Develop Reference

How to use AntiXss with a Web API

This is a question that has been asked before, but I've not found the information I'm looking for or maybe I'm just missing the point so please bear with me. I can always adjust my question if I'm asking it the wrong way.
If for example, I have a POST endpoint that use a simply DTO object with 2 properties (i.e. companyRequestDto) and contains a script tag in one of its properties. When I call my endpoint from Postman I use the following:
{
"company": "My Company<script>alert(1);</script>",
"description": "This is a description"
}
When it is received by the action in my endpoint,
public void Post(CompanyRequestDto companyRequestDto)
my DTO object will automatically be set and its properties will be set to:
companyDto.Company = "My Brand<script>alert(1);</script>";
companyDto.Description = "This is a description";
I clearly don't want this information to be stored in our database as is, nor do I want it stored as an escaped string as displayed above.
1) Request: So my first question is how do I throw an error if the DTO posted contains some invalid content such as the tag?
I've looked at Microsoft AntiXss but I don't understand how to handle this as the data provided in the properties of a DTO object is not an html string but just a string, so What I am missing here as I don't understand how this is helping sanitizing or validating the passed data.
When I call
var test = AntiXss.AntiXssEncoder.HtmlEncode(companyRequestDto.Company, true);
It returns an encoded string, but then what??
Is there a way to remove disallowed keywords or just simply throw an error?
2) Response: Assuming 1) was not implemented or didn't work properly and it ended up being stored in our database, am I suppose to return encoded data as a json string, so instead of returning:
"My company"
Am I suppose to return:
"My Company<script>alert(1)</script>"
Is the browser (or whatever app) just supposed to display as below then?:
"My Company<script>alert(1)</script>"
3) Code: Assuming there is a way to sanitize or throw an error, should I use this at the property level using attribute on all the properties of my various DTO objects or is there a way to apply this at the class level using an attribute that will validate and/or sanitize all string properties of a DTO object for example?
I found interesting articles but none really answering my problems or I'm having other problems with some of the answers:
asp.net mvc What is the difference between AntiXss.HtmlEncode and HttpUtility.HtmlEncode?
Stopping XSS when using WebAPI (currently looking into this one but don't see how example is solving problem as property is always failing whether I use the script tag or not)
how to sanitize input data in web api using anti xss attack (also looking at this one but having a problem calling ReadFromStreamAsync from my project at work. Might be down to some of the settings in my web.config but haven't figured out why but it always seems to return an empty string)
Thanks.
UPDATE 1:
I've just finished going through the answer from Stopping XSS when using WebAPI
This is probably the closest one to what I am looking for. Except I don't want to encode the data, as I don't want to store it in my database, so I'll see if I can figure out how to throw an error but I'm not sure what the condition will be. Maybe I should just look for characters such as <, >, ; , etc... as these will not likely be used in any of our fields.

You need to consider where your data will be used when you think about encoding, so that data with in it is only a problem if it's rendered as HTML so if you are going to display data that has been provided by users anywhere, it's probably at the point you are going to display it that you would want to html encode it for display (you want to avoid repeatedly html encoding the same string when saving it for example).
Again, it depends what the response is going to be used for... you probably want to html encode it at the point it's going to be displayed... remember if you are encoding something in the response it may not match whats in data so if the calling code could do something like call your API to search for a company with that name that could cause problems. If the browser does display the html encoded version it might look ugly but it's better than users being compromised by XSS attacks.
It's quite difficult to sanitize text for things like tags if you allow most characters for normal use. It's easier if you can whitelist characters allowed and only allow, say, alphanumeric but that isn't often possible. This can be done using a regex validation attribute on the DTO object. The best approach I think is to encode values for display if you can't stop certain characters. It's really difficult to try to allow all characters but avoid things like as people can start using ascii characters etc.

Is it possible to change the way Attribute.Add formats the addition of the attribute?

My Question: Is it possible to change the way Attribute.Add formats the addition of the attribute?
I have an ASP.net website that loads a widget in a div, and I'm trying to find a way to add a data-options attribute to the div with my codebehind. I need the attribute to be created with a single quote around the data-options value instead of double quotes, because the value I'm assigning is a JSON pair.
What I need the attribute to look like:
data-options='{“post_message_origin”:”https://www.mysite.com/MyWidget.aspx”}'
What it looks like when using Attribute.Add("data-options"):
My code:
string dataoptions = "{\"post_message_origin\":\""+ HttpContext.Current.Request.Url.AbsoluteUri + "\"}";
MYWIDGET.Attributes.Add("data-options", dataoptions);
The attribute result:
data-options="{“post_message_origin”:”https://www.mysite.com/MyWidget.aspx”}"
The set of double quotes encompassing the data-options value is preventing the JSON pair from being read correctly, hence my question.
I'm doing my best to avoid using hard coding so that I can easily load the page from development servers to production servers without changing the code, which is why I'm using HttpContext.Current.Request.Url.AbsoluteUri in the code behind instead of writing the data-options value straight to the div in the ASP markup.

I would suggest using single quotes with the JSON, in this case. Either is acceptable, as long as they are in open-close pairs. This sidesteps the issue.
EDIT: Unfortunately, Attribute.Add encodes the quotes...
This has been brought up before. It looks like the long term solution is implementing your own encoder...

I would recommend including neither kind of quote in your data:
string dataoptions = "{\"post_message_origin\":\""+
HttpContext.Current.Request.Url.AbsoluteUri + "\"}";
dataoptions = dataoptions.Replace("\"", """).Replace("'", "'");
MYWIDGET.Attributes.Add("data-options", dataoptions);

Regular expression to get the Name from URL link

I have a Hyperlink field (aka column) in SharePoint 2010.
Say it's called SalesReportUrl. The url looks like:
http://portal.cab.com/SalessiteCollection/October2012Library/Forms/customview.aspx
The hyperlink field stores values in two fields (the link and description).
What would be the RegEx if I want to get the October2012Library out of the Url?
I tried this but it's definitely not working:
#"<a[\s]+[^>]*?href[\s]?=[\s\"\']+(.*?)[\"\']+.*?>([^<]+|.*?)?<\/a>";
I also tried:
^(.*?/)?Forms/$
but no luck.
I think sharepoint stores hyperlink like this:
http://portal.cab.com/SalessiteCollection/October2012Library/Forms/customview.aspx, some description
Looks like this has a solution. but what's the syntax substring get the list or library name ?https://sharepoint.stackexchange.com/questions/40712/get-list-title-in-sharepoint-designer-workflow

How about this (as Daniel suggested) :
string url = #"http://portal.cab.com/SalessiteCollection/October2012Library/Forms/customview.aspx";
Uri uri = new Uri(url);
if(uri.Segments.Length > 2))
Console.WriteLine(uri.Segments[2]); // will output "October2012Library/"
you can add .Replace("/", string.Empty) if you want to get rid of the "/"
Console.WriteLine(uri.Segments[2].Replace("/", string.Empty));

http://[^/]+/[^/]+/([^/]+)/
match's group[1] is the value you need. it gets the 3rd part (divided by /) in the url. if you need make sure it is followed by other parts, i.e. forms, add it at the end.

try using this new RegEx("SalessiteCollection/(.+?)/Forms").match(<urlString>).groups[1].value
Though it is a rough answer, you might have to make few corrections but I hope you understand what I am trying to explain.

maybe this?
http:\/\/([a-zA-Z0-9]([a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])?\.)+[a-zA-Z]{2,6}\/[a-zA-Z]*\/([a-zA-Z0-9]*)\/
http://rubular.com/r/LuuuORPRXt

Request.QueryString[] vs. Request.Query.Get() vs. HttpUtility.ParseQueryString()

I searched SO and found similar questions, but none compared all three. That surprised me, so if someone knows of one, please point me to it.
There are a number of different ways to parse the query string of a request... the "correct" way (IMO) should handle null/missing values, but also decode parameter values as appropriate. Which of the following would be the best way to do both?
Method 1
string suffix = Request.QueryString.Get("suffix") ?? "DefaultSuffix";
Method2
string suffix = Request.QueryString["suffix"] ?? "DefaultSuffix";
Method 3
NameValueCollection params = HttpUtility.ParseQueryString(Request.RawUrl);
string suffix = params.Get("suffix") ?? "DefaultSuffix";
Method 4
NameValueCollection params = HttpUtility.ParseQueryString(Request.RawUrl);
string suffix = params["suffix"] ?? "DefaultSuffix";
Questions:
Would Request.QueryString["suffix"] return a null if no suffix was specified?
(Embarrassingly basic question, I know)
Does HttpUtility.ParseQueryString() provide any extra functionality over accessing Request.QueryString directly?
The MSDN documentation lists this warning:
The ParseQueryString method uses query strings that might contain user input, which is a potential security threat. By default, ASP.NET Web pages validate that user input does not include script or HTML elements. For more information, see Script Exploits Overview.
But it's not clear to me if that means ParseQueryString() should be used to handle that, or is exposed to security flaws because of it... Which is it?
ParseQueryString() uses UTF8 encoding by default... do all browsers encode the query string in UTF8 by default?
ParseQueryString() will comma-separate values if more than one is specified... does Request.QueryString() do that as well, or what happens if it doesn't?
Which of those methods would correctly decode "%2b" to be a "+"?
Showing my Windows development roots again... and I would be a much faster developer if I didn't wonder about these things so much... : P

Methods #1 and #2 are the same thing, really. (I think the .Get() method is provided for language compatibility.)
ParseQueryString returns you something that is the functional equivalent of Request.Querystring. You would usually use it when you have a raw URL and no other way to parse the query string parameters from it. Request.Querystring does that for you, so in this case, it's not needed.
You can't leave off "suffix". You either have to pass a string or an index number. If you leave off the [] entirely, you get the whole NameValueCollection. If you mean what if "suffix" was not one of the QueryString values then yes; you would get null if you called Request.QueryString["suffix"].
No. The most likely time you would use it is if you had an external URL and wanted to parse the query string parameters from it.
ParseQueryString does not handle it... neither does pulling the values straight from Request.QueryString. For ASP.NET, you usually handle form values as the values of controls, and that is where ASP.NET usually 'handles' these things for you. In other words: DON'T TRUST USER INPUT Ever. No matter what framework is doing what ever for you.
I have no clue (I think no). However, I think what you are reading is telling you that ParseQueryString is returning UTF-8 encoded text - regardless if it was so encoded when it came in.
Again: ParseQueryString returns basically the same thing you get from Request.QueryString. In fact, I think ParseQueryString is used internally to provide Request.QueryString.
They would produce the equivalent; they will all properly decode the values submitted. If you have URL: http://site.com/page.aspx?id=%20Hello then call Request.QueryString["id"] the return value will be " Hello", because it automatically decodes.

Example 1:
string itsMeString = string.IsNullOrEmpty(Request.QueryString["itsMe"]) ? string.Empty : HttpUtillity.UrlDecode(Request.QueryString["itsMe"]);
Stright to your questions:
Not quite sure what do you mean by suffix, if you are asking what happens if the key is not present(you don't have it in the QueryString) - yes it will return null.
My GUESS here is that when constructed, Request.QueryString internally calls HttpUtillity.ParseQueryString() method and caches the NameValueCollection for subsequential access. I think the first is only left so you can use it over a string that is not present in the Request, for example if you are scrapping a web page and need to get some arguments from a string you've found in the code of that page. This way you won't need to construct an Uri object but will be able to get just the query string as a NameValueCollection if you are sure you only need this. This is a wild guess ;).)
This is implemented on a page level so if you are accessing the QueryString let's say in Page_Load event handler, you are having a valid and safe string (ASP.NET will throw an exception otherwise and will not let the code flow enter the Page_Load so you are protected from storing XSS in your database, the exception will be: "A potentially dangerous Request.QueryString value was detected from the client, same as if a post variable contains any traces of XSS but instead Request.Form the exception says Request.QueryString."). This is so if you let the "validateRequest" switched on (by default it is). The ASP.NET pipeline will throw an exception earlier, so you don't have the chance to save any XSS things to your store (Database). Switching it off implies you know what you're doing so you will then need to implement the security yourself (by checking what's comming in).
Probably it will be safe to say yes. Anyway, since you will in most cases generating the QueryString on your own (via JavaScript or server side code - be sure to use HttpUtillity.UrlEncode for backend code and escape for JavaScript). This way the browser will be forced to turn "It's me!" to "It%27s%20me%21". You can refer to this article for more on Url Encoding in JavaScript: http://www.javascripter.net/faq/escape.htm.
Please elaborate on that, couldn't quite get what do you mean by "will comma-separate values if more than one is specified.".
As far as I remember, none of them will. You will probably need to call HttpUtillity.UrlDecode / HttpUtillity.HtmlDecode (based on what input do you have) to get the string correctly, in the above example with "It's me!" you will do something like (see Example 1 as something's wrong with the code formatting if I put it after the numbered list).

How do I use a pattern Url to extract a segment from an actual Url?

If I have a series of "pattern" Urls of the form:
http://{username}.sitename.com/
http://{username}.othersite.net/
http://mysite.com/{username}
and I have an actual Url of the form:
http://joesmith.sitename.com/
Is there any way that I can match a pattern Url and in turn use it to extract the username portion out the actual Url? I've thought of nasty ways to do it, but it just seems like there should be a more intuitive way to accomplish this.
ASP.NET MVC uses a similar approach to extract the various segments of the URL when it is building its routes. Given the example:
{controller}/{action}
So given the Url of the form, Home/Index, it knows that it is the Home controller calling the Index action method.

Not sure I understand this question correctly but you can just use a regular expression to match anything between 'http://' and the first dot.

A very simple regex will do:
':https?://([a-z0-9\.-]*[a-z0-9])\.sitename\.com'
This will allow any subdomain that only contains valid subdomain characters. Example of allowed subdomains:
joesmith.sitename.com
joe.smith.sitename.com
joe-smith.sitename.com
a-very-long-subdomain.sitename.com
As you can see, you might want to complicate the regex slightly. For instance, you could limit it to only allow a certain amount of characters in the subdomain.

It seems the the quickest and easiest solution is going off of Machine's answer.
var givenUri = "http://joesmith.sitename.com/";
var patternUri = "http://{username}.sitename.com/";
patternUri = patternUri.Replace("{username}", #"([a-z0-9\.-]*[a-z0-9]");
var result = Regex.Match(givenUri, patternUri, RegexOptions.IgnoreCase).Groups;
if(!String.IsNullOrEmpty(result[1].Value))
return result[1].Value;
Seems to work great.

Well, this "pattern URL" is a format you've made up, right? You basically you'll just need to process it.
If the format of it is:
anything inside "{ }" is a thing to capture, everything else must be as is
Then you'd just find the start/end index of those brackets, and match everything else. Then when you get to a place where one is, make sure you only look for chars such that they don't match whatever 'token' comes after the next ending '}'.

There are definitely different ways - ultimately though your server must be configured to handle (and possibly route) these different subdomain requests.
What I would do would be to answer all subdomain requests (except maybe some reserved words, like 'www', 'mail', etc.) on sitename.com with a single handler or page (I'm assuming ASP.NET here based on your C# tag).
I'd use the request path, which is easy enough to get, with some simple string parsing/regex routines (remove the 'http://', grab the first token up until '.' or '/' or '\', etc.) and then use that in a session, making sure to observe URL changes.
Alternately, you could map certain virtual paths to request urls ('joesmith.sitename.com' => 'sitename.com/index.aspx?username=joesmith') via IIS but that's kind of nasty too.
Hope this helps!