Welcome!

Microsoft Cloud Authors: Janakiram MSV, Yeshim Deniz, David H Deans, Andreas Grabner, Stackify Blog

Related Topics: Microsoft Cloud

Microsoft Cloud: Article

SQL Injection

Defeat one of the most common attacks

Many applications (Web-based applications and forms-based ["smart client"] applications) typically use data stored in a database. While you may have firewalls and other protections established when running your application, your application can still be open to an attacker gaining direct (or indirect) access to information in your database. The most common and dangerous attack technique is to use SQL injection.

SQL injection occurs when an attacker is able to insert a series of SQL statements into a "query" by manipulating data input into an application. This can happen because data input is not checked or "sanitized" before being entered into the database. All it takes is one input point through your application that can allow an attacker to retrieve sensitive and private information, change data, drop tables, and possibly shut down your database.

In this article, I will talk mostly about how SQL injection can be performed against SQL Server 2000 (using Transact-SQL code), but these techniques also apply to other databases such as Oracle, DB2, and MySQL, which vary only slightly in SQL syntax. I will also talk about best practices in writing correct code to counter SQL injection, as well as some ways to help audit your code for these problems.

How It's Done
SQL injection is primarily caused by developers who use "string-building" techniques for SQL statements that are executed in a database. An attacker can take advantage of code developed this way by passing commands directly to a database and then take advantage of a poorly secured system to leverage access privileges.

As an example of the simplest form of SQL injection, let's talk about a common entry point into most applications: a login or authentication form. The form may look something like Figure 1.

In order to identify the user's account, an SQL query may be written to look up the username and password in a users table in the database. For example, you may have C# code that builds the SQL statement this way:


string sql = "SELECT userid, first
name, lastname FROM users WHERE
username = '" + txtUsername.Text +
"' and password = '" + txtPassword.
Text + "'";

The user authenticates (i.e., "logs in") to the application by supplying their credentials, in this case, username and password. After this, if those credentials match what is stored in the database, the user is considered authenticated and the user's information is returned. For example, if the user name is "JohnSmith" and the password is "hsl33s7%," then the aforementioned SQL command that is sent into the database would look like this:


SELECT userid, firstname, last
name FROM users WHERE username =
'JohnSmith' and password = 'hsl33s7%'

Notice how the SQL query is formed. As it is written, I am only returning one user's information. However, what if I don't know the password or the username? This is the strategy of an attacker - figure out a way to exploit any vulnerability. In this case, the vulnerability is that I can send in valid SQL statements that will significantly change the final SQL query sent to the database. If you know a little SQL, you should know what happens when I send in this value in the Username field:


' OR 1=1 --

You get this result SQL statement:


SELECT userid, firstname, lastname
FROM users WHERE username = '' OR
1=1 --

What's happening here is the quote (') is used to end the open quote in the first username check and this is combined with a logical statement that will always evaluate to true ("OR 1=1"). Finally, SQL comments (--) are used to make SQL Server ignore the rest of the SQL query. By using this logical SQL query result, you can get a list of all users without knowing any username or password!

Advanced SQL Injection
At this point, if an attacker has found an entry point with problem code similar to the code shown above, then many valid SQL statements can be sent into the database. Taking the original SQL query above, I could send in this statement in the Username field:


' UNION SELECT null, name, null from
dbo.sysobjects where xtype = 'U' --

As you may know, when you form a UNION statement, you must have the same number of fields as the other SQL statement you are forming the UNION with. In this case, we knew there were three columns, but an attacker can use this method to determine how many fields there are to get the correct results. This particular statement gives you all the names of the tables contained within the database! Again, this is SQL Server-specific, but a similar construct could be used for Oracle, DB2, etc. An attacker can continue sending in SQL statements including INSERTs, UPDATEs, DELETEs, and even the Transact SQL command "SHUTDOWN" (yes, that will shutdown the database!).

Unfortunately, it doesn't end there. Many databases run with a high privileged account, or applications connect to the database with a high privileged account (i.e., "sa" - system administrator account). An attacker can use this fact to further make calls to extended stored procedures such as xp_cmdshell to drop down into a command shell on the database server to then call applications, import files, download password files, and ultimately use the database server as a starting point to attack other computers within the internal network. The possibilities are almost endless.

Protecting Against SQL Injection
How do you guard against SQL injection? One key technique that security experts talk about often and loud is Don't Trust User Input. In our example above, we trust the user to enter the correct credential information in the correct format (no SQL statements imbedded in the input). We are not validating the format of that input, and are therefore blindly letting that data go through the database. Don't do this!

When validating any data input, you should always check for what is expected and correct, and throw away the rest. What does that mean? One obvious method that many applications try to foil this type of attack is to set up what are called signature checks. For example, if I look for "' OR 1=1 -- " in the input data, I will certainly catch that SQL injection attempt. However, these are all equivalent expressions:


' OR 'Test'='Test' --
' OR 2=2 --
' OR 2>1 --
' OR 'Test' IN ('Test') --
etc.

You could try to look for a space between UNION and SELECT as above, but sending in the following could thwart this move:


' UNION /* */SELECT null, name, null
from dbo.sysobjects where xtype =
'U' --

In this case, the /* */ is another comment form. It will be ignored by the database, and you will again have a UNION SELECT construct being sent in. The bottom line regarding signature checks is that almost no matter what invalid input you try to check for (i.e., in this case, valid SQL statements), another variation of an SQL statement can be sent in and will be missed by your checks.

The key is to check for the good input and reject the rest. In our case, the best way to do this is to sanitize the input by using parameterized queries. The earlier code example can be rewritten this way:


string sql = "SELECT userid, first
name, lastname FROM users WHERE
username = @username and password =
@password";
SqlCommand cmd = new SqlCommand(sql);
cmd.Parameters.Add("@username",
SqlDbType.NVarChar).Value = txtUser
name.Text;
cmd.Parameters.Add("@password",
SqlDbType.NVarChar).Value = txtPass
word.Text;

This method will perform type-checking for us, as well as convert the data input (in this case) into a literal string value rather than an SQL statement to be executed. In other words, with this technique, the exploit "' OR 1=1 --" becomes:


SELECT userid, firstname, lastname
FROM users WHERE username = ''' OR
1=1 -- ''' and password = ''

which will return no records because the SQL query will try to do a literal match on "' OR 1=1 --".

Another technique an attacker will use is inspecting error messages to determine what's available through your application. Never display raw error messages to the user when an SQL statement fails. Also, don't display messages that are too helpful to the attacker. For example, when a login fails, rather than saying, "User name 'JohnSmith' has been found, but the password is incorrect. Please try again," say something like, "Login failed. Please try again." This indicates there was a problem, but doesn't tell the attacker any extra information they don't already know about your data.

One other key security technique is to develop your applications following the Principle of Least Privilege. This means not running your database with a high-privileged account and not setting your applications to connect to the database with a high-privileged account. Use a low-privileged SQL user, or, even better; use Windows Security with a domain or local user account to connect to the database. By restricting the privileges of the account with which your application connects to the database as well as what account the database is running under, you can minimize the privileged access if an attacker is able to get through your application using one of the SQL injection techniques above. Using both methods of parameterized queries and secured database access, you are also practicing the security technique of Defense in Depth.

Tools
Though nothing replaces due diligence in applying the secure coding best practices covered above and securing your database, there are some tools you can use to help you pinpoint possible vulnerabilities that currently exist in your code. One nice tool, and it is free, is Microsoft FxCop (at the time of this writing, the latest release is 1.312 for the .NET Framework 1.1 and it can be found here: www.gotdotnet.com/team/fxcop/). This is a great auditing tool to run against your existing .NET code to determine if it conforms to a set of rules that more or less match Microsoft's coding best practices for the .NET Framework. Among the set of rules in the latest version are some Security Rules that watch for SQL injection vulnerabilities.

One of the FxCop Security Rules found the original concatenated strings code above and described the rule used this way:

SQL queries built up from user input are potentially vulnerable to injection attacks. SQL Server, as well as other database servers, supports parameterized SQL queries, a feature that will reduce the risk for injection attacks.

By changing the code to use parameterized queries, this FxCop rule was satisfied.

Conclusion
Every developer should understand how SQL injection works and the best ways to defend against it. Inspect your own code for some of the weaknesses mentioned in this article and be diligent about fixing your code if you find these problems. Don't get caught not knowing these fundamental secure programming techniques. You can find out more about other advanced SQL injection techniques at www.nextgenss.com/papers/advanced_sql_injection.pdf and security-papers.globint.com.ar/oracle_security/sql_injection_in_oracle.php, among other great resources from Application Security, Inc. (www.appsecinc.com/techdocs/whitepapers/research.html) and SPI Dynamics (www.spidynamics.com/support/whitepapers/index.html).

More Stories By Robert Hurlbut

Robert Hurlbut is an independent software architect/developer, consultant, and owner of Hurlbut Consulting, Inc. He is also a Microsoft MVP for Visual C#. Robert specializes in application security, distributed architectures, and database architecture and development. He speaks at industry conferences, including WIN-DEV,
New England Code Camp, Heartland Developers Conference, and DevTeach. He has a .NET blog at http://weblogs.asp.net/rhurlbut.

Comments (0)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


@ThingsExpo Stories
A strange thing is happening along the way to the Internet of Things, namely far too many devices to work with and manage. It has become clear that we'll need much higher efficiency user experiences that can allow us to more easily and scalably work with the thousands of devices that will soon be in each of our lives. Enter the conversational interface revolution, combining bots we can literally talk with, gesture to, and even direct with our thoughts, with embedded artificial intelligence, whic...
Coca-Cola’s Google powered digital signage system lays the groundwork for a more valuable connection between Coke and its customers. Digital signs pair software with high-resolution displays so that a message can be changed instantly based on what the operator wants to communicate or sell. In their Day 3 Keynote at 21st Cloud Expo, Greg Chambers, Global Group Director, Digital Innovation, Coca-Cola, and Vidya Nagarajan, a Senior Product Manager at Google, discussed how from store operations and ...
In his session at 21st Cloud Expo, Carl J. Levine, Senior Technical Evangelist for NS1, will objectively discuss how DNS is used to solve Digital Transformation challenges in large SaaS applications, CDNs, AdTech platforms, and other demanding use cases. Carl J. Levine is the Senior Technical Evangelist for NS1. A veteran of the Internet Infrastructure space, he has over a decade of experience with startups, networking protocols and Internet infrastructure, combined with the unique ability to it...
It is of utmost importance for the future success of WebRTC to ensure that interoperability is operational between web browsers and any WebRTC-compliant client. To be guaranteed as operational and effective, interoperability must be tested extensively by establishing WebRTC data and media connections between different web browsers running on different devices and operating systems. In his session at WebRTC Summit at @ThingsExpo, Dr. Alex Gouaillard, CEO and Founder of CoSMo Software, presented ...
Gemini is Yahoo’s native and search advertising platform. To ensure the quality of a complex distributed system that spans multiple products and components and across various desktop websites and mobile app and web experiences – both Yahoo owned and operated and third-party syndication (supply), with complex interaction with more than a billion users and numerous advertisers globally (demand) – it becomes imperative to automate a set of end-to-end tests 24x7 to detect bugs and regression. In th...
"Space Monkey by Vivent Smart Home is a product that is a distributed cloud-based edge storage network. Vivent Smart Home, our parent company, is a smart home provider that places a lot of hard drives across homes in North America," explained JT Olds, Director of Engineering, and Brandon Crowfeather, Product Manager, at Vivint Smart Home, in this SYS-CON.tv interview at @ThingsExpo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
"Cloud Academy is an enterprise training platform for the cloud, specifically public clouds. We offer guided learning experiences on AWS, Azure, Google Cloud and all the surrounding methodologies and technologies that you need to know and your teams need to know in order to leverage the full benefits of the cloud," explained Alex Brower, VP of Marketing at Cloud Academy, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clar...
"MobiDev is a software development company and we do complex, custom software development for everybody from entrepreneurs to large enterprises," explained Alan Winters, U.S. Head of Business Development at MobiDev, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
SYS-CON Events announced today that CrowdReviews.com has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5–7, 2018, at the Javits Center in New York City, NY. CrowdReviews.com is a transparent online platform for determining which products and services are the best based on the opinion of the crowd. The crowd consists of Internet users that have experienced products and services first-hand and have an interest in letting other potential buye...
"There's plenty of bandwidth out there but it's never in the right place. So what Cedexis does is uses data to work out the best pathways to get data from the origin to the person who wants to get it," explained Simon Jones, Evangelist and Head of Marketing at Cedexis, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
"IBM is really all in on blockchain. We take a look at sort of the history of blockchain ledger technologies. It started out with bitcoin, Ethereum, and IBM evaluated these particular blockchain technologies and found they were anonymous and permissionless and that many companies were looking for permissioned blockchain," stated René Bostic, Technical VP of the IBM Cloud Unit in North America, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Conventi...
WebRTC is great technology to build your own communication tools. It will be even more exciting experience it with advanced devices, such as a 360 Camera, 360 microphone, and a depth sensor camera. In his session at @ThingsExpo, Masashi Ganeko, a manager at INFOCOM Corporation, introduced two experimental projects from his team and what they learned from them. "Shotoku Tamago" uses the robot audition software HARK to track speakers in 360 video of a remote party. "Virtual Teleport" uses a multip...
SYS-CON Events announced today that Telecom Reseller has been named “Media Sponsor” of SYS-CON's 22nd International Cloud Expo, which will take place on June 5-7, 2018, at the Javits Center in New York, NY. Telecom Reseller reports on Unified Communications, UCaaS, BPaaS for enterprise and SMBs. They report extensively on both customer premises based solutions such as IP-PBX as well as cloud based and hosted platforms.
SYS-CON Events announced today that Evatronix will exhibit at SYS-CON's 21st International Cloud Expo®, which will take place on Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA. Evatronix SA offers comprehensive solutions in the design and implementation of electronic systems, in CAD / CAM deployment, and also is a designer and manufacturer of advanced 3D scanners for professional applications.
Leading companies, from the Global Fortune 500 to the smallest companies, are adopting hybrid cloud as the path to business advantage. Hybrid cloud depends on cloud services and on-premises infrastructure working in unison. Successful implementations require new levels of data mobility, enabled by an automated and seamless flow across on-premises and cloud resources. In his general session at 21st Cloud Expo, Greg Tevis, an IBM Storage Software Technical Strategist and Customer Solution Architec...
To get the most out of their data, successful companies are not focusing on queries and data lakes, they are actively integrating analytics into their operations with a data-first application development approach. Real-time adjustments to improve revenues, reduce costs, or mitigate risk rely on applications that minimize latency on a variety of data sources. In his session at @BigDataExpo, Jack Norris, Senior Vice President, Data and Applications at MapR Technologies, reviewed best practices to ...
An increasing number of companies are creating products that combine data with analytical capabilities. Running interactive queries on Big Data requires complex architectures to store and query data effectively, typically involving data streams, an choosing efficient file format/database and multiple independent systems that are tied together through custom-engineered pipelines. In his session at @BigDataExpo at @ThingsExpo, Tomer Levi, a senior software engineer at Intel’s Advanced Analytics gr...
When talking IoT we often focus on the devices, the sensors, the hardware itself. The new smart appliances, the new smart or self-driving cars (which are amalgamations of many ‘things’). When we are looking at the world of IoT, we should take a step back, look at the big picture. What value are these devices providing? IoT is not about the devices, it’s about the data consumed and generated. The devices are tools, mechanisms, conduits. In his session at Internet of Things at Cloud Expo | DXWor...
Everything run by electricity will eventually be connected to the Internet. Get ahead of the Internet of Things revolution. In his session at @ThingsExpo, Akvelon expert and IoT industry leader Sergey Grebnov provided an educational dive into the world of managing your home, workplace and all the devices they contain with the power of machine-based AI and intelligent Bot services for a completely streamlined experience.
SYS-CON Events announced today that Synametrics Technologies will exhibit at SYS-CON's 22nd International Cloud Expo®, which will take place on June 5-7, 2018, at the Javits Center in New York, NY. Synametrics Technologies is a privately held company based in Plainsboro, New Jersey that has been providing solutions for the developer community since 1997. Based on the success of its initial product offerings such as WinSQL, Xeams, SynaMan and Syncrify, Synametrics continues to create and hone inn...