $array = array();
    $res->data_seek(0);
    while ($row = $res->fetch_assoc()) {
        $count++;
        foreach ($row as $k => $v) { $array[$count][$k] = $v; }
    }

    return $array;
}

Program output

SQL INJECTION - Plain
RES[log_id] = 1
RES[date] = 2015-03-25 10:40:18
RES[entry] = SHOW full columns FROM log

SQL INJECTION = Hexadecimal
RES[log_id] = 2
RES[date] = 2015-03-25 10:40:18
RES[entry] = SELECT * FROM log ORDER BY title ASC

The first part of the output from the program is for the SQL command to be sent without any checks or modifications. The original request is for the second record to be returned but with the SQL injection in the command the first record is returned. The second part of the output shows the result from converting all incoming text into hexadecimal. When this is done all of the characters in the command, including the SQL injection, are converted to hexadecimal and are no longer a threat because they are not interpreted as a command but instead just as a string. Because all of the characters are treated as a part of the overall string, MySQL determines it needs a number and converts the string into a number. The rules for conversion of a string into a number is to convert the string until it reaches a non-numeric character or the end of the string. When either of these two events occur the conversion stops at that point. Thus, the "2" is first seen and then the single quote (') is seen which tells MySQL to stop the conversion of the string. So the numeric value of two(2) is used to determine which record to return. This process or method of evaluating the incoming information is why SQL injection attacks can not happen with hexadecimal conversion.

Additional considerations

Additionally the usage of BIN2HEX and UNHEX can require less time to execute than the other methods presented. This is mainly because of the simplistic nature of both BIN2HEX and UNHEX. As shown in the following JavaScript snippet, converting to hexadecimal is fairly simple and straight forwards:

Example code

File: toHex.js
////////////////////////////////////////////////////////////////////////////////
//	toHex().  Convert a string to hex.
////////////////////////////////////////////////////////////////////////////////
function toHex(s)
{
	var l = "0123456789ABCDEF";
	var o = "";

	if (typeof s != "string") { s = s.toString(); }
	for (var i = 0; i < s.length; i++) {
		var c = s.charCodeAt(i);
		o = o + l[(c >> 4)] + l[(c & 0xf)];
	}

	return o;
}

As depicted, unlike mysqli_real_escape_string() which must test for each of the different characters to be escaped, bin2hex() simply converts all characters into their hexadecimal equivalent. Just as the unhex() function performs the opposite operation. Since no tests are made for a particular character or characters the conversion cycle is quite small and efficient. Further, unlike mysqli_real_escape_string(), if there is some new method discovered in the future - the bin2hex() function will automatically catch and disable the character combination because it returns a hexadecimal string. An example of this is the Control-D ASCII character code. Control-D can cause an "End of Transmission" condition in some languages. If the bin2hex() function is used then Control-D simply becomes the plain ASCII text "04" which will not cause a problem.

Articles

There are several articles on the internet which talk about Hexadecimal Conversion and how it stops SQL injection attacks. Some of these articles are:

1. Is hexing input sufficient to sanitize SQL Queries?
2. Use bin2hex and unhex as simple sql injection prevention
3. Best way to prevent SQL injection?
4. SQL Injections - The final solution to

Examples

• In February 2002, Jeremiah Jacks discovered that Guess.com was vulnerable to an SQL injection attack, permitting anyone able to construct a properly-crafted URL to pull down 200,000+ names, credit card numbers and expiration dates in the site's customer database.
• On November 1, 2005, a teenage hacker used SQL injection to break into the site of a Taiwanese information security magazine from the Tech Target group and steal customers' information.
• On January 13, 2006, Russian computer criminals broke into a Rhode Island government website and allegedly stole credit card data from individuals who have done business online with state agencies.
• On March 29, 2006, a hacker discovered an SQL injection flaw in an official Indian government's tourism site.
• On June 29, 2007, a computer criminal defaced the Microsoft UK website using SQL injection. UK website The Register quoted a Microsoft spokesperson acknowledging the problem.
• 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.
• In July 2008, Kaspersky's Malaysian site was hacked by a Turkish hacker going by the handle of "m0sted", who said to have used an SQL injection.
• In February 2013, a group of Maldivian hackers, hacked the website " UN-Maldives" using SQL Injection.
• In May 28, 2009 Anti-U.S. Hackers Infiltrate Army Servers Investigators believe the hackers used a technique called SQL injection to exploit a security vulnerability in Microsoft's SQL Server database to gain entry to the Web servers. "m0sted" is known to have carried out similar attacks on a number of other websites in the past—including against a site maintained by Internet security company Kaspersky Lab.
• On April 13, 2008, the Sexual and Violent Offender Registry of Oklahoma shut down its website for "routine maintenance" after being informed that 10,597 Social Security numbers belonging to sex offenders had been downloaded via an SQL injection attack
• 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.

• 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 does not 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.
• On August 17, 2009, the United States Department of Justice 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.
• In December 2009, an attacker breached a RockYou plaintext database containing the unencrypted usernames and passwords of about 32 million users using an SQL injection attack.
• On July 2010, a South American security researcher who goes by the handle "Ch Russo" obtained sensitive user information from popular BitTorrent site The Pirate Bay. He gained access to the site's administrative control panel and exploited a SQL injection vulnerability that enabled him to collect user account information, including IP addresses, MD5 password hashes and records of which torrents individual users have uploaded.
• From July 24 to 26, 2010, attackers from Japan and China used an SQL injection to gain access to customers' credit card data from Neo Beat, an Osaka-based company that runs a large online supermarket site. The attack also affected seven business partners including supermarket chains Izumiya Co, Maruetsu Inc, and Ryukyu Jusco Co. The theft of data affected a reported 12,191 customers. As of August 14, 2010 it was reported that there have been more than 300 cases of credit card information being used by third parties to purchase goods and services in China.
• On September 19 during the 2010 Swedish general election a voter attempted a code injection by hand writing SQL commands as part of a write‑in vote.
• On November 8, 2010 the British Royal Navy website was compromised by a Romanian hacker named TinKode using SQL injection.
• On February 5, 2011 HBGary, a technology security firm, was broken into by LulzSec using a SQL injection in their CMS-driven website
• On March 27, 2011, mysql.com, the official homepage for MySQL, was compromised by a hacker using SQL blind injection
• On April 11, 2011, Barracuda Networks was compromised using an SQL injection flaw. Email addresses and usernames of employees were among the information obtained.
• Over a period of 4 hours on April 27, 2011, an automated SQL injection attack occurred on Broadband Reports website that was able to extract 8% of the username/password pairs: 8,000 random accounts of the 9,000 active and 90,000 old or inactive accounts.
• On June 1, 2011, "hacktivists" of the group LulzSec were accused of using SQLI to steal coupons, download keys, and passwords that were stored in plaintext on Sony's website, accessing the personal information of a million users.
• In June 2011, PBS was hacked, mostly likely through use of SQL injection; the full process used by hackers to execute SQL injections was described in this Imperva blog.
• In May 2012, the website for Wurm Online, a massively multiplayer online game, was shut down from an SQL injection while the site was being updated.
• In July 2012 a hacker group was reported to have stolen 450,000 login credentials from Yahoo!. The logins were stored in plain text and were allegedly taken from a Yahoo subdomain, Yahoo! Voices. The group breached Yahoo's security by using a "union-based SQL injection technique".
• On October 1, 2012, a hacker group called "Team GhostShell" published the personal records of students, faculty, employees, and alumni from 53 universities including Harvard, Princeton, Stanford, Cornell, Johns Hopkins, and the University of Zurich on pastebin.com. The hackers claimed that they were trying to "raise awareness towards the changes made in today’s education", bemoaning changing education laws in Europe and increases in tuition in the United States.
• On June 27, 2013, hacker group "RedHack" breached Istanbul Administration Site. They claimed that, they’ve been able to erase people's debts to water, gas, Internet, electricity, and telephone companies. Additionally, they published admin user name and password for other citizens to log in and clear their debts early morning. They announced the news from Twitter.
• On November 4, 2013, hacktivist group "RaptorSwag" allegedly compromised 71 Chinese government databases using an SQL injection attack on the Chinese Chamber of International Commerce. The leaked data was posted publicly in cooperation with Anonymous.
• On February 2, 2014, AVS TV had 40,000 accounts leaked by a hacking group called @deletesec
• On February 21, 2014, United Nations Internet Governance Forum had 3,215 account details leaked.
• On February 21, 2014, Hackers of a group called @deletesec hacked Spirol International after allegedly threatening to have the hackers arrested for reporting the security vulnerability. 70,000 user details were exposed over this conflict.
• On March 7, 2014, officials at Johns Hopkins University publicly announced that their Biomedical Engineering Servers had become victim to an SQL injection attack carried out by an Anonymous hacker named "Hooky" and aligned with hacktivist group "RaptorSwag". The hackers compromised personal details of 878 students and staff, posting a press release and the leaked data on the internet.
• In August 2014, Milwaukee-based computer security company Hold Security disclosed that it uncovered a theft of confidential information from nearly 420,000 websites through SQL injections. The New York Times confirmed this finding by hiring a security expert to check the claim.
• In October 2015, SQL injection was believed to be used to attack the British telecommunications company Talk Talk's servers, stealing the personal details of up to four million customers.

In popular culture

• Unauthorized login to web sites by means of SQL injection forms the basis of one of the subplots in J.K. Rowling's novel The Casual Vacancy, published in 2012.
• An xkcd cartoon involved a character "Robert'); DROP TABLE students;--" named to carry out a SQL injection. As a result of this cartoon, SQL injection is sometimes informally referred to as 'Bobby Tables'.
• In 2014, an individual in Poland legally renamed his business to Dariusz Jakubowski x'; DROP TABLE users; SELECT '1 in an attempt to disrupt operation of spammers’ harvesting bots.

See also

• Code injection
• Cross-site scripting
• Metasploit Project
• OWASP Open Web Application Security Project
• Uncontrolled format string
• w3af
• Web application security
• XML external entity

External links

• Manual Sql Injection Tutorial By The Ajay Devgan
• SQL Injection Knowledge Base, by Websec.
• SQL Injection Wiki
• Blind Sql injection with Regular Expression
• WASC Threat Classification - SQL Injection Entry, by the Web Application Security Consortium.
• Why SQL Injection Won't Go Away, by Stuart Thomas.
• SQL Injection Attacks by Example, by Steve Friedl
• SQL Injection Prevention Cheat Sheet, by OWASP.
• SQL Injection Tutorial, by BTS.
• sqlmap: automatic SQL injection and database takeover tool
• SDL Quick security references on SQL injection by Bala Neerumalla.
• Backdoor Web-server using MySQL SQL Injection By Yuli Stremovsky
• Defacing website with SQL injection by sploitswiki
• Attacking web App with SQL injection by Abhinav Sejpal