How Does SQL-Injection Work and How to Protect Against It

How can prepared statements protect from SQL injection attacks?

The idea is very simple - the query and the data are sent to the database server separately.

That's all.

The root of the SQL injection problem is in the mixing of the code and the data.

In fact, our SQL query is a legitimate program.
And we are creating such a program dynamically, adding some data on the fly. Thus, the data may interfere with the program code and even alter it, as every SQL injection example shows it (all examples in PHP/Mysql):

$expected_data = 1;
$query = "SELECT * FROM users where id=$expected_data";

will produce a regular query

SELECT * FROM users where id=1

while this code

$spoiled_data = "1; DROP TABLE users;"
$query        = "SELECT * FROM users where id=$spoiled_data";

will produce a malicious sequence

SELECT * FROM users where id=1; DROP TABLE users;

It works because we are adding the data directly to the program body and it becomes a part of the program, so the data may alter the program, and depending on the data passed, we will either have a regular output or a table users deleted.

While in case of prepared statements we don't alter our program, it remains intact
That's the point.

We are sending a program to the server first

$db->prepare("SELECT * FROM users where id=?");

where the data is substituted by some variable called a parameter or a placeholder.

Note that exactly the same query is sent to the server, without any data in it! And then we're sending the data with the second request, essentially separated from the query itself:

$db->execute($data);

so it can't alter our program and do any harm.

Quite simple - isn't it?

The only thing I have to add that always omitted in the every manual:

Prepared statements can protect only data literals, but cannot be used with any other query part.

So, once we have to add, say, a dynamical identifier - a field name, for example - prepared statements can't help us. I've explained the matter recently, so I won't repeat myself.

How does SQL-injection work and how do I protect against it

I cannot resist aswell.

SQL Injection is "a code injection technique that exploits a security vulnerability occurring in the database layer of an application". In other words it's SQL code injected in as user input inside a query.

SQL Injections can manipulate data (delete, update, add ecc...) and corrupt or delete tables of the database. I'm not aware of SQL Injections manipulating scripts though.

Let's say in your PHP script you are expecting (as user input) a username and a password from the login form that are later used inside a query such as:

SELECT Id FROM Users WHERE Name = $name AND Password = $password;

The user can insert inside $name and as $password whatever he likes (for example trough an <input>). Let's imagine he adds a name such as "1 OR 1 = 1; --", the query will now look like:

SELECT Id FROM Users WHERE Name = 1 OR 1 = 1; -- AND Password = $password;

and then, after the ; I could add another query or make the script think that the username and the password actually exists.

Notice that -- AND Password = $password; is a SQL comment and will therefore be ignored.

If you are using PHP < 5 then you should look for mysql_real_escape_string() and use it to escape user inputs before embedding it inside a query.

If you are using PHP5+ you should use PDO or the mysqli extension which can prevent this problem via prepared statements.

How spring/hibernate gives us guarantee to protect from SQL injection and how it's handled internally?

Hibernate does provide security from SQL injection if you use the API properly.

From: https://www.owasp.org/index.php/Hibernate#A_note_about_SQL_injection

A note about SQL injection
Since it is the hot topic, I will address it now but discuss in detail later.
Hibernate does not grant immunity to SQL Injection, one can misuse the API as they please.
There is nothing special about HQL (Hibernates subset of SQL) that makes it any more or less susceptible.
Functions such as createQuery(String query) and createSQLQuery(String query) create a Query object that will be executed when the call to commit() is made. If the query string is tainted you have SQL injection. The details of these functions are covered later.

Always use the PreparedStatement to prevent SQL injection, it is part of JDBC API and Hibernate itself uses this API see.

For example:

String query1 = "select * from MyBean where id = "+ id;//Not secure
String query2 = "select * from MyBean where id = :id";//Secure

A useful article on this topic: http://software-security.sans.org/developer-how-to/fix-sql-injection-in-java-hibernate

How can I prevent SQL injection in PHP?

The correct way to avoid SQL injection attacks, no matter which database you use, is to separate the data from SQL, so that data stays data and will never be interpreted as commands by the SQL parser. It is possible to create an SQL statement with correctly formatted data parts, but if you don't fully understand the details, you should always use prepared statements and parameterized queries. These are SQL statements that are sent to and parsed by the database server separately from any parameters. This way it is impossible for an attacker to inject malicious SQL.

You basically have two options to achieve this:

Using PDO (for any supported database driver):

$stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');
$stmt->execute([ 'name' => $name ]);

foreach ($stmt as $row) {
    // Do something with $row
}

Using MySQLi (for MySQL):

Since PHP 8.2+ we can make use of execute_query() which prepares, binds parameters, and executes SQL statement in one method:

$result = $dbConnection->execute_query('SELECT * FROM employees WHERE name = ?', [$name]);

while ($row = $result->fetch_assoc()) {
    // Do something with $row
}

Up to PHP8.1:

$stmt = $dbConnection->prepare('SELECT * FROM employees WHERE name = ?');
$stmt->bind_param('s', $name); // 's' specifies the variable type => 'string'
$stmt->execute();

$result = $stmt->get_result();
while ($row = $result->fetch_assoc()) {
    // Do something with $row
}

If you're connecting to a database other than MySQL, there is a driver-specific second option that you can refer to (for example, pg_prepare() and pg_execute() for PostgreSQL). PDO is the universal option.

Correctly setting up the connection

PDO

Note that when using PDO to access a MySQL database real prepared statements are not used by default. To fix this you have to disable the emulation of prepared statements. An example of creating a connection using PDO is:

$dbConnection = new PDO('mysql:dbname=dbtest;host=127.0.0.1;charset=utf8mb4', 'user', 'password');

$dbConnection->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$dbConnection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

In the above example, the error mode isn't strictly necessary, but it is advised to add it. This way PDO will inform you of all MySQL errors by means of throwing the PDOException.

What is mandatory, however, is the first setAttribute() line, which tells PDO to disable emulated prepared statements and use real prepared statements. This makes sure the statement and the values aren't parsed by PHP before sending it to the MySQL server (giving a possible attacker no chance to inject malicious SQL).

Although you can set the charset in the options of the constructor, it's important to note that 'older' versions of PHP (before 5.3.6) silently ignored the charset parameter in the DSN.

Mysqli

For mysqli we have to follow the same routine:

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT); // error reporting
$dbConnection = new mysqli('127.0.0.1', 'username', 'password', 'test');
$dbConnection->set_charset('utf8mb4'); // charset

Explanation

The SQL statement you pass to prepare is parsed and compiled by the database server. By specifying parameters (either a ? or a named parameter like :name in the example above) you tell the database engine where you want to filter on. Then when you call execute, the prepared statement is combined with the parameter values you specify.

The important thing here is that the parameter values are combined with the compiled statement, not an SQL string. SQL injection works by tricking the script into including malicious strings when it creates SQL to send to the database. So by sending the actual SQL separately from the parameters, you limit the risk of ending up with something you didn't intend.

Any parameters you send when using a prepared statement will just be treated as strings (although the database engine may do some optimization so parameters may end up as numbers too, of course). In the example above, if the $name variable contains 'Sarah'; DELETE FROM employees the result would simply be a search for the string "'Sarah'; DELETE FROM employees", and you will not end up with an empty table.

Another benefit of using prepared statements is that if you execute the same statement many times in the same session it will only be parsed and compiled once, giving you some speed gains.

Oh, and since you asked about how to do it for an insert, here's an example (using PDO):

$preparedStatement = $db->prepare('INSERT INTO table (column) VALUES (:column)');

$preparedStatement->execute([ 'column' => $unsafeValue ]);

Can prepared statements be used for dynamic queries?

While you can still use prepared statements for the query parameters, the structure of the dynamic query itself cannot be parametrized and certain query features cannot be parametrized.

For these specific scenarios, the best thing to do is use a whitelist filter that restricts the possible values.

// Value whitelist
// $dir can only be 'DESC', otherwise it will be 'ASC'
if (empty($dir) || $dir !== 'DESC') {
   $dir = 'ASC';
}

Can I protect against SQL injection by escaping single-quote and surrounding user input with single-quotes?

First of all, it's just bad practice. Input validation is always necessary, but it's also always iffy.

Worse yet, blacklist validation is always problematic, it's much better to explicitly and strictly define what values/formats you accept. Admittedly, this is not always possible - but to some extent it must always be done.

Some research papers on the subject:

http://www.imperva.com/docs/WP_SQL_Injection_Protection_LK.pdf
http://www.it-docs.net/ddata/4954.pdf (Disclosure, this last one was mine ;) )
https://www.owasp.org/images/d/d4/OWASP_IL_2007_SQL_Smuggling.pdf (based on the previous paper, which is no longer available)

Point is, any blacklist you do (and too-permissive whitelists) can be bypassed. The last link to my paper shows situations where even quote escaping can be bypassed.

Even if these situations do not apply to you, it's still a bad idea. Moreover, unless your app is trivially small, you're going to have to deal with maintenance, and maybe a certain amount of governance: how do you ensure that its done right, everywhere all the time?

The proper way to do it:

Whitelist validation: type, length, format or accepted values
If you want to blacklist, go right ahead. Quote escaping is good, but within context of the other mitigations.
Use Command and Parameter objects, to preparse and validate
Call parameterized queries only.
Better yet, use Stored Procedures exclusively.
Avoid using dynamic SQL, and dont use string concatenation to build queries.
If using SPs, you can also limit permissions in the database to executing the needed SPs only, and not access tables directly.
you can also easily verify that the entire codebase only accesses the DB through SPs...

Can I protect against SQL injection attacks by wrapping SQL in PostgreSQL functions?

Problem (exploit)

client.query(`select create_user(${someUserInput})`

The problem there is what happens if

let someUserInput = `'foo'); DROP DATABASE bar;`;

That will get sent to your call as,

client.query("select create_user('foo'); DROP DATABASE bar;")`

And, that would be bad. Yes, the argument to create_user is protected against injection, but the call to that isn't.

Solutions

Use placeholders (obvious choice: most failsafe and secure solution.)
Ensure someUserInput is properly quoted
1. Use the client-library to quote it with something like PQescapeLiteral
2. Use a second run to the server to quote it with quote_literal (requires placeholders anyway). SELECT quote_literal($1);

I would not try to create the quoting-mechanism myself.

what sql injection can do to harm you

Potentially, sure. If you can inject a DROP TABLE table; into the SQL statement that is executed, you could just as easily inject an UPDATE statement that modified whatever rows of whatever tables you'd like. You can also frequently add or modify a SELECT statement to show you information that you're interested in. For example, if you have a query like

select name
  from people
 where person_id = '$person'

you could inject something like

anything` union all select table_name from information_schema.tables

to produce a statement like

select name
  from people
 where person_id = 'anything'
union all
select table_name
  from information_schema.tables

to show you all the tables. You can do the same sort of thing to get a list of columns in the tables and then start running queries to see what data is in the various tables.