Do stored procedures prevent SQL injection?

Question

Is it true that stored procedures prevent SQL injection attacks against PostgreSQL databases? I did a little research and found out that SQL Server, Oracle and MySQL are not safe against SQL injection even if we only use stored procedures. However, this problem does not exist in PostgreSQL.

Does the stored procedure implementation in PostgreSQL core prevent SQL injection attacks or is it something else? Or is PostgreSQL also susceptible to SQL injection even if we only use stored procedures? If so, please show me an example (e.g. book, site, paper, etc).

Answer 1

No, stored procedures do not prevent SQL injection. Here's an actual example (from an in-house app someone created where I work) of a stored procedure that unfortunately permits SQL injection:

This sql server code:

CREATE PROCEDURE [dbo].[sp_colunmName2]   
    @columnName as nvarchar(30),
    @type as nvarchar(30), 
    @searchText as nvarchar(30)           
AS
BEGIN
    DECLARE @SQLStatement NVARCHAR(4000)
    BEGIN
        SELECT @SQLStatement = 'select * from Stations where ' 
            + @columnName + ' ' + @type + ' ' + '''' + @searchText + '''' 
        EXEC(@SQLStatement)
    END      
END
GO

roughly equivalent to postgres:

CREATE or replace FUNCTION public.sp_colunmName2 (
    columnName  varchar(30),
    type varchar(30), 
    searchText  varchar(30) ) RETURNS SETOF stations LANGUAGE plpgsql            
AS
$$
DECLARE SQLStatement VARCHAR(4000);
BEGIN
    SQLStatement = 'select * from Stations where ' 
            || columnName || ' ' || type || ' ' || ''''|| searchText || '''';
    RETURN QUERY EXECUTE  SQLStatement;
END
$$;

The developer's idea was to create a versatile search procedure, but the result is that the WHERE clause can contain anything the user wants, allowing a visit from little Bobby Tables.

Whether you use SQL statements or stored procedure doesn't matter. What matters is whether your SQL uses parameters or concatenated strings. Parameters prevent SQL injection; concatenated strings allow SQL injection.

Answer 2

SQL-Injection attacks are those where untrusted input is directly appended queries, allowing the user to effectively execute arbitrary code, as illustrated in this canonical XKCD comic.

Thus, we get the situation:

userInput = getFromHTML # "Robert ') Drop table students; --"

Query = "Select * from students where studentName = " + userInput

Stored Procedures are, in general, good defenses against SQL injection attacks because the incoming parameters are never parsed.

In a stored procedure, in most DBs (and programs, don't forget that precompiled queries count as stored procedures) look like the following:

 

create Stored procdure foo (
select * from students where studentName = :1
);

Then, when the program desires access, it calls foo(userInput) and happily retrieves the result.

A stored procedure is not a magical defense against SQL-Injection, as people are quite able to write bad stored procedures. However, pre-compiled queries, be they stored in the database or in the program, are much more difficult to open security holes in if you understand how SQL-Injection works.

You can read more about SQL-Injection:

• In this discussion by Jeff Atwood
• Prevention cheat sheet
• How to attack your own code (Make sure your QA includes security testing. If it doesn't, your site will be security tested by the outside. This is a Bad Thing.)

Answer 3

Yes, to some extent.
Stored Procedures alone will not prevent SQL Injection.

Let me first quote about SQL Injection from OWASP

A SQL injection attack consists of insertion or "injection" of a SQL query via the input data from the client to the application. A successful SQL injection exploit can read sensitive data from the database, modify database data (Insert/Update/Delete), execute administration operations on the database (such as shutdown the DBMS), recover the content of a given file present on the DBMS file system and in some cases issue commands to the operating system. SQL injection attacks are a type of injection attack, in which SQL commands are injected into data-plane input in order to effect the execution of predefined SQL commands.

You have to sanitize user inputs and do not concatenate SQL statements, even if you are using stored procedure.

Jeff Attwood explained consequences of concatenating sql in "Give me parameterized SQL, or give me death"

Following is the interesting cartoon which comes to my mind whenever I hear SQL Injection I think you got the point :-)

Have a look at SQL Injection Prevention Cheat Sheet, prevention methods are neatly explained...

Answer 4

String concatenation is the cause of SQL Injection. This is avoided using parametrisation.

Stored procedures add an additional layer of security by enforcing invalid syntax when you concatenate, but are not "safer" if you use, say, dynamic SQL in them.

So, your code above is caused by concatenation of these strings

• exec sp_GetUser '
• x' AND 1=(SELECT COUNT(*) FROM Client); --
• ' , '
• monkey
• '

This gives invalid syntax, luckily

Parametrising it would give

exec sp_GetUser 'x'' AND 1=(SELECT COUNT(*) FROM Client); --' , 'monkey'

This means

• @UserName = x' AND 1=(SELECT COUNT(*) FROM Client); --
• @Password = monkey

Now, in the code above you'll get no rows because I assume you have no user x' AND 1=(SELECT COUNT(*) FROM Client); --

If the stored proc looked like this (using concatenated dynamic SQL), then your parametrised stored proc call will still allow SQL Injection

...
SET @sql = 'SELECT userName from users where userName = ''' + 
               @UserName + 
               ''' and userPass = ''' +
               @Password +
               ''''
EXEC (@sql)
....

So, as demonstrated, string concatenation is the main enemy for SQL injection

Stored procedures do add encapsulation, transaction handling, reduced permissions etc, but they can still be abused for SQL injection.

You can look on Stack Overflow for more about parametrisation

Answer 5

"SQL injection attacks happen when user input is improperly encoded. Typically, the user input is some data the user sends with her query, i.e. values in the $_GET, $_POST, $_COOKIE, $_REQUEST, or $_SERVER arrays. However, user input can also come from a variety of other sources, like sockets, remote websites, files, etc.. Therefore, you should really treat everything but constants (like 'foobar') as user input."

I've been investigating thoroughly on this subject recently and would like to share with others quite interesting material, thus, making this post more complete and instructive for everyone.

• Preventing SQL Injection with PHP by John Nebel
• Security Corner - SQL Injection by Chris Shiflett
• The Unexpected SQL Injection by Alexander Andonov
• Mysql_real_escape_string() versus Prepared Statements by Ilia Alshanetsky
• SQL Injection Attack and Defense by Sagar Joshi
• SQL Injection Attacks by Prof. Jim Whitehead
• addslashes() vs mysql_real_escape_string() by Chris Shiflett
• What's a SQL Injection Bug by Joel Spolsky
  
  
• MySQL - SQL injection prevention
• SQL Injection Walkthrough
• SQL Injection Cheat Sheet
• Prepared Statements in PHP and MySQLi

---

From YouTube

• SQL Injection Myths & Fallacies: Best practices of defense by Bill Karwin
• PHP Tutorials: Security - SQL Injection
• How to SQL Inject with SQLMAP on Backtrack5 RC1

---

From Wikipedia

• Wikipedia - SQL injection
• Wikipedia - SQL

---

From OWASP

• SQL Injection
• Guide to SQL Injection
• OWASP - Avoiding SQL Injection
• SQL Injection Prevention Cheat Sheet
• Testing for SQL Injection

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From PHP Manual

• SQL Injection
• PDO class - Prepared statements and stored procedures
• MySQL Improved Extension
• mysql_real_escape_string()

---

From Microsoft and Oracle

• What's the Right Way to Prevent SQL Injection in PHP Scripts by Microsoft
• Stop SQL Injection Attacks Before They Stop You by Microsoft
• Defending Against SQL Injection Attacks by Oracle

---

Stack Overflow

• SQL Injections and ADOdb library! General PHP website security with examples
• Best way to prevent SQL Injection in PHP
• XKCD SQL injection — please explain
• What is the best way to avoid SQL injection attacks?
• What is SQL injection?
• SQL injection on INSERT
• How do I protect this function from SQL injection?
• Are Parameters really enough to prevent Sql injections?
• Is SQL injection a risk today?
• SQL Injection
• SQL Injection ethical hacking
• Does this code prevent SQL injection?

---

SQL injection scanner

• Top 15 SQL injection scanner
• Netsparker Community Edition, Free SQL Injection Scanner & XSS Scanner

Answer 6

Stored procedures don't magically prevent SQL injection, but they do make preventing it a heck of a lot easier. All you have to do is something like the following (Postgres example):

CREATE OR REPLACE FUNCTION my_func (
  IN in_user_id INT 
)
[snip]
  SELECT user_id, name, address FROM my_table WHERE user_id = in_user_id; --BAM! SQL INJECTION IMMUNE!!
[snip]

That's it! The problem only comes up when forming a query via string concatenation (i.e. dynamic SQL), and even in those cases you may be able to bind! (Depends on the database.)

How to avoid SQL injection in your dynamic query:

Step 1) Ask yourself if you really need a dynamic query. If you're sticking strings together just to set the input, then you're probably doing it wrong. (There are exceptions to this rule -- one exception is for reporting queries on some databases, you may have performance issues if you don't force it to compile a new query with each execution. But research this issue before you jump into that.)

Step 2) Research the proper way to set the variable for your particular RDBMS. For example Oracle lets you do the following (quoting from their docs):

sql_stmt := 'UPDATE employees SET salary = salary + :1 WHERE ' 
           || v_column || ' = :2';
EXECUTE IMMEDIATE sql_stmt USING amount, column_value; --INJECTION IMMUNE!!

Here you are still not concatenating the input. You are safely binding! Hooray!

If your database does not support something like the above (hopefully none of them are still this bad, but I wouldn't be surprised) - or if you still really must concatenate your input (like in the "sometimes" case of reporting queries as I hinted at above), then you must use a proper escaping function. Don't write it yourself. For example postgres provides the quote_literal() function. So you'd run:

sql_stmt := 'SELECT salary FROM employees WHERE name = ' || quote_literal(in_name);

This way if in_name is something devious like '[snip] or 1=1' (the "or 1=1" part means select all rows, allowing the user to see salaries he shouldn't!), then quote_literal saves your butt by making the resulting string:

SELECT salary FROM employees WHERE name = '[snip] or 1=1'

No results will be found (unless you have some employees with really weird names.)

That's the gist of it! Now let me just leave you with a link to a classic post by Oracle guru Tom Kyte on the subject of SQL Injection, to drive the point home: Linky