How do prepared statements help us prevent SQL injection attacks?
Wikipedia says:
Prepared statements are resilient against SQL injection, because
parameter values, which are transmitted later using a different
protocol, need not be correctly escaped. If the original statement
template is not derived from external input, SQL injection cannot
occur.
I cannot see the reason very well. What would be a simple explanation in an easy English and some examples?
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.
Here is an SQL statement for setting up an example:
CREATE TABLE employee(name varchar, paymentType varchar, amount bigint);
INSERT INTO employee VALUES('Aaron', 'salary', 100);
INSERT INTO employee VALUES('Aaron', 'bonus', 50);
INSERT INTO employee VALUES('Bob', 'salary', 50);
INSERT INTO employee VALUES('Bob', 'bonus', 0);
The Inject class is vulnerable to SQL injection. The query is dynamically pasted together with user input. The intent of the query was to show information about Bob. Either salary or bonus, based on user input. But the malicious user manipulates the input corrupting the query by tacking on the equivalent of an 'or true' to the where clause so that everything is returned, including the information about Aaron which was supposed to be hidden.
import java.sql.*;
public class Inject {
public static void main(String[] args) throws SQLException {
String url = "jdbc:postgresql://localhost/postgres?user=user&password=pwd";
Connection conn = DriverManager.getConnection(url);
Statement stmt = conn.createStatement();
String sql = "SELECT paymentType, amount FROM employee WHERE name = 'bob' AND paymentType='" + args[0] + "'";
System.out.println(sql);
ResultSet rs = stmt.executeQuery(sql);
while (rs.next()) {
System.out.println(rs.getString("paymentType") + " " + rs.getLong("amount"));
}
}
}
Running this, the first case is with normal usage, and the second with the malicious injection:
c:\temp>java Inject salary
SELECT paymentType, amount FROM employee WHERE name = 'bob' AND paymentType='salary'
salary 50
c:\temp>java Inject "salary' OR 'a'!='b"
SELECT paymentType, amount FROM employee WHERE name = 'bob' AND paymentType='salary' OR 'a'!='b'
salary 100
bonus 50
salary 50
bonus 0
You should not build your SQL statements with string concatenation of user input. Not only is it vulnerable to injection, but it has caching implications on the server as well (the statement changes, so less likely to get a SQL statement cache hit whereas the bind example is always running the same statement).
Here is an example of Binding to avoid this kind of injection:
import java.sql.*;
public class Bind {
public static void main(String[] args) throws SQLException {
String url = "jdbc:postgresql://localhost/postgres?user=postgres&password=postgres";
Connection conn = DriverManager.getConnection(url);
String sql = "SELECT paymentType, amount FROM employee WHERE name = 'bob' AND paymentType=?";
System.out.println(sql);
PreparedStatement stmt = conn.prepareStatement(sql);
stmt.setString(1, args[0]);
ResultSet rs = stmt.executeQuery();
while (rs.next()) {
System.out.println(rs.getString("paymentType") + " " + rs.getLong("amount"));
}
}
}
Running this with the same input as the previous example shows the malicious code does not work because there is no paymentType matching that string:
c:\temp>java Bind salary
SELECT paymentType, amount FROM employee WHERE name = 'bob' AND paymentType=?
salary 50
c:\temp>java Bind "salary' OR 'a'!='b"
SELECT paymentType, amount FROM employee WHERE name = 'bob' AND paymentType=?
Basically, with prepared statements the data coming in from a potential hacker is treated as data - and there's no way it can be intermixed with your application SQL and/or be interpreted as SQL (which can happen when data passed in is placed directly into your application SQL).
This is because prepared statements "prepare" the SQL query first to find an efficient query plan, and send the actual values that presumably come in from a form later - at that time the query is actually executed.
More great info here:
Prepared statements and SQL Injection
I read through the answers and still felt the need to stress the key point which illuminates the essence of Prepared Statements. Consider two ways to query one's database where user input is involved:
Naive Approach
One concatenates user input with some partial SQL string to generate a SQL statement. In this case the user can embed malicious SQL commands, which will then be sent to the database for execution.
String SQLString = "SELECT * FROM CUSTOMERS WHERE NAME='"+userInput+"'"
For example, malicious user input can lead to SQLString being equal to "SELECT * FROM CUSTOMERS WHERE NAME='James';DROP TABLE CUSTOMERS;'
Due to the malicious user, SQLString contains 2 statements, where the 2nd one ("DROP TABLE CUSTOMERS") will cause harm.
Prepared Statements
In this case, due to the separation of the query & data, the user input is never treated as a SQL statement, and thus is never executed. It is for this reason, that any malicious SQL code injected would cause no harm. So the "DROP TABLE CUSTOMERS" would never be executed in the case above.
In a nutshell, with prepared statements malicious code introduced via user input will not be executed!
When you create and send a prepared statement to the DBMS, it's stored as the SQL query for execution.
You later bind your data to the query such that the DBMS uses that data as the query parameters for execution (parameterization). The DBMS doesn't use the data you bind as a supplemental to the already compiled SQL query; it's simply the data.
This means it's fundamentally impossible to perform SQL injection using prepared statements. The very nature of prepared statements and their relationship with the DBMS prevents this.
In SQL Server, using a prepared statement is definitely injection-proof because the input parameters don't form the query. It means that the executed query is not a dynamic query.
Example of an SQL injection vulnerable statement.
string sqlquery = "select * from table where username='" + inputusername +"' and password='" + pass + "'";
Now if the value in the inoutusername variable is something like a' or 1=1 --, this query now becomes:
select * from table where username='a' or 1=1 -- and password=asda
And the rest is commented after --, so it never gets executed and bypassed as using the prepared statement example as below.
Sqlcommand command = new sqlcommand("select * from table where username = #userinput and password=#pass");
command.Parameters.Add(new SqlParameter("#userinput", 100));
command.Parameters.Add(new SqlParameter("#pass", 100));
command.prepare();
So in effect you cannot send another parameter in, thus avoiding SQL injection...
The key phrase is need not be correctly escaped. That means that you don't need to worry about people trying to throw in dashes, apostrophes, quotes, etc...
It is all handled for you.
ResultSet rs = statement.executeQuery("select * from foo where value = " + httpRequest.getParameter("filter");
Let’s assume you have that in a Servlet you right. If a malevolent person passed a bad value for 'filter' you might hack your database.
The simple example:
"select * from myTable where name = " + condition;
And if user input is:
'123'; delete from myTable; commit;
The query will be executed like this:
select * from myTable where name = '123'; delete from myTable; commit;
Root Cause #1 - The Delimiter Problem
Sql injection is possible because we use quotation marks to delimit strings and also to be parts of strings, making it impossible to interpret them sometimes. If we had delimiters that could not be used in string data, sql injection never would have happened. Solving the delimiter problem eliminates the sql injection problem. Structure queries do that.
Root Cause #2 - Human Nature, People are Crafty and Some Crafty People Are Malicious And All People Make Mistakes
The other root cause of sql injection is human nature. People, including programmers, make mistakes. When you make a mistake on a structured query, it does not make your system vulnerable to sql injection. If you are not using structured queries, mistakes can generate sql injection vulnerability.
How Structured Queries Resolve the Root Causes of SQL Injection
Structured Queries Solve The Delimiter Problem, by by putting sql commands in one statement and putting the data in a separate programming statement. Programming statements create the separation needed.
Structured queries help prevent human error from creating critical security holes.
With regard to humans making mistakes, sql injection cannot happen when structure queries are used. There are ways of preventing sql injection that don't involve structured queries, but normal human error in that approaches usually leads to at least some exposure to sql injection. Structured Queries are fail safe from sql injection. You can make all the mistakes in the world, almost, with structured queries, same as any other programming, but none that you can make can be turned into a ssstem taken over by sql injection. That is why people like to say this is the right way to prevent sql injection.
So, there you have it, the causes of sql injection and the nature structured queries that makes them impossible when they are used.
I have gone through various document (SO post as well) about how exactly Prepared statement of PDO protect user from SQL injection.
Although,I understand it protect user because in prepared statement,user record is directly not executing on server insted we are sending positional / named parameter ( ? / :name) and then we send actual data in execute statement, and because of that it saves us from SQL Injection.
Well, Now if I have below code for SQL :
$query = "select * from user where id = $user_input_id";
and user input id = 1
So query will be something like :
$query = "select * from user where id = 1";
This is perfect till now. But if user entre $id = "1; DROP TABLE users;" so query will be something like :
$query = "SELECT * FROM users where id=$id";
and hence ,it will execute
$query = "SELECT * FROM users where id=1; DROP TABLE users;";
It works and out user table will drop because this query directly execute:
Well,I have read that prepared statement can save user from this :
and prepared statement working like :
$data = "1; DROP TABLE users;"
$db->prepare("SELECT * FROM users where id=?");
$db->execute($data);
In execute statement as well,record with Drop table is passing,so how exactly it won't execute drop table statament ? execute also performing some part on server right ?
Anyone can please explain how exactly prepared statement here save user from SQL injection ?
Thanks
Without explicitly setting a type (see PDOStatement::bindValue() for an example), it will treat the passed value as a string, so it will do this effectively:
SELECT * FROM users where id='1; DROP TABLE users;'
Btw, this would actually happen if you're using emulated prepared statements (PDO::ATTR_EMULATE_PREPARES); without this, it will send the parametrised query first followed by the actual data.
That is why you can additionally set the type of binded data to the type you need.
$stm->bindParam(":id", $id, PDO:PARAM_INT)
Additionally, PDO does some escaping of the data, and the string you provided will not break the query at ;, but will be inserted as plain string in the db.
SQL injection is an attack against the SQL parsing step, not the statement execution step. In this, it has similarities to other parse attacks such as cross site scripting and XML injection attacks.
SQL injection works because the common (broken) technique of creating SQL statements by using string concatenation operators to combine both code and (untrusted) data in a single string allows for the possibility of a specially crafted string to violate the statement data protocol (typically by breaking out of a data context using string delimiters embedded in data), and allowing the attacker to manipulate the SQL parser into executing different code to that originally intended.
When one uses a prepared statement, one is telling the parser 'treat the statement purely as trusted code, and provide some slots into which I will insert the data for execution'.
When you drop the string '1; drop table users' into the data slot you created using the '?' placeholder, that string is not processed by the parser, and hence it has no opportunity to influence the parsing of the string : you made it impossible for the contents of the string to break out of a data context.
Using your example, the database will execute the equivalent statement to :
SELECT * FROM users where id="1; drop table users;"
This is a perfectly valid select statement, which may or may not return rows depending on the data in your tables, but which is almost certainly not going to work properly.
Nevertheless, the approach bypassed the attempt at SQL injection.
Be aware : using prepared statements is the ONLY generalised way to avoid SQL injection attacks. In general, attempts to filter untrusted input data are broken.
This question already has answers here:
How can I prevent SQL injection in PHP?
(27 answers)
Closed 9 years ago.
I've build a website that will be going live soon and just have a couple questions about preventing SQL injection, I understand how to use mysqli_real_escape_string but I'm just wondering if I have to use that on all variables that I'm getting for my SQL statement and do I have to use it when I'm doing select statements also or just on insert update and delete? Also what other security would you recommend me implementing before I put the site live, thanks in advance for any help!
Any query can be injected whether it's read or write, persistent or transient. Injections can be performed by ending one query and running a separate one (possible with mysqli), which renders the intended query irrelevant.
Any input to a query from an external source whether it is from users or even internal should be considered an argument to the query, and a parameter in the context of the query. Any parameter in a query needs to be parameterized. This leads to a properly parameterized query that you can create a prepared statement from and execute with arguments. For example:
SELECT col1 FROM t1 WHERE col2 = ?
? is a placeholder for a parameter. Using mysqli, you can create a prepared statement using prepare, bind a variable (argument) to a parameter using bind_param, and run the query with execute. You don't have to sanitize the argument at all (in fact it's detrimental to do so). mysqli does that for you. The full process would be:
$stmt = $mysqli->prepare("SELECT col1 FROM t1 WHERE col2 = ?");
$stmt->bind_param("s", $col2_arg);
$stmt->execute();
There is also an important distinction between parameterized query and prepared statement. This statement, while prepared, is not parameterized and is thus vulnerable to injection:
$stmt = $mysqli->prepare("INSERT INTO t1 VALUES ($_POST[user_input])");
To summarize:
All Queries should be properly parameterized (unless they have no parameters)
All arguments to a query should be treated as hostile as possible no matter their source
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
MySQL Injection - Use SELECT query to UPDATE/DELETE
So I have found in my site bug that allows to perform sql injection
http://mysite.com/script.php?id=1 union select 1,2,3 will output all fields that has Id property equal to one plus one additional row with 1,2,3. I know that I have to validate user input to close my bug.
However my question is quite another. Is it possible to perform update query or insert query? I am able to comment query using --, however I cannot use multiple statements that are delimited by ;. So is it possible to perform update query in my case. I can show PHP code and SQL query if needed.
$sql = "SELECT id, title, text from table where cId=$val";
$result = mysql_query($sql);
$array = mysql_fetch_array($result);
//echo rows in table
Judging from MySQL Injection - Use SELECT query to UPDATE/DELETE
all that is protecting you is a limitation of mysql_query. I would not rely on this, and in particular not that it remains this way over time. You should never rely on a feature to be disabled by default. Maybe the next version already allows statements such as.
SELECT id, title, text from table where cId=1; DROP table table
Nope it is not possible. Most probably you ar running mysql_query, that would not allow multiple queries to be run in one pass. And hence if your query starts with SELECT (as it does), it would not allow any UPDATE injection
Edit: Use mysql_real_escape_string on your input even then
By default this should not be possible. Although there are options for mysql_query to run multiple statements in one string since MySQL 5.0 which you have to set with mysql_set_server_option.
Please consider changing your statement command like this to use mysql_real_escape_string:
$q = mysql_fetch_array(mysql_query("SELECT id, title, text from table where cId = " . mysql_real_escape_string($val)));
At the very best you change your code to use PDO since all mysql_* functions are officially deprecated.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
PDO Prepared Statements
I'm using the mysqli extension in PHP and I'm wondering, is there possibly any way to see a prepared query as it will be executed on the server, e.g. The query is something like this
select * from table1 where id = ? and name = ?
but I want to see the query after the values are filled in, like this:
select * from table1 where id = 20 and name = "John"
Turn on mysql query logging and it will log all queries to a text file for you to review.
Duplicate of PDO Prepared Statements
Short answer: no. A prepared query will never be converted to the query you expect. It's executed directly by the database server. You can use mysql's query log or PDO's undocumented function debugDumpParams, but both are just approximations.
See it where?
If it's your code you have the query and you have the prepared parameters, log them separately or replace in the original query string.
If the binding will fail you will get an error, otherwise you should expect the same values to be "filled" in as you specified them.
Its the way most of the times I am debugging mysql quires:
$q = "select * from table1 where id = ".$id." and name = ".$name;
echo $q;
The output generates all variables assigned to the query.
Hope I understood you exactly, what you wanted.