Related
I am very new to working with databases. Now I can write SELECT, UPDATE, DELETE, and INSERT commands. But I have seen many forums where we prefer to write:
SELECT empSalary from employee where salary = #salary
...instead of:
SELECT empSalary from employee where salary = txtSalary.Text
Why do we always prefer to use parameters and how would I use them?
I wanted to know the use and benefits of the first method. I have even heard of SQL injection but I don't fully understand it. I don't even know if SQL injection is related to my question.
Using parameters helps prevent SQL Injection attacks when the database is used in conjunction with a program interface such as a desktop program or web site.
In your example, a user can directly run SQL code on your database by crafting statements in txtSalary.
For example, if they were to write 0 OR 1=1, the executed SQL would be
SELECT empSalary from employee where salary = 0 or 1=1
whereby all empSalaries would be returned.
Further, a user could perform far worse commands against your database, including deleting it If they wrote 0; Drop Table employee:
SELECT empSalary from employee where salary = 0; Drop Table employee
The table employee would then be deleted.
In your case, it looks like you're using .NET. Using parameters is as easy as:
string sql = "SELECT empSalary from employee where salary = #salary";
using (SqlConnection connection = new SqlConnection(/* connection info */))
using (SqlCommand command = new SqlCommand(sql, connection))
{
var salaryParam = new SqlParameter("salary", SqlDbType.Money);
salaryParam.Value = txtMoney.Text;
command.Parameters.Add(salaryParam);
var results = command.ExecuteReader();
}
Dim sql As String = "SELECT empSalary from employee where salary = #salary"
Using connection As New SqlConnection("connectionString")
Using command As New SqlCommand(sql, connection)
Dim salaryParam = New SqlParameter("salary", SqlDbType.Money)
salaryParam.Value = txtMoney.Text
command.Parameters.Add(salaryParam)
Dim results = command.ExecuteReader()
End Using
End Using
Edit 2016-4-25:
As per George Stocker's comment, I changed the sample code to not use AddWithValue. Also, it is generally recommended that you wrap IDisposables in using statements.
You are right, this is related to SQL injection, which is a vulnerability that allows a malicioius user to execute arbitrary statements against your database. This old time favorite XKCD comic illustrates the concept:
In your example, if you just use:
var query = "SELECT empSalary from employee where salary = " + txtSalary.Text;
// and proceed to execute this query
You are open to SQL injection. For example, say someone enters txtSalary:
1; UPDATE employee SET salary = 9999999 WHERE empID = 10; --
1; DROP TABLE employee; --
// etc.
When you execute this query, it will perform a SELECT and an UPDATE or DROP, or whatever they wanted. The -- at the end simply comments out the rest of your query, which would be useful in the attack if you were concatenating anything after txtSalary.Text.
The correct way is to use parameterized queries, eg (C#):
SqlCommand query = new SqlCommand("SELECT empSalary FROM employee
WHERE salary = #sal;");
query.Parameters.AddWithValue("#sal", txtSalary.Text);
With that, you can safely execute the query.
For reference on how to avoid SQL injection in several other languages, check bobby-tables.com, a website maintained by a SO user.
In addition to other answers need to add that parameters not only helps prevent sql injection but can improve performance of queries. Sql server caching parameterized query plans and reuse them on repeated queries execution. If you not parameterized your query then sql server would compile new plan on each query(with some exclusion) execution if text of query would differ.
More information about query plan caching
Two years after my first go, I'm recidivating...
Why do we prefer parameters? SQL injection is obviously a big reason, but could it be that we're secretly longing to get back to SQL as a language. SQL in string literals is already a weird cultural practice, but at least you can copy and paste your request into management studio. SQL dynamically constructed with host language conditionals and control structures, when SQL has conditionals and control structures, is just level 0 barbarism. You have to run your app in debug, or with a trace, to see what SQL it generates.
Don't stop with just parameters. Go all the way and use QueryFirst (disclaimer: which I wrote). Your SQL lives in a .sql file. You edit it in the fabulous TSQL editor window, with syntax validation and Intellisense for your tables and columns. You can assign test data in the special comments section and click "play" to run your query right there in the window. Creating a parameter is as easy as putting "#myParam" in your SQL. Then, each time you save, QueryFirst generates the C# wrapper for your query. Your parameters pop up, strongly typed, as arguments to the Execute() methods. Your results are returned in an IEnumerable or List of strongly typed POCOs, the types generated from the actual schema returned by your query. If your query doesn't run, your app won't compile. If your db schema changes and your query runs but some columns disappear, the compile error points to the line in your code that tries to access the missing data. And there are numerous other advantages. Why would you want to access data any other way?
In Sql when any word contain # sign it means it is variable and we use this variable to set value in it and use it on number area on the same sql script because it is only restricted on the single script while you can declare lot of variables of same type and name on many script. We use this variable in stored procedure lot because stored procedure are pre-compiled queries and we can pass values in these variable from script, desktop and websites for further information read Declare Local Variable, Sql Stored Procedure and sql injections.
Also read Protect from sql injection it will guide how you can protect your database.
Hope it help you to understand also any question comment me.
Old post but wanted to ensure newcomers are aware of Stored procedures.
My 10¢ worth here is that if you are able to write your SQL statement as a stored procedure, that in my view is the optimum approach. I ALWAYS use stored procs and never loop through records in my main code. For Example: SQL Table > SQL Stored Procedures > IIS/Dot.NET > Class.
When you use stored procedures, you can restrict the user to EXECUTE permission only, thus reducing security risks.
Your stored procedure is inherently paramerised, and you can specify input and output parameters.
The stored procedure (if it returns data via SELECT statement) can be accessed and read in the exact same way as you would a regular SELECT statement in your code.
It also runs faster as it is compiled on the SQL Server.
Did I also mention you can do multiple steps, e.g. update a table, check values on another DB server, and then once finally finished, return data to the client, all on the same server, and no interaction with the client. So this is MUCH faster than coding this logic in your code.
Other answers cover why parameters are important, but there is a downside! In .net, there are several methods for creating parameters (Add, AddWithValue), but they all require you to worry, needlessly, about the parameter name, and they all reduce the readability of the SQL in the code. Right when you're trying to meditate on the SQL, you need to hunt around above or below to see what value has been used in the parameter.
I humbly claim my little SqlBuilder class is the most elegant way to write parameterized queries. Your code will look like this...
C#
var bldr = new SqlBuilder( myCommand );
bldr.Append("SELECT * FROM CUSTOMERS WHERE ID = ").Value(myId);
//or
bldr.Append("SELECT * FROM CUSTOMERS WHERE NAME LIKE ").FuzzyValue(myName);
myCommand.CommandText = bldr.ToString();
Your code will be shorter and much more readable. You don't even need extra lines, and, when you're reading back, you don't need to hunt around for the value of parameters. The class you need is here...
using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.SqlClient;
public class SqlBuilder
{
private StringBuilder _rq;
private SqlCommand _cmd;
private int _seq;
public SqlBuilder(SqlCommand cmd)
{
_rq = new StringBuilder();
_cmd = cmd;
_seq = 0;
}
public SqlBuilder Append(String str)
{
_rq.Append(str);
return this;
}
public SqlBuilder Value(Object value)
{
string paramName = "#SqlBuilderParam" + _seq++;
_rq.Append(paramName);
_cmd.Parameters.AddWithValue(paramName, value);
return this;
}
public SqlBuilder FuzzyValue(Object value)
{
string paramName = "#SqlBuilderParam" + _seq++;
_rq.Append("'%' + " + paramName + " + '%'");
_cmd.Parameters.AddWithValue(paramName, value);
return this;
}
public override string ToString()
{
return _rq.ToString();
}
}
I am looking at some SQL code:
txtUserId = getRequestString("UserId");
txtSQL = "SELECT * FROM Users WHERE UserId = " + txtUserId;
I know in php it uses $_GET or $_POST to retrieve the values entered in a form so I am just wondering what is the language the first statement is written in that is retrieving the values?
I'm going to answer this definitively. No, not the language question, but the actual, important part of that code snippet.
Do Not Open Yourself To SQL Injection Attacks.
That code puts in the text contents of UserId directly into the SQL statement. Which means that someone can enter something like:
UserId=-1 or (1=1)
... and get the entire table. Or:
UserId=-1; NewSQLStatementStartsHere
... and start running malicious SQL statements on your server.
Never inject raw values into SQL. Always use parameterized values. SQL Injection Attack is still the #1 cause of security vulnerabilities in software.
SELECT verlog.bearerid, detail.snum
FROM verlog
INNER JOIN detail
ON verlog.txnid = detail.txnid
WHERE verlog.bearerid ='"+ TextBox1.Text +"'
is this the right way to obtain the value of a text box and use it in a where clause?
By constructing the statement in this way you open yourself up to SQL injection attacks in which users may be able to run additional sql against your database.
You can protect against this by using parameterised sql. Then pass both the query and the parameters when executing.
SELECT verlog.bearerid, detail.snum
FROM verlog
INNER JOIN detail
ON verlog.txnid = detail.txnid
WHERE verlog.bearerid = #bearerid
Further details can be read here: http://www.csharp-station.com/Tutorial/AdoDotNet/lesson06
Alternatively you can use an ORM which will handle this for you.
I seggest not to write scripts as you mentioned; that would pave the way of injection. If not using any ORM technology such as Entity framework which in turn calls for its own security issues as well, always stick into parametric ado.net queries. Look at this link
I am very new to working with databases. Now I can write SELECT, UPDATE, DELETE, and INSERT commands. But I have seen many forums where we prefer to write:
SELECT empSalary from employee where salary = #salary
...instead of:
SELECT empSalary from employee where salary = txtSalary.Text
Why do we always prefer to use parameters and how would I use them?
I wanted to know the use and benefits of the first method. I have even heard of SQL injection but I don't fully understand it. I don't even know if SQL injection is related to my question.
Using parameters helps prevent SQL Injection attacks when the database is used in conjunction with a program interface such as a desktop program or web site.
In your example, a user can directly run SQL code on your database by crafting statements in txtSalary.
For example, if they were to write 0 OR 1=1, the executed SQL would be
SELECT empSalary from employee where salary = 0 or 1=1
whereby all empSalaries would be returned.
Further, a user could perform far worse commands against your database, including deleting it If they wrote 0; Drop Table employee:
SELECT empSalary from employee where salary = 0; Drop Table employee
The table employee would then be deleted.
In your case, it looks like you're using .NET. Using parameters is as easy as:
string sql = "SELECT empSalary from employee where salary = #salary";
using (SqlConnection connection = new SqlConnection(/* connection info */))
using (SqlCommand command = new SqlCommand(sql, connection))
{
var salaryParam = new SqlParameter("salary", SqlDbType.Money);
salaryParam.Value = txtMoney.Text;
command.Parameters.Add(salaryParam);
var results = command.ExecuteReader();
}
Dim sql As String = "SELECT empSalary from employee where salary = #salary"
Using connection As New SqlConnection("connectionString")
Using command As New SqlCommand(sql, connection)
Dim salaryParam = New SqlParameter("salary", SqlDbType.Money)
salaryParam.Value = txtMoney.Text
command.Parameters.Add(salaryParam)
Dim results = command.ExecuteReader()
End Using
End Using
Edit 2016-4-25:
As per George Stocker's comment, I changed the sample code to not use AddWithValue. Also, it is generally recommended that you wrap IDisposables in using statements.
You are right, this is related to SQL injection, which is a vulnerability that allows a malicioius user to execute arbitrary statements against your database. This old time favorite XKCD comic illustrates the concept:
In your example, if you just use:
var query = "SELECT empSalary from employee where salary = " + txtSalary.Text;
// and proceed to execute this query
You are open to SQL injection. For example, say someone enters txtSalary:
1; UPDATE employee SET salary = 9999999 WHERE empID = 10; --
1; DROP TABLE employee; --
// etc.
When you execute this query, it will perform a SELECT and an UPDATE or DROP, or whatever they wanted. The -- at the end simply comments out the rest of your query, which would be useful in the attack if you were concatenating anything after txtSalary.Text.
The correct way is to use parameterized queries, eg (C#):
SqlCommand query = new SqlCommand("SELECT empSalary FROM employee
WHERE salary = #sal;");
query.Parameters.AddWithValue("#sal", txtSalary.Text);
With that, you can safely execute the query.
For reference on how to avoid SQL injection in several other languages, check bobby-tables.com, a website maintained by a SO user.
In addition to other answers need to add that parameters not only helps prevent sql injection but can improve performance of queries. Sql server caching parameterized query plans and reuse them on repeated queries execution. If you not parameterized your query then sql server would compile new plan on each query(with some exclusion) execution if text of query would differ.
More information about query plan caching
Two years after my first go, I'm recidivating...
Why do we prefer parameters? SQL injection is obviously a big reason, but could it be that we're secretly longing to get back to SQL as a language. SQL in string literals is already a weird cultural practice, but at least you can copy and paste your request into management studio. SQL dynamically constructed with host language conditionals and control structures, when SQL has conditionals and control structures, is just level 0 barbarism. You have to run your app in debug, or with a trace, to see what SQL it generates.
Don't stop with just parameters. Go all the way and use QueryFirst (disclaimer: which I wrote). Your SQL lives in a .sql file. You edit it in the fabulous TSQL editor window, with syntax validation and Intellisense for your tables and columns. You can assign test data in the special comments section and click "play" to run your query right there in the window. Creating a parameter is as easy as putting "#myParam" in your SQL. Then, each time you save, QueryFirst generates the C# wrapper for your query. Your parameters pop up, strongly typed, as arguments to the Execute() methods. Your results are returned in an IEnumerable or List of strongly typed POCOs, the types generated from the actual schema returned by your query. If your query doesn't run, your app won't compile. If your db schema changes and your query runs but some columns disappear, the compile error points to the line in your code that tries to access the missing data. And there are numerous other advantages. Why would you want to access data any other way?
In Sql when any word contain # sign it means it is variable and we use this variable to set value in it and use it on number area on the same sql script because it is only restricted on the single script while you can declare lot of variables of same type and name on many script. We use this variable in stored procedure lot because stored procedure are pre-compiled queries and we can pass values in these variable from script, desktop and websites for further information read Declare Local Variable, Sql Stored Procedure and sql injections.
Also read Protect from sql injection it will guide how you can protect your database.
Hope it help you to understand also any question comment me.
Old post but wanted to ensure newcomers are aware of Stored procedures.
My 10¢ worth here is that if you are able to write your SQL statement as a stored procedure, that in my view is the optimum approach. I ALWAYS use stored procs and never loop through records in my main code. For Example: SQL Table > SQL Stored Procedures > IIS/Dot.NET > Class.
When you use stored procedures, you can restrict the user to EXECUTE permission only, thus reducing security risks.
Your stored procedure is inherently paramerised, and you can specify input and output parameters.
The stored procedure (if it returns data via SELECT statement) can be accessed and read in the exact same way as you would a regular SELECT statement in your code.
It also runs faster as it is compiled on the SQL Server.
Did I also mention you can do multiple steps, e.g. update a table, check values on another DB server, and then once finally finished, return data to the client, all on the same server, and no interaction with the client. So this is MUCH faster than coding this logic in your code.
Other answers cover why parameters are important, but there is a downside! In .net, there are several methods for creating parameters (Add, AddWithValue), but they all require you to worry, needlessly, about the parameter name, and they all reduce the readability of the SQL in the code. Right when you're trying to meditate on the SQL, you need to hunt around above or below to see what value has been used in the parameter.
I humbly claim my little SqlBuilder class is the most elegant way to write parameterized queries. Your code will look like this...
C#
var bldr = new SqlBuilder( myCommand );
bldr.Append("SELECT * FROM CUSTOMERS WHERE ID = ").Value(myId);
//or
bldr.Append("SELECT * FROM CUSTOMERS WHERE NAME LIKE ").FuzzyValue(myName);
myCommand.CommandText = bldr.ToString();
Your code will be shorter and much more readable. You don't even need extra lines, and, when you're reading back, you don't need to hunt around for the value of parameters. The class you need is here...
using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.SqlClient;
public class SqlBuilder
{
private StringBuilder _rq;
private SqlCommand _cmd;
private int _seq;
public SqlBuilder(SqlCommand cmd)
{
_rq = new StringBuilder();
_cmd = cmd;
_seq = 0;
}
public SqlBuilder Append(String str)
{
_rq.Append(str);
return this;
}
public SqlBuilder Value(Object value)
{
string paramName = "#SqlBuilderParam" + _seq++;
_rq.Append(paramName);
_cmd.Parameters.AddWithValue(paramName, value);
return this;
}
public SqlBuilder FuzzyValue(Object value)
{
string paramName = "#SqlBuilderParam" + _seq++;
_rq.Append("'%' + " + paramName + " + '%'");
_cmd.Parameters.AddWithValue(paramName, value);
return this;
}
public override string ToString()
{
return _rq.ToString();
}
}
I was trying to stress test my system's login unit. My system is designed like this -
if the user enters userid - abcd and password pass then the server takes these parameters and prepares an sql command and fires it to the microsoft database-
select password from UserInformationTable where userid = 'abcd';
The returned value is compared with the given password pass and result is then sent to the client.
I successfully broken into the system using the following method -
user enters userid - <abcd1 or drop table UserInformationTable >. This worked and my complete UserInformationTable got dropped.
Is there any graceful way of handling such a hacking problem. One way is to detect 'or' substring in the userid, but I did not find this very graceful. Is there any way I can restrict the no. of queries in a statement in microsoft sql ?
Thanks and Regards,
Radz
This is the SQL injection problem illustrated by the famous "Bobby Tables" cartoon.
Here is some info on how to fix it for MS SQL. Read especially #Rook's answer.
You have two problems.
You are subject to SQL injection attack as described by other users. Solve that problem by sanitizing your input and/or using parameterized queries.
You should neither store nor transmit plain text passwords. Why? See this story on Slashdot. The solution to this problem is to use one-way encryption to create a password hash to store in the database. When the user tries to log in use the same encryption on the password he or she provides, then search your database to see if a row exists with the same userid and password hash.
Use parameters in SQL queries. They automatically prevent this. Something like this in C#:
SqlCommand comm = new SqlCommand(conn);
comm.CommandText = "SELECT * FROM foo WHERE bar = #id";
comm.CommandType = CommandType.Text;
SqlParameter param = new SqlParameter("#id", DbType.Int64);
param.Value = 3; // The actual value that replaces #id
comm.Parameters.Add(param);
// ...
This not only applies to C#, but basically all modern DB systems and languages support parameterized queries <- google it.
And second, your first query can be changed to:
SELECT userid FROM users WHERE username = 'foo' AND password = 'bar'
Then just count how many rows you got returned, if it's 0, then the user entered the wrong password.