Posts in category .net - page 13

5 signs your ASP.NET application may be vulnerable to HTML injection

If you don’t encode data when using any of the following methods to output to HTML your application could be compromised by unexpected HTML turning up in the page and modifying everything from formatting though to capturing and interfering with form data via remote scripts (XSS). Such vulnerabilities are incredibly dangerous.

Using MonoRail or Microsoft’s MVC does not make you automatically immune – use {! } in MonoRail’s Brail engine and the HtmlHelpers in Microsoft’s MVC to ensure correct encoding.

Just imagine post.Author contains “><script src=””></script> after an unscrupulous user entered that into a field your application uses and it got into the database. The following typical ASP.NET techniques would leave you open.

1. You use <%= %> or <%# %> tags to output data

Example showing outputting literals with <%= %> :

// Vulnerable
<p>Posted by <%= post.Author %></p>
// Secure
<p>Posted by <%= HttpUtility.HtmlEncode(post.Author) %></p>

2. You use Response.Write

Example showing writing out attributes with Response.Write and String.Format, again post.Author could contain

// Vulnerable
Response.Write(String.Format("<input type=\"text\" value=\"{0}\" />", post.Author);
// Secure
Response.Write(String.Format("<input type=\"text\" value=\"{0}\" />", HttpUtility.HtmlAttributeEncode(post.Author));

3. You set HRef or Src on HtmlAnchor, HtmlImage or HtmlnputImage controls

In general the HtmlControls namespace are very well behaved with encoding but there is a bug in the code that attempts to adjust the relative url’s for href and src attributes which causes those properties to bypass encoding (I’ve reported this to Microsoft).

Example showing anchor HRef attribute abuse:

// Vulnerable
outputDiv.Controls.Add(new HtmlAnchor() { Text = "Test", HRef = post.Author } );
// Secure
outputDiv.Controls.Add(new HtmlAnchor() { Text = "Test", HRef = HttpUtility.HtmlAttributeEncode(post.Author) } );

4. You set the Text property of WebControls/WebForms

You would imagine the high-level WebForms controls would take care of encoding and you’d be wrong.

Example showing the Label control being so easily taken advantage of:

// Vulnerable
outputDiv.Controls.Add(new Label() { Text = post.Author } );
// Secure
outputDiv.Controls.Add(new Label() { Text = HttpUtility.HtmlEncode(post.Author) } );

The one exception to this is the Text property of input controls – as they put the value into an attribute and therefore call HttpUtility.HtmlAttributeEncode for you.

5. You use the LiteralControl

LiteralControl is a useful control for adding text to the output stream that doesn’t require it’s own tag. It also helpfully, and uncharacteristically, provides a useful constructor. Unfortunately it fails encode the output.

Example showing poor LiteralControl wide open:

// Vulnerable
outputDiv.Controls.Add(new LiteralControl(post.Author));
// Secure
outputDiv.Controls.Add(new LiteralControl(HttpUtility.HtmlEncode(post.Author)));
Do not: 1. Encode data in the database – your contaminated data will be difficult to use elsewhere and will end up double-encoded 2. Look for script on submit – you won’t catch every combination and it might prevent valid data 3. Trap entry with client-side code – it is trivially bypassed

Just encode the output :)


PS: The samples use .NET 3.5 object initializer syntax for brevity as many affected controls do not have useful constructors)

ASP.NET MVC preview available

The first public preview of Microsoft’s ASP.NET MVC (model view controller) framework is now available.

Download ASP.NET 3.5 Extensions (EXE) (3.7 MB)

Download MVC Toolkit (ZIP) (400 KB)

The project takes cues from Ruby on Rail’s success and looks to address dissatisfaction with the testability and maintainability of WebForms applications and provides an alternative approach that is centered around views, models, controllers with a clear separation of concern and the ability to mock test the individual elements.The official documentation is online and there is a great four-part series over at Scott Guthrie’s blog which covers:

Phil Hack and Rob Conery are both now at Microsoft and working on the framework, they have some interesting things to say on it too:

A few other people have already written about the subject too:

Most of the examples and many of the routines/helpers fail to encode output which opens them up to HTML and script injection vulnerabilities. Remember to HttpUtility.HtmlEncode output and use Reflector if you’re unsure whether a function is encoding correctly.

The CTP requires Visual Studio 2008 to get the most out if it so either head over to MSDN Subscriber Downloads or grab a 90-day trial edition if you don’t already have it installed.


Shrinking JS or CSS is premature optimization

Rick Strahl has a post on a JavaScript minifier utility the sole job of which is to shrink the size of your JavaScript whilst making it almost impossible to read in order to save a few kilobytes.I thought I’d take a quick look at what the gain would be and fed it the latest version (1.6) of the very popular Prototype library:

File (KB) GZip (KB)
Standard 121.0 26.7
Shrunk/minified 90.5 22.0
Saving 30.7 4.7

The 30.7 KB saving looks great at first glance but bear in mind that external JavaScript files are cached on the client between page requests and it looses some appeal.If you also consider the fact that most browsers and clients support GZip compression and the savings there are around 4.7 KB and you might wonder if you are wasting your time.In computer science there is a term for blindly attempting to optimize systems without adequate measurement or justification and that term is premature optimization.As Sir Tony Hoare wrote (and Donald Knuth paraphrased)

We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.

And he was working on computers throughout the 60’s and 70’s that had much less resources than those today.By all means if your server bandwidth is an issue delve into the stats, identify the cause and take it from there. Going with Yahoo’s YSlow plug-in for Firefox/Firebug is a great starting point but remember to analyze the statistics from your own context.

Rick’s tool had shortcomings with non-ASCII characters such as accents, symbols and non-US currency symbols which goes to show how optimization can have other unintended and undesirable effects.


Calculating Elf-32 in C# and .NET

GitHub has the latest version of Elf32

Because you can never have enough hashing algorithms at your disposal this one is compatible with the elf_hash function that forms part of the Executable and Linkable Format.

using System;
using System.Security.Cryptography;

public class Elf32 : HashAlgorithm {
  private UInt32 hash;

  public Elf32() {

  public override void Initialize() {
    hash = 0;

  protected override void HashCore(byte[] buffer, int start, int length) {
    hash = CalculateHash(hash, buffer, start, length);

  protected override byte[] HashFinal() {
    byte[] hashBuffer = UInt32ToBigEndianBytes(hash);
    this.HashValue = hashBuffer;
    return hashBuffer;

  public override int HashSize { get { return 32; } }

  public static UInt32 Compute(UInt32 polynomial, UInt32 seed, byte[] buffer) {
    return CalculateHash(seed, buffer, 0, buffer.Length);

  private static UInt32 CalculateHash(UInt32 seed, byte[] buffer, int start, int size) {
    UInt32 hash = seed;

    for (int i = start; i < size; i++)
       unchecked {
          hash = (hash << 4) + buffer[i];
          UInt32 work = (hash & 0xf0000000);
          if (work != 0)
             hash ^= (work >> 24);
          hash &= ~work;
    return hash;

  private byte[] UInt32ToBigEndianBytes(UInt32 x) {
    return new byte[] {
       (byte)((x >> 24) & 0xff),
       (byte)((x >> 16) & 0xff),
       (byte)((x >> 8) & 0xff),
       (byte)(x & 0xff)