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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.


How dangerous is HTML injection?

A few years ago I believed that HTML and SQL injection vulnerabilities were headed for extinction. Thanks to object-relational mapping tools SQL injection continues to die but HTML and script injection vulnerabilities are as popular as ever.

Part of the problem stems from the “back-to-basics” approach to rendering web pages, throwing out classes and controls for string-based libraries (primitive obsession) and helpers which do not encode HTML or even offer a concise simple syntax to do so.

MonoRail was one such project but they took feedback on board and addressed the issue although I was surprised it had got as far as release candidate 2 with such a serious oversight.

Other projects have been less reactive when advised of the problem and I can’t help but wonder if I am not getting the severity of the issue across. This isn’t just an annoyance but a real security problem.

If you are not familiar with:

  • HttpUtility.HtmlEncode (.NET)
  • Server.HtmlEncode (ASP)
  • htmlentities/htmlspecialchars (PHP)
  • html_escape (Rails)
  • {! } (MonoRail Brail)

and your web apps output data then they are likely open to HTML & script injection vulnerabilities.

Vulnerable code often looks like this:

myLabel.Text = Request.Form["Something"];
<%= myDataReader[0] %>
<? php echo get_the_title() ?>

For more ASP.NET examples check out 5 signs your ASP.NET application may be vulnerable to HTML injection.

Let’s start by considering the actors involved:

Visitor to visitor

If your site stores input from an external user (visitor) and displays it to another then you could be exposed to this scenario. Many sites do this although it is not always immediately recognized – an internet banking site does not seem an obvious candidate until you consider that you may put a textual reference on payments made to another person. If you know they use a vulnerable internet banking solution…

A worst-case scenario here would be that one visitor could steal another’s login credentials and exploit whatever rights that might give him – anything from posting messages to stealing funds.

Visitor to staff

Not all sites exchange data between users but if your site collects information from visitors chances are it presents this information to staff. Internal systems used to examine it are often considered less vulnerable which is a mistake. Remember all data provided from a user should be considered to be a potential avenue for a dangerous payload, e.g. even the language-accepts or user-agent strings.

When exploited internal systems can reveal information in bulk about the users, the system and the administration accounts used to manage it. Gaining access to these details brings all the privileges those accounts have to offer which can be catastrophic.

Staff to visitor

It is easy to forget that many frauds are perpetuated by people on the inside. A staff member given the ability to present text to the user via a website has the ability to modify any page that the content is presented on which if it includes a login page (perhaps for system status messages) then capturing login details to a server of their own choice is easy.

Security operators with access to reset (but not view) passwords would find this attack particularly enticing given that they do not need to reset the users account and therefore raise any awareness. An insider can perpetuated the fraud and may be in a position to further conceal it within the organization.

Next steps?

I can envisage a sequence of steps that start with discovery of injectable systems through detection of script-enabled into form capture-and-forward and async logging of passwords through XmlHttp.

Detailing those steps would certainly raise awareness and help developers appreciate the severity of the issue but how do I make sure that information isn’t abused?

Disclosure is a double-edged sword but then you can’t have security through obscurity… I wonder how many crackers/black hackers already utilize these techniques for nefarious means.

.NET developers might like to check out the [slides from the Web Application Security talk](/blog/2007/08/16/gsdf-august-2007-postmortem) I gave at the Guernsey Software Developer Forum which demonstrates exploitable, exploits and safe alternatives for preventing HTML and SQL injection.


Security vulnerabilities are not acceptable in sample code

Earlier this week the ASP.NET article of the day linked to 4-Tier Architecture in ASP.NET with C# which I noticed suffered from both HTML and SQL injection. I promptly informed the author and the ASP.NET site (who pulled the link) but the author was rather unconcerned and wrote (after editing my comment):

Thanks for your feedback Damieng. Sql statement has been used here to make the tutorial simple, it is informed in the DAL description.

The problem is people borrowing this code may not notice the vulnerability or understand how to fix it. This isn’t the first time I’ve seen easily exploited sample code, responded and been buffed off with the it’s just sample code excuse.

Writing secure code isn’t difficult, time consuming or confusing to read.

Microsoft’s forthcoming LINQ toS QL and Entity Framework provide object-relational mapping that takes care of the SQL, as do other well-known ORM tools such as SubSonic and NHibernate.

If you must write your own data-access-layer (DAL) code use parameterized queries and not string concatenation.

When outputting values be 100% sure whether your technique will encode the values for you or not and be aware of what encoding tools are available to you.

ASP & ASP.NET’s Response.Write and <%= %> methods do NOT encode for you and you should be using HttpUtility.HtmlEncode to output data to a HTML stream.

Samples of vulnerable and secure code are in my presentation on Web Security I gave at the Guernsey Software Developer Forum a few months ago.