Posts in category .net - page 31

Avoiding SQL injection

Back in ’98 I was developing an extranet site for a local company when I realized that it would be open for exploit if somebody put single quotes in text fields. It was early in the development cycle so I fixed it and moved on, unable to find out how other people were avoiding the problem.

It turned out many were not and it became a well-known exploit called SQL injection. Unfortunately there are many developers who don’t know or appreciate the problem, and it is this:

If you build SQL by appending strings and data without correct encoding your application can be exploited. These exploits can range from exposing sensitive information, through to modification and deletion of data.

This problem is very real and applies to:

  • All SQL statements, not just SELECT
  • All database systems, not just MS SQL or MySQL
  • All programming environments, not just C#, PHP or ASP
  • All data, most essentially that obtained from end-users, regardless of client-side checking

Let’s walk through an example and see how it works and what can be done to avoid it.

Example: User login

We have a user-name and password from a web form and want to get the users ID from the database, or nothing if it wasn’t valid. We want to send something like this SQL statement to our database.

SELECT UserID FROM Users WHERE UserName='Bob' AND Password='test'

And so a developer might do something like this (in C# using .NET);

var dr = connection.Execute("SELECT UserID FROM Users WHERE UserName='" + Request("UserName") + "' AND Password='" + Request("Password") + "'");
if (dr.Read()) userId = dr.GetInt32(dr.GetOrdinal("UserID"));

The problem here is that if there is a ‘ in the form fields it effectively breaks out of the selection criteria and allows the end user to add extra criteria or even commands to what we are sending to the database server. Should they enters the following into the password form field…

    ' OR ''='

Then our code above will send the following SQL to the database:

SELECT UserID FROM Users WHERE UserName='aaa' AND Password='' OR ''=''

Which will return every record in the database and our code will let him log in as the first user it finds – normally a developer or administrator account. Ouch!

Bad solution: Encode it yourself

One solution often adopted is to always ensure all string input has a single-quote replaced by two single-quotes, which is what SQL server expects if you really want to send it a single quote.

This solution fails in that it doesn’t handle numbers or dates and falls apart in that both numbers and dates are often regionally formatted.

Good solution: Let the DB client encode it

A much better solution is to use your environment to perform all the proper encoding for you. As well as protecting you from such exploits you’ll also avoid localization problems where the string representation of something on your client is interpreted differently in your database. This can be a real problem in the UK where dates formatted by a UK web-server are sent to a misconfigured SQL server expecting US formatting and the days and months become transposed without error.

var cmd = new SqlCommand("SELECT UserID FROM Users WHERE UserName=@UserName AND Password=@Password");
cmd.Parameters.Add(new SqlParameter("@UserName", System.Data.SqlDbType.NVarChar, 255, Request("UserName")))
cmd.Parameters.Add(new SqlParameter("@Password", System.Data.SqlDbType.NVarChar, 255, Request("Password")))
dr = cmd.ExecuteReader();
if (dr.Read()) userId = dr.GetInt32(dr.GetOrdinal("UserID"));

Okay there is no doubt this is a little longer but it takes care of all our encoding and localization worries, and if for example you want to insert a lot of data into the database, creating the command and parameters once, then just setting the parameters for each insert (and executing it) will run faster than lots of string building inserts…

Moral of the story

Always encode data properly. If you can use the provided methods and functions to do so. If none are provided grab the specification and find out all the special characters used. Learn what encoding and escape sequences are used and apply them properly.

A few places where data should be encoded properly:

  • HTML – Obviously < and > have special meanings and need to be escaped. ASP.NET controls will take care of this if you set the .Text or .InnerText properties but set .InnerHTML at your own peril. Old ASP has the Server.HTMLEncode() function.
  • URL – A whole host of rules but the query string is often modified in code. Use URLEncode() or something similar especially if you want XHTML compliance too.
  • XML – Again a whole host of rules for what is valid data. Either use an XML object to write out your data (MSXML, Xerces etc) or maybe even store it in [[CDATA sections.
  • CSV – Even comma-separated value files have encoding rules. What do you need to do if text is going to have a ” in a field. What happens if a number contains a comma! Find out or use a well regarded library to do it for you.

Notes about the example

A better login system would not allow the web server direct access to sensitive data such as the user table. All access to sensitive information should be through stored procs that enforce those restrictions.

Such a login system would therefore call a stored procedure that logged the attempted, decided if it was valid, and locked out the user if too many incorrect attempts. I’ll blog that if anyone is interested.

Even if you don’t want to do that, returning a single field is better achieved by using ExecuteScalar() and forgetting a data reader.

Microsoft have a developer how-to on injection.

[)amien

Visual Studio 2003 – System.ArgumentException in debugger

I recently ran into a problem while debugging inside Visual Studio 2003.Net. Google couldn’t find me an answer, only a few other people with the same problem. Here’s my solution in the hope it might save somebody else some time.

Symptoms

Whenever debugging a specific VB.NET application that used a C# class library I would receive the following error certain objects in the C# class library:

<error: an exception of type: {System.ArgumentException} occurred>

Strangely only the ASP.NET application was affected, the WinForms application that also used this class library was unaffected. It also seemed to be localized to the debugger only, run-time behavior appeared to be just fine.

A reboot, rebuild project or a clear down of the ASP temp directory had no effect and another developer on the project had exactly the same problem despite the fact we do not share any binaries…

Solution

Close VS and delete all bin and obj directories from all projects in your solution if you want to be totally sure.

If you really don’t want to do this, deleting the .pdb’s from the project where the messed up object lives may be enough.

You may think forcing a rebuild with “rebuild solution” would achieve this, but it does not.

Cause (optional)

While debugging I noticed that I couldn’t create the messed up object interactively either. It was complaining that no constructor took 4 arguments and that it took 3. Well, it used to take 3 – over a week ago.

This leads me to believe that some combination of changing your C# source can make either VS or the C# compiler believe the debugging symbols are still up to date when they are not. This may possibly only affect VB.NET applications compiled against it.

It must be a reproducible bug because it happened on two different machines.

[)amien

VB.NET to C# conversion

I recently converted some components on a project from VB.NET to C#, mainly for overloading and better tool support (such as ReSharper). Some of the existing code was generated from my own CodeSmith templates, so a small rewrite to generate C# handled most of that.

VB.NET to C# Converter 1.31

The remaining code, while not extensive in size, is a rather complex affair and the prospect of debugging this code when hand-converted was a little daunting so I decided to give the demo version of VBConversion’s VB.NET to C# Converter 1.31 a spin.

I was rather disappointed it made some rather obvious and stupid mistakes especially when it had converted everything else so well – so much for the 99% accuracy claim. I thought it might just be our project be we adhere to many of Microsoft’s guidelines

Problem 1: Ignores all instance/class variables default assignments and constructors.

Public Class Mine
  Public myObj As MyClass = New MyClass()
  Private myVar As MyEnumeration = MyEnumeration.MyDefault
End Class

Suddenly becomes;

public class Mine {
  public MyClass myObj;
  private MyEnumeration myVar;
}

While the object not being created will soon throw an exception, the defaults for value types is a little harder to track down.

Problem 2: Gives up on Select Case statements after the first case – commenting out the others.

In at least one case it got so confused it commented out substantially more code after the case statement too.

Problem 3: Declares additional unnecessary name-spaces including the one for Microsoft.VisualBasic, despite not needing it.

Either we hadn’t used the CDate/CInt/CLng functions or it had converted them)…

Problem 4: All with statements are converted to be variable with1, even when the old with clause was a simple case.

For example:

With myObj
  .doThis()
End With

becomes;

MyClass with1 = myObj;
with1.doThis();

Using both VB.NET and C# together

If you are going to use both in a project you can bring the syntax a little closer in format, here’s a few tips.

  • Don’t use the Visual Basic CInt/CDate/IsNumber etc functions. Use the .NET Framework equivalents such as Int.Parse, Date.Parse etc. which will work in both languages and are normally faster than these legacy functions.
  • Bring the source closer together by;
    • Putting brackets round if conditions in VB.NET
    • Putting quotes round region declarations in C#
    • Putting attributes on separate lines in VB.NET with a _ suffix
    • Dropping the implementation and interface declarations onto a new line in C#
    • Dropping the base(whatever) onto a new line in C# constructors (good idea anyway)
  • Check out the “Differences Between Visual Basic .NET and Visual C# .NET” white paper on MSDN
  • Use VBCommenter to get C# style XML comment documentation.
  • Use Monitor.Enter() instead of VB.NET’s synclock and C#’s lock.
  • Be aware of the differences regarding math! VB.NET defaults to floating point precision and rounding when converting with CInt/CLng etc.. C# normally uses integer division and casting truncates.

[)amien

My .NET toolkit

Every developer has his own favorite set of development tools and libraries that he’s come to rely on. Here’s a round-up of some I use or am looking at.

Actively using

  • CodeSmith – Now in version 3 with it’s own IDE this tool lets you write templates that are executed against database structures. The C# ASP.NET style syntax is easy to get to grips with, allowing you to easily generate stored procedures, database scripts, class templates, simple object-relational-mappings, collection classes and the works. Will be less useful when .NET 2.0 arrives with Generics support but until then snap it up. A free version is available.
  • NullableTypes – A shortcoming in .NET 1.1 is the lack of null support for value types outside the SQL Server name space. Until this is addressed in .NET 2.0 then check out this open-source library that does a sterling job. (Disclosure, I’ve contributed to this project)
  • Subversion – A great source control tool, the replacement for the aging CVS product. Gaining some good support now you can run the server directly or inside Apache. Combine with TortoiseSVN for Explorer shell based integration or AnkhSVN for integration inside the Visual Studio IDE.
  • NUnit – Unit testing is a great way to prevent regressions and NUnit is both simple to learn and fast to use.
  • VisualStyles – I’ve mentioned this before but .NET’s support for XP themes is abysmal with drawing problems all over the place especially when it comes to tab controls. VisualStyles is a free component you just drop on your form that magically fixes everything.

On my radar

  • Trac – Provides wiki, issue tracking and source browsing over the top of a Subversion repository. I installed on my server at the weekend and it looks quite promising although its convoluted installation procedure had far too many dependencies for my liking. Will be testing this on a project I’m working on to see how it goes.
  • PageMethods – Previously called SharpURL’s but now free and without an expiration this provides a way of identifying methods into a web page and the parameters they require.

I’m also on the lookout for a replacement for my object-relational CodeSmith templates I knocked up a while back and have been refining since. While they are functional and incredibly fast I’m now wanting features they don’t provide such as database independence, lazy loading and locking patterns.

[)amien