One of the most common web server attacks attempts to compromise backend databases in order to expose confidential information such as customer information, company data, and so on. Since the stakes are so high, whether you’re an application owner or a casual user, answering questions like “what is SQL injection?” and “what are some of the most frequently used SQL injection types?” would hopefully help us learn all about SQL injection attacks, how they function, and how to avoid them.
As a result, once we have a better understanding of what constitutes a high-risk vulnerability, we will explore how to avoid SQL injection attacks in this post.
What Is SQL Injection and How Does It Work?
SQL (structured query language, pronounced “sequel” or “S-Q-L” depending on the user) injection attacks often appear on the OWASP vulnerabilities list, which lists the industry’s top 10 web application security threats. SQLi attacks, as they’re also called, depend on flaws in input validation that can be exploited to run a designed query as a SQL command on the site’s backend database.
The backend database server can be accessed using user-supplied data (a malicious string sent as a search query). The question may be performed on the server without sufficient input validation tests. Aside from data protection concerns, attackers may use this flaw to delete tables and cause chaos, seriously disrupting the business operations. SQL injection bugs may result in a variety of undesirable and catastrophic outcomes, ranging from verbose error messages that could unwittingly reveal helpful information to a potential attacker to confidential data stored on the server being leaked.
Let’s take a look at what a standard SQL query looks like and how it’s handled by the database. This will help you grasp the concept of SQL injection and how it functions. Consider an unreliable e-commerce programme that has no safeguards against SQL injection attacks. When a user tries to log in, their browser sends the following request:
https://insecure-app.com/login
The SQL query that runs on the backend database to log in the user after the credentials are submitted is shown below:
SELECT * FROM Users WHERE username=‘admin’ AND password=’adminpass’;The asterisk symbol (*) indicates that all the columns from the table “Users” will display if the login is successful. But now consider that an attacker tries to manipulate the database to log in as the admin user without knowing the password and enters the username as admin’–:
SELECT * FROM Users WHERE username=‘admin’--’ AND password=’passw0rd’;A common trick used in SQL injection attacks is to use line comments (–) to trick the database into disregarding the remaining query in order to avoid filling out mandatory parameters or avoiding syntax errors. The issue with the above question is that the database has commented out the “AND” argument, causing the password parameter to be ignored. In a SQL argument, an input parameter serves as a placeholder for data that can be transferred at runtime.
So, how can we find out if our applications have this flaw? A popular research tool called sqlmap can be used to recognise and exploit six types of SQL injection vulnerabilities in addition to manual testing. Detection may also be done with other instruments, such as:
Types of SQL Injection Attacks
SQL injection attacks can be divided into three categories when it comes to their types:
In-Band SQLi
In-band SQL injection is one of the most popular SQL injection forms, in which the data appears on the same channel as the malicious code. Two of the most popular in-band SQL injection attack strategies are error-based and union-based SQL injection attacks. A verbose error (retrieved data) appears on the web page in response to a malformed or unexpected query in error-based SQLi attacks (the malicious user input). An in-band SQL injection attack is demonstrated here.
based on nion SQLi involves combining the results of two or more SELECT statements into a single query using the UNION SQL operator, and it can be used to retrieve data from several tables within the database.
Out-of-Band SQLi
Out-of-Band is a term that refers to a musician who is SQLi is a form of SQL injection that is less common. When an intruder is unable to retrieve data using the same channel that he used to initiate the attack, this is known as a failure. The response to the attack is sent via other means, such as email, or it relies on the application’s database server’s ability to send DNS or HTTP requests to a server managed by the attacker.
Users with DBA permissions, for example, can use xp cmdshell to run commands on a remote database host. Attackers may email the results or find a way to pass the appropriate information to themselves once the question is executed.
Inferential SQLi
There is no real data transfer in this form of SQL injection, also known as blind SQLi. Attackers, on the other hand, will monitor how the application responds to payloads, gaining insight into whether the query is performed or how the database handles the queries. Although it’s much easier to reconstruct the logic of the original query with verbose errors, an attacker who successfully executes a blind SQL injection attack on the application can reverse engineer the logic to get to the original query.
Another commonly used approach is a time-based blind SQLi attack, which involves analysing the pause in response to see whether the query is executed. Consider the following scenario:
https://insecure-app.com/attack.php?id=1’ and sleep(10)--There is no delay in page loading after submitting the above message. If the comment is changed, as seen below, and the answer is delayed, it means the question was executed. The intruder now understands how to comment out portions of a query using the application’s syntax. If they have this information, they will use it to continue targeting the application in future techniques.
https://insecure-app.com/attack.php?id=1’ and sleep(10)#Now that we know what a SQL injection is and what kinds of SQL injection attacks exist, it’s time to look at ways to avoid them.
How to Prevent SQL Injection Attacks
When it comes to preventing SQL injection attacks, the most critical precautionary steps are probably the use of parameterized queries and proper input validation tests, in addition to using web application firewalls (like ModSecurity or NAXSI on NGINX) to filter out malicious or dangerous requests. They aren’t, though, the only protective measures you can take.
Let’s look at a few other tips to help you avoid SQL injection attacks in addition to these tools:
1. Don’t Trust Any User-Supplied Input
The primary access points used to carry out SQLi attacks are user input channels, and it is here that input validation comes into play. Only inputs that meet a particular set of requirements are permitted, thanks to strict context-dependent validation checks applied early in the data flow (which verify that the sort, length, format, allowed characters, and so on match expected values). Apart from using context-based semantic validation, implementing syntactic validation to ensure accurate syntax is often useful for structured data.
Consider escaping all user-controllable data using the supported character escaping scheme, but this isn’t as successful as input validation.
2. Refrain From Using Dynamic SQL Queries Whenever Possible
Since the SQL code and issuing logic are designed while the user input is being processed, dynamic queries used in an unstable manner may lead to SQL injection vulnerabilities. Using a parameterized query, on the other hand, the SQL logic is specified before the user input is passed as parameters. As a consequence, user feedback has no effect on the logic and is simply passed as a parameter dependent on the data form it represents.
Another alternative is to use stored procedures that are implemented safely without the use of unsafe dynamic SQL generation. Stored procedures are SQL statements with parameters (that are automatically parameterized) that are stored in the database and only called during execution rather than being written again and again. However, stored procedures that lack adequate input validation checks and execution rights can actually increase an application’s risk potential rather than minimise it.
3. Use Accounts With Least Privileges to Restrict Access in Case of a Breach
The risks of unauthorised access can be minimised by restricting access and reducing privileges for each account. This can be achieved by:
- Identifying the extent of access rights that each account requires, as well as the particular tables or parts of it to which it requires access.
- Creating several database users for various purposes.
- Holding administrative rights out of the picture.
- Views are used to restrict access to underlying tables.
- Wherever possible, assigning only read access.
- Before granting build or remove access to any database account, think about the consequences.
4. Rely More on Whitelists Since Most Persistent Attackers Can Find a Way Around Blacklists
We spoke about input validation earlier, and there are two approaches to it: blacklists and whitelists. Whitelisting compares user feedback to a list of allowed characters, while blacklisting is used to block known malicious characters. These two methods are often used to block traffic from questionable IP addresses or to restrict access to whitelisted addresses.
Whitelists are usually considered a more successful solution when it comes to protection due to emerging attack tactics that can subvert filters.
5. Use Appropriate Technology and Frameworks When Developing Your Application
The majority of today’s software frameworks have built-in SQL injection security mechanisms that aren’t present in older development technologies. Building your application with modern frameworks and environments will protect you from such attacks from the start.
To minimise risks, for example, use the object relational mapping (ORM) system or PHP data objects (PDO) instead of MySQLi in PHP. However, not all of these are foolproof, and it is more often than not sufficient to use a mixture of techniques. The vulnerability of the Sequelize ORM to SQLi highlights how ORM-generated code can often be vulnerable to injection attacks.
6. Conduct Regular Vulnerability Scans and Code Reviews to Detect Potential Second-Order Attacks
Your application will be extensively investigated for any SQL injection or other possible vulnerabilities that could lead to a security incident using automated vulnerability scanning software and manual web application security assessments. A second-order SQL injection attack occurs when a user input is inserted into the database but does not elicit an immediate response unless triggered by a future event.
So, how do you keep these kinds of attacks from happening? To rule out the possibility of second-order SQL injection attacks, secure code reviews can be performed.
7. Install Updates and Patches Regularly to Your Applications
You can defend yourself against known vulnerabilities by using updated software and applying patches to your applications — and even the underlying systems on which they operate. When developers fix a bug discovered or recorded in previous releases, they issue a security patch or an upgrade. Until these updates are installed, the application would remain vulnerable to attacks based on security vulnerabilities that could have been prevented with a simple update.
8. Organize Security Awareness Workshops to Educate Your Staff
If you’re serious about security, you’ve probably already developed security awareness as a part of your company’s culture, which goes beyond holding annual or biannual workshops. It’s important to persuade the staff to be aware of the fact that cyber threats are on the rise. This makes any employee of a company feel accountable for ensuring protection on a regular basis.
General security issues or unique questions — such as what is SQL injection and how to avoid SQL injection attacks — can be discussed at awareness seminars to keep the staff up to date on the types of security incidents that are normal. To prevent negligent activities, however, protection necessitates a shift in attitude. Mandatory preparation and seminars can only get you so far before that happens.
Final Thoughts
Since its inception in 2003, the OWASP top 10 list has included SQL injection attacks. It’s been around for a while, and it’s still important, thanks to a misguided emphasis when it comes to putting security controls in place. Since attackers continue to find new ways to circumvent existing controls, this relevance is unlikely to fade anytime soon.
Hopefully, we were able to answer your questions about SQL injection and the different forms of SQL injection. We’ve also given you some tips for preventing SQL injection attacks, including using a combination of parameterized queries, whitelists, input validation, and database controls (like the LIMIT SQL clause to prevent mass disclosure in case of a successful breach). Subscribing to our blog will keep you up to date on future posts about SQL injection attacks.