I have purposfully (for testing) assigned the following variable in WebMatrix C#:
string val = "<script type='text/javascript'>alert('XSS Vector')</script>";
Later in the page I have used razor to write that value directly to the page.
<p>
#val
</p>
It writes the text, but in a safe manner (i.e., no alert scripts run)
This, coupled with the fact that if 'val' contains an html entity (e.g., <) it also writes exactly "<" and not "<" as I would expect the page to render.
Is this because C# runs first, then html is rendered?
More importantly, is using razor in this fashion a suitable replacement for html encoding, when used like this?
The #Variable syntax will HtmlEncode any text you pass to it; hence you seeing literally what you set to the string value. You are correct in that this is for XSS protection. It is part of Razor that does this; the #Variable syntax itself.
So basically, using the #Variable syntax is not so much a 'replacement' for Html Encoding; it is HTML encoding.
Related: If you ever want some string to render the HTML, you would use this syntax in Razor:
#Html.Raw(Variable)
That causes the Html Encoding not to be done. Obviously, this is dangerous to do with user-supplied input.
Related
I have a controller which generates a string containing html markup. When it displays on views, it is displayed as a simple string containing all tags.
I tried to use an Html helper to encode/decode to display it properly, but it is not working.
string str= "seeker has applied to Job floated by you.</br>";
On my views,
#Html.Encode(str)
You are close you want to use #Html.Raw(str)
#Html.Encode takes strings and ensures that all the special characters are handled properly. These include characters like spaces.
You should be using IHtmlString instead:
IHtmlString str = new HtmlString("seeker has applied to Job floated by you.</br>");
Whenever you have model properties or variables that need to hold HTML, I feel this is generally a better practice. First of all, it is a bit cleaner. For example:
#Html.Raw(str)
Compared to:
#str
Also, I also think it's a bit safer vs. using #Html.Raw(), as the concern of whether your data is HTML is kept in your controller. In an environment where you have front-end vs. back-end developers, your back-end developers may be more in tune with what data can hold HTML values, thus keeping this concern in the back-end (controller).
I generally try to avoid using Html.Raw() whenever possible.
One other thing worth noting, is I'm not sure where you're assigning str, but a few things that concern me with how you may be implementing this.
First, this should be done in a controller, regardless of your solution (IHtmlString or Html.Raw). You should avoid any logic like this in your view, as it doesn't really belong there.
Additionally, you should be using your ViewModel for getting values to your view (and again, ideally using IHtmlString as the property type). Seeing something like #Html.Encode(str) is a little concerning, unless you were doing this just to simplify your example.
you can use
#Html.Raw(str)
See MSDN for more
Returns markup that is not HTML encoded.
This method wraps HTML markup using the IHtmlString class, which
renders unencoded HTML.
I had a similar problem with HTML input fields in MVC. The web paged only showed the first keyword of the field.
Example: input field: "The quick brown fox" Displayed value: "The"
The resolution was to put the variable in quotes in the value statement as follows:
<input class="ParmInput" type="text" id="respondingRangerUnit" name="respondingRangerUnit"
onchange="validateInteger(this.value)" value="#ViewBag.respondingRangerUnit">
I had a similar problem recently, and google landed me here, so I put this answer here in case others land here as well, for completeness.
I noticed that when I had badly formatted html, I was actually having all my html tags stripped out, with just the non-tag content remaining. I particularly had a table with a missing opening table tag, and then all my html tags from the entire string where ripped out completely.
So, if the above doesn't work, and you're still scratching your head, then also check you html for being valid.
I notice even after I got it working, MVC was adding tbody tags where I had none. This tells me there is clean up happening (MVC 5), and that when it can't happen, it strips out all/some tags.
I am using the shorthand for HttpUtility.HtmlEncode to encode the data going into my textboxs.
<asp:TextBox ID="txtProperty" runat="server" Text='<%#: Bind("Property")%>'></asp:TextBox>
My understanding of how encoded characters behave is that when your web browser renders them, they should display as the characters they represent and not the actual encoded characters. As this example code on the MSDN website suggests.
However my encoded characters does not behave this way.
For example a '£' character being retrieved from a database, displays in the textbox as:
And not:
I don't think it has anything to do with how my website is configured to handle encoding, because if I manually set the text as the encoded characters in the HTML:
<asp:TextBox ID="txtProperty" runat="server" Text="£"></asp:TextBox>
It renders the encoded characters correctly as:
This indicates to me that it is a problem with the way I am using HtmlEncode.
Still I tried explicitly setting the encoding to UTF-8 in my webconfig and it made no difference.
Could someone explain this behavior, or what might be the problem here?
When you do <%#: Bind("Property")%> ASP.NET will already take care of HTML-encoding the string, if you pre-encode it you'll fall in the double-encoding scenario.
See ScottGu's New <%: %> Syntax for HTML Encoding Output in ASP.NET 4 (and ASP.NET MVC 2):
ASP.NET 4 introduces a new IHtmlString interface (along with a concrete implementation: HtmlString) that you can implement on types to indicate that its value is already properly encoded (or otherwise examined) for displaying as HTML, and that therefore the value should not be HTML-encoded again.
The <%: %> code-nugget syntax checks for the presence of the IHtmlString interface and will not HTML encode the output of the code expression if its value implements this interface.
This allows developers to avoid having to decide on a per-case basis whether to use <%= %> or <%: %> code-nuggets.
Instead you can always use <%: %> code nuggets, and then have any properties or data-types that are already HTML encoded implement the IHtmlString interface.
I'm looking for a way to write a value in my razor view, without encoding it, AND without using the Html Helper.
I'm rendering the view in a hybrid website, where I parse my View programmatically, like this:
string html = "<html>#("Write something <strong>unencoded</strong>"</html>")
html = Razor.Parse<TModel>(html, model);
So essentially, my html variable contains a template containing razor c# code. Because I am compiling my view like this, I have no access to the Html helper (the accepted answer in this post implies this is indeed the case: How to render a Razor View to a string in ASP.NET MVC 3?)
However, my html variable also contains a statement like this:
#Html.Raw("<strong>This should be printed unencoded</strong>")
This does not work but gives "Html is not available in this context".
How can I achieve the same behavior? Using Response.Write gives the exact same error.
Are there any other ways?
Note: this is a hybrid website, containing both classic ASP webforms and some newer Web API and MVC stuff. The View I'm using is NOT accessed through conventional MVC ways.
HtmlString type of string should work for you.
Represents an HTML-encoded string that should not be encoded again.
Sample with creating such string inline (normally you'd have such values in Model set by controler):
#(new HtmlString("<strong>This should be printed unencoded</strong>"))
It took me a while to figure out, but this is the final solution (which works for me, but has some security implications, so do read on!)
Compile your view like this
var config = new TemplateServiceConfiguration();
config.EncodedStringFactory = new RawStringFactory();
var service = RazorEngineService.Create(config);
html = service.RunCompile(html, "templateNameInTheCache", null, model);
As you can see, I employed the RawStringFactory to make sure no HTML at all gets encoded. Of course, MVC automatically encodes HTML as a safety precaution, so doing this isn't very safe. Only do this if you're 100% sure that all of the output in the entire Razor view is safe!
Why doesn't this work?
<input type="button" id="btnAccept" value="Accept" onclick='<%# String.Format("accept('{0}','{1}','{2}','{3}-{4}');", Container.DataItem("PositionID"), Container.DataItem("ApplicantID"), Container.DataItem("FullName"), Container.DataItem("DepartmentName"), Container.DataItem("PositionTitle"))%>' />
The onclick doesn't do anything.
Your best bet is to look at the generated HTML. I think it's a really good habit to check the generated HTML in text format and how it renders on-screen, all the time. Besides errors such as this (which can easily be spotted in the generated HTML), it will help you catch other possible invalid uses of HTML which may render as intended in one browser while rendering terribly in another. HTML rendering engines employ many tricks to try and make invalid HTML look okay.
Anyway, all things aside (such as, assuming accept(...) exists, and all other calls in the tag are correct) I think the issue you are having is as follows:
onclick='<%# String.Format("accept('{0}','{1}','{2}','{3}-{4}');", ... )%>'
This line is probably going to evaluate to look something like this:
onclick='accept('{0}','{1}','{2}','{3}-{4}');'
With all single quotes, all the onclick attribute will see is onclick='accept(' which is not a valid javascript method call. You're going to want to use the "" strings, which you can embed in the format string by escaping them.
String.Format("accept(\"{0}\",\"{1}\",\"{2}\",\"{3}-{4}\");", ... )
Then, you should be able to get the correct combination of ' and " within the attribute:
onclick='accept("{0}","{1}","{2}","{3}-{4}");'
I have a legacy ASP.Net site (recently upgraded to .NET 4.0) which never had Request Validation turned on and it doesn't Html encode any user input at all.
My solution was to turn on request validation and to catch the HttpRequestValidationException in Global.asax and redirect the user to an error page. I don't Html Encode the user input as I'll have to do it in thousands of places. I hope my approach will stop any XSS vectors getting saved into database.
However, in case if there is already any XSS vector stored in database I reckon I should also Html encode all output. Unfortunately I have very limited dev and test resource to successfully achieve this. I came up with a list of changes I need to go through:
Search and Replace all <%= %> with <%: %>.
Search and Replace all Labels with Literals and add Mode="Encode".
Wrap all eval() with HtmlEncode.
My questions are:
Is there any simpler way of turning on all output to be automatically Html encoded?
Am I missing anything from above list?
Any pitfalls I should be careful about?
Thanks.
Search and Replace all <%= %> with <%: %>.
Don't forget the <%# and Response.Write which will be harder to replace
Search and Replace all Labels with Literals and add Mode="Encode".
But you will loose all formatting on the previously generated spans, break the DOM
and the corresponding js/css
You would also have to search all Literals with Mode="PassThrough" and set them to Encode
Wrap all eval() with HtmlEncode.
Yes, it seems like a subset of the <%# matter above
Also, you could have some custom controls with funky render method
Assuming there is "only" a relational DB as back-end, If I had access to the DB, I would first go on identifying the problematic tables and columns which values contain markup.
I would then :
cleanup as best as I can those values in DB.
ensure HtmlEncoding of the corresponding outputs in my pages
I could then go for a basic global replace <%= becoming <%: and sanitize outputs on the long run.