SQL Injection flaws are introduced when software developers create dynamic database queries that include user supplied input. To avoid SQL injection flaws is simple. Developers need to either:
stop writing dynamic queries; and/or
prevent user supplied input which contains malicious SQL from affecting the logic of the executed query.
Let us look into a set of simple techniques for preventing #SQL #Injection vulnerabilities by avoiding these two problems. These techniques can be used with practically any kind of programming language with any type of database. There are other types of databases, like XML databases, which can have similar problems (e.g., XPath and XQuery injection) and these techniques can be used to protect them as well.
Primary Defenses:
Use of Prepared Statements (with Parameterized Queries)
Use of Stored Procedures
Whitelist Input Validation
Escaping All User Supplied Input
SQL injection flaws typically look like this:
The following (Java) example is UNSAFE, and would allow an attacker to inject code into the query that would be executed by the database. The unvalidated "customerName" parameter that is simply appended to the query allows an attacker to inject any SQL code they want. Unfortunately, this method for accessing databases is all too common.
String query="SELECT account_balance FROM user_data WHERE user_name = "+request.getParameter("customerName");
try{
Statement statement = connection.createStatement(...);
ResultSet results = statement.executeQuery(query);
}Defense Option 1: Prepared Statements (with Parameterized Queries)
The use of prepared statements with variable binding (parameterized queries) is how all developers should first be taught how to write database queries. They are simple to write, and easier to understand than dynamic queries. Parameterized queries force the developer to first define all the SQL code, and then pass in each parameter to the query later. This coding style allows the database to distinguish between code and data, regardless of what user input is supplied.
Prepared statements ensure that an attacker is not able to change the intent of a query, even if SQL commands are inserted by an attacker. In the safe example below, if an attacker were to enter the userID of tom' or '1'='1, the parameterized query would not be vulnerable and would instead look for a username which literally matched the entire string tom' or '1'='1.
Language specific recommendations:
Java – use PreparedStatement() with bind variables
Hibernate - use createQuery() with bind variables (called named parameters in #Hibernate)
Sometimes prepared statements can harm performance. In that case, it is best to either
strongly validate all data OR
escape all user supplied input using an escaping routine specific to your database vendor as described below, rather than using a prepared statement.
Safe Java Prepared Statement Example:
The following code example uses a PreparedStatement, Java's implementation of a parameterized query, to execute the same database query.
// This should also be validated
String custname =request.getParameter("customerName");
// Perform input validation to detect attacks
String query="SELECT account_balance FROM user_data WHERE user_name = ? ";
PreparedStatement pstmt = connection.prepareStatement(query);
pstmt.setString(1,custname);
ResultSet results=pstmt.executeQuery();
Hibernate Query Language (HQL) Prepared Statement (Named Parameters) Examples:
Unsafe HQL Statement:
Query unsafeHQLQuery=session.createQuery("from Inventory where productID='"+userSuppliedParameter+"'");Safe version of the same query using named parameters:
Query safeHQLQuery=session.createQuery("from Inventory where productID=:productid");
safeHQLQuery.setParameter("productid",userSuppliedParameter);Developers tend to like the Prepared Statement approach because all the SQL code stays within the application. This makes your application relatively database independent.
Defense Option 2: Stored Procedures
Stored procedures are not always safe from SQL injection. However, certain standard stored procedure programming constructs have the same effect as the use of parameterized queries when implemented safely which is the norm for most stored procedure languages.
They require the developer to just build SQL statements with parameters which are automatically parameterized unless the developer does something largely out of the norm. The difference between prepared statements and stored procedures is that the SQL code for a stored procedure is defined and stored in the database itself, and then called from the application. Both of these techniques have the same effectiveness in preventing SQL injection so your organization should choose which approach makes the most sense for you.
Safe Java Stored Procedure Example:
The following code example uses a CallableStatement, Java's implementation of the stored procedure interface, to execute the same database query. The sp_getAccountBalance stored procedure would have to be predefined in the database and implement the same functionality as the query defined above.
// This should also be validated
String custname=request.getParameter("customerName");
try{
CallableStatementcs = connection.prepareCall("{
call sp_getAccountBalance(?)
}");
cs.setString(1,custname);
ResultSetresults=cs.executeQuery();
// … result set handling
}catch(SQLExceptionse){
// … logging and error handling
}Defense Option 3: Whitelist Input Validation
Various parts of SQL queries aren't legal locations for the use of bind variables, such as the names of tables or columns, and the sort order indicator (ASC or DESC). In such situations, input validation or query redesign is the most appropriate defense. For the names of tables or columns, ideally those values come from the code, and not from user parameters.
If user parameter values are used for targeting different table names and column names, then the parameter values should be mapped to the legal/expected table or column names to make sure unvalidated user input doesn't end up in the query. Please note, this is a symptom of poor design and a full rewrite should be considered if time allows.
Here is an example of table name validation.
String tableName;
switch(PARAM):
case "Value1": tableName = "fooTable";
break;
case "Value2": tableName = "barTable";
break;
...
default : throw new InputValidationException("unexpected value provided for table name");The tableName can then be directly appended to the SQL query since it is now known to be one of the legal and expected values for a table name in this query. Keep in mind that generic table validation functions can lead to data loss as table names are used in queries where they are not expected.
For something simple like a sort order, it would be best if the user supplied input is converted to a boolean, and then that boolean is used to select the safe value to append to the query. This is a very standard need in dynamic query creation.
For example:
public String someMethod(boolean sortOrder) {
String SQLquery = "some SQL ... order by Salary " + (sortOrder ? "ASC" : "DESC");`Any time user input can be converted to a non-String, like a date, numeric, boolean, enumerated type, etc. before it is appended to a query, or used to select a value to append to the query, this ensures it is safe to do so. Input validation is also recommended as a secondary defense in ALL cases.
Defense Option 4: Escaping All User-Supplied Input
This technique should only be used as a last resort, when none of the above are feasible. Input validation is probably a better choice as this methodology is frail compared to other defenses and we cannot guarantee it will prevent all SQL Injection in all situations.
This technique is to escape user input before putting it in a query. It is very database specific in its implementation. It's usually only recommended to retrofit legacy code when implementing input validation isn't cost effective. Applications built from scratch, or applications requiring low risk tolerance should be built or re-written using parameterized queries, stored procedures, or some kind of Object Relational Mapper (ORM) that builds your queries for you.
Each DBMS supports one or more character escaping schemes specific to certain kinds of queries. If you then escape all user supplied input using the proper escaping scheme for the database you are using, the DBMS will not confuse that input with SQL code written by the developer, thus avoiding any possible SQL injection vulnerabilities.
The OWASP Enterprise Security API (ESAPI) is a free, open source, web application security control library that makes it easier for programmers to write lower-risk applications. The ESAPI libraries are designed to make it easier for programmers to retrofit security into existing applications. The ESAPI libraries also serve as a solid foundation for new development.
Hex-encoding all input
A somewhat special case of escaping is the process of hex-encode the entire string received from the user (this can be seen as escaping every character). The web application should hex-encode the user input before including it in the SQL statement. The SQL statement should take into account this fact, and accordingly compare the data.
For example, if we have to look up a record matching a sessionID, and the user transmitted the string abc123 as the session ID, the select statement would be:
SELECT ... FROM session WHERE hex_encode(sessionID) = '616263313233'
hex_encode should be replaced by the particular facility for the database being used. The string 606162313233 is the hex encoded version of the string received from the user (it is the sequence of hex values of the ASCII/UTF-8 codes of the user data).
If an attacker were to transmit a string containing a single-quote character followed by their attempt to inject SQL code, the constructed SQL statement will only look like:
... WHERE hex_encode ( ... ) = '2720 ... '27 being the ASCII code (in hex) of the single-quote, which is simply hex-encoded like any other character in the string. The resulting SQL can only contain numeric digits and letters a to f, and never any special character that could enable an SQL injection.
Additional Defenses:
Beyond adopting one of the four primary defenses, we also recommend adopting all of these additional defenses in order to provide defense in depth. These additional defenses are:
Least Privilege
Whitelist Input Validation
Least Privilege
To minimize the potential damage of a successful SQL injection attack, you should minimize the privileges assigned to every database account in your environment. Do not assign DBA or admin type access rights to your application accounts. We understand that this is easy, and everything just 'works' when you do it this way, but it is very dangerous.
Start from the ground up to determine what access rights your application accounts require, rather than trying to figure out what access rights you need to take away. Make sure that accounts that only need read access are only granted read access to the tables they need access to.
If an account only needs access to portions of a table, consider creating a view that limits access to that portion of the data and assigning the account access to the view instead, rather than the underlying table. Rarely, if ever, grant create or delete access to database accounts.
If you adopt a policy where you use stored procedures everywhere, and don't allow application accounts to directly execute their own queries, then restrict those accounts to only be able to execute the stored procedures they need. Don't grant them any rights directly to the tables in the database.
SQL injection is not the only threat to your database data. Attackers can simply change the parameter values from one of the legal values they are presented with, to a value that is unauthorized for them, but the application itself might be authorized to access. As such, minimizing the privileges granted to your application will reduce the likelihood of such unauthorized access attempts, even when an attacker is not trying to use SQL injection as part of their exploit.
While you are at it, you should minimize the privileges of the operating system account that the DBMS runs under. Don't run your DBMS as root or system! Most DBMSs run out of the box with a very powerful system account. For example, MySQL runs as system on Windows by default! Change the DBMS's OS account to something more appropriate, with restricted privileges.
Multiple DB Users
The designer of web applications should not only avoid using the same owner/admin account in the web applications to connect to the database. Different DB users could be used for different web applications.
In general, each separate web application that requires access to the database could have a designated database user account that the web-app will use to connect to the DB. That way, the designer of the application can have good granularity in the access control, thus reducing the privileges as much as possible. Each DB user will then have select access to what it needs only, and write-access as needed.
As an example, a login page requires read access to the username and password fields of a table, but no write access of any form (no insert, update, or delete). However, the sign-up page certainly requires insert privilege to that table; this restriction can only be enforced if these web apps use different DB users to connect to the database.
Views
You can use SQL views to further increase the granularity of access by limiting the read access to specific fields of a table or joins of tables. It could potentially have additional benefits: for example, suppose that the system is required (perhaps due to some specific legal requirements) to store the passwords of the users, instead of salted-hashed passwords.
The designer could use views to compensate for this limitation; revoke all access to the table (from all DB users except the owner/admin) and create a view that outputs the hash of the password field and not the field itself. Any SQL #injection attack that succeeds in stealing DB information will be restricted to stealing the hash of the passwords (could even be a keyed hash), since no DB user for any of the web applications has access to the table itself.
Whitelist Input Validation
In addition to being a primary defense when nothing else is possible (e.g., when a bind variable isn't legal), input validation can also be a secondary defense used to detect unauthorized input before it is passed to the SQL #query. For more information please see the Input Validation Cheat Sheet. Proceed with caution here. Validated data is not necessarily safe to insert into SQL queries via string building.
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