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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
$stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');
$stmt->execute([ 'name' => $name ]);
foreach ($stmt as $row) {
// Do something with $row
}
Using
Since PHP 8.2+ we can make use of
$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() andpg_execute() for PostgreSQL). PDO is the universal option.
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 thePDOException.
What is mandatory, however, is the firstsetAttribute() 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 thecharset 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.
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');
$dbConnection->set_charset('utf8mb4'); // charset
The SQL statement you pass toprepare is parsed and compiled by the database server. By specifying parameters (either a? or a named parameter like:name in the example above) you tell the database engine where you want to filter on. Then when you callexecute, the prepared statement is combined with the parameter values you specify.
The important thing here is that the parameter values are combined with the compiled statement, not an SQL string. SQL injection works by tricking the script into including malicious strings when it creates SQL to send to the database. So by sending the actual SQL separately from the parameters, you limit the risk of ending up with something you didn't intend.
Any parameters you send when using a prepared statement will just be treated as strings (although the database engine may do some optimization so parameters may end up as numbers too, of course). In the example above, if the$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 ]);
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';
}
To use the parameterized query, you need to use either Mysqli or PDO. To rewrite your example with mysqli, we would need something like the following.
<?php
mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli("server", "username", "password", "database_name");
$variable = $_POST["user-input"];
$stmt = $mysqli->prepare("INSERT INTO table (column) VALUES (?)");
// "s" means the database expects a string
$stmt->bind_param("s", $variable);
$stmt->execute();
The key function you'll want to read up on there would be
Also, as others have suggested, you may find it useful/easier to step up a layer of abstraction with something like PDO.
Please note that the case you asked about is a fairly simple one and that more complex cases may require more complex approaches. In particular:
mysql_real_escape_string. In this kind of case, you would be better off passing the user's input through a whitelist to ensure only 'safe' values are allowed through.Every answer here covers only part of the problem. In fact, there are four different query parts which we can add to SQL dynamically: -
And prepared statements cover only two of them.
But sometimes we have to make our query even more dynamic, adding operators or identifiers as well. So, we will need different protection techniques.
In general, such a protection approach is based on whitelisting.
In this case, every dynamic parameter should be hardcoded in your script and chosen from that set. For example, to do dynamic ordering:
$orders = array("name", "price", "qty"); // Field names
$key = array_search($_GET['sort'], $orders)); // if we have such a name
$orderby = $orders[$key]; // If not, first one will be set automatically.
$query = "SELECT * FROM `table` ORDER BY $orderby"; // Value is safe
To ease the process I wrote a whitelist helper function that does all the job in one line:
$orderby = white_list($_GET['orderby'], "name", ["name","price","qty"], "Invalid field name");
$query = "SELECT * FROM `table` ORDER BY `$orderby`"; // sound and safe
There is another way to secure identifiers - escaping but I rather stick to whitelisting as a more robust and explicit approach. Yet as long as you have an identifier quoted, you can escape the quote character to make it safe. For example, by default for mysql you have to double the quote character to escape it. For other other DBMS escaping rules would be different.
Still, there is an issue with SQL syntax keywords (such asAND,DESC and such), but white-listing seems the only approach in this case.
So, a general recommendation may be phrased as
- Any variable that represents an SQL data literal, (or, to put it simply - an SQL string, or a number) must be added through a prepared statement. No Exceptions.
- Any other query part, such as an SQL keyword, a table or a field name, or an operator - must be filtered through a white list.
Although there is a general agreement on the best practices regarding SQL injection protection, there are still many bad practices as well. And some of them too deeply rooted in the minds of PHP users. For instance, on this very page there are (although invisible to most visitors) more than 80 deleted answers - all removed by the community due to bad quality or promoting bad and outdated practices. Worse yet, some of the bad answers aren't deleted, but rather prospering.
For example, there(1) are(2) still(3) many(4) answers(5), including the second most upvoted answer suggesting you manual string escaping - an outdated approach that is proven to be insecure.
Or there is a slightly better answer that suggests just another method of string formatting and even boasts it as the ultimate panacea. While of course, it is not. This method is no better than regular string formatting, yet it keeps all its drawbacks: it is applicable to strings only and, like any other manual formatting, it's essentially optional, non-obligatory measure, prone to human error of any sort.
I think that all this because of one very old superstition, supported by such authorities like OWASP or the PHP manual, which proclaims equality between whatever "escaping" and protection from SQL injections.
Regardless of what PHP manual said for ages, *_escape_string by no means makes data safe and never has been intended to. Besides being useless for any SQL part other than string, manual escaping is wrong, because it is manual as opposite to automated.
And OWASP makes it even worse, stressing on escaping user input which is an utter nonsense: there should be no such words in the context of injection protection. Every variable is potentially dangerous - no matter the source! Or, in other words - every variable has to be properly formatted to be put into a query - no matter the source again. It's the destination that matters. The moment a developer starts to separate the sheep from the goats (thinking whether some particular variable is "safe" or not) he/she takes his/her first step towards disaster. Not to mention that even the wording suggests bulk escaping at the entry point, resembling the very magic quotes feature - already despised, deprecated and removed.
So, unlike whatever "escaping", prepared statements is the measure that indeed protects from SQL injection (when applicable).
I'd recommend using PDO (PHP Data Objects) to run parameterized SQL queries.
Not only does this protect against SQL injection, but it also speeds up queries.
And by using PDO rather thanmysql_,mysqli_, andpgsql_ functions, you make your application a little more abstracted from the database, in the rare occurrence that you have to switch database providers.
UsePDO and prepared queries.
($conn is aPDO object)
$stmt = $conn->prepare("INSERT INTO tbl VALUES(:id, :name)");
$stmt->bindValue(':id', $id);
$stmt->bindValue(':name', $name);
$stmt->execute();
As you can see, people suggest you use prepared statements at the most. It's not wrong, but when your query is executed just once per process, there would be a slight performance penalty.
I was facing this issue, but I think I solved it in very sophisticated way - the way hackers use to avoid using quotes. I used this in conjunction with emulated prepared statements. I use it to prevent all kinds of possible SQL injection attacks.
If you expect input to be integer make sure it's really integer. In a variable-type language like PHP it is this very important. You can use for example this very simple but powerful solution:sprintf("SELECT 1,2,3 FROM table WHERE 4 = %u", $input);
If you expect anything else from integer hex it. If you hex it, you will perfectly escape all input. In C/C++ there's a function called
Don't worry about that the escaped string will have a 2x size of its original length because even if you usemysql_real_escape_string, PHP has to allocate same capacity((2*input_length)+1), which is the same.
This hex method is often used when you transfer binary data, but I see no reason why not use it on all data to prevent SQL injection attacks. Note that you have to prepend data with0x or use the MySQL functionUNHEX instead.
So, for example, the query:
SELECT password FROM users WHERE name = 'root';
Will become:
SELECT password FROM users WHERE name = 0x726f6f74;
or
SELECT password FROM users WHERE name = UNHEX('726f6f74');
Hex is the perfect escape. No way to inject.
There was some discussion in comments, so I finally want to make it clear. These two approaches are very similar, but they are a little different in some ways:
The0x prefix can only be used for data columns such aschar,varchar,text,block,binary, etc.
Also, its use is a little complicated if you are about to insert an empty string. You'll have to entirely replace it with'', or you'll get an error.
UNHEX() works on any column; you do not have to worry about the empty string.
Note that this hex method is often used as an SQL injection attack where integers are just like strings and escaped just withmysql_real_escape_string. Then you can avoid the use of quotes.
For example, if you just do something like this:
"SELECT title FROM article WHERE id = " . mysql_real_escape_string($_GET["id"])
an attack can inject you very easily. Consider the following injected code returned from your script:
SELECT ... WHERE id = -1 UNION ALL SELECT table_name FROM information_schema.tables;
and now just extract table structure:
SELECT ... WHERE id = -1 UNION ALL SELECT column_name FROM information_schema.column WHERE table_name = __0x61727469636c65__;
And then just select whatever data ones want. Isn't it cool?
But if the coder of an injectable site would hex it, no injection would be possible because the query would look like this:
SELECT ... WHERE id = UNHEX('2d312075...3635');
