How to avoid XSS in this asp.net code, url : http://localhost/Tiles/ShowPage.aspx?u=javascript:alert(1234)?

When this url is used in browser , an alert comes out .
http://localhost/Tiles/ShowPage.aspx?u=javascript:alert(1234)
I added "javascript:alert(1234)" part , it was spmething else .

The customer asked to avoid XSS kind of attck (like javascript:alert(1234))
I am very new to this, sorry it maybe lame to ask :
Where exacly this url is called in project, I dont know if it is in ShowPage.aspx or some where else.
I know I just need to use the code below to avoid XSS , but don't know where to use it!
HttpUtility.HtmlEncode(http://localhost/Tiles/ShowPage.aspx?u=javascript:alert(1234));

Thx in advance .

this is a massive topic, and way beyond a simple Q+A forum like SO.

You don't show the markup in that web page (maybe it is blank and defaulted, and you don't have any markup - but EVEN that information was BEYOND important.

You need basic asp.net skills here. So, what do you mean you don't know where the above is coming from? You typing in some js code, and it is being allowed.

Worse yet? You don't even bother to mention what framework version you are using here.

However, given that you NOT included some basic information here? That tells me you are in way beyond your skill levels here.

However, assuming say .net 4.x framework, and web forms (but ,again, we are clueless here), then I would check the web config:

<system.web>
   <httpRuntime targetFramework="4.8" requestValidationMode="4.8"/>

This should prevent your example "injection" example code from working, but as I stated, script injection and web site security is a WHOLE career path, and generally requires someone with years of experience in this area.

But then again, as noted, with such "feeble" information provided here, and worse being that you think such simple details are not required here?

We really do need more details here.

That JavaScript should not be able to run by default. now if that query URL parameter takes that input and displays that on the page (say as markup), then the JavaScript will run.

In most cases, asp.net already has protection built in against such injection.

Say we have a simple text box and markup like this:

<h3>Please enter some text</h3>
<asp:TextBox ID="TextBox1" runat="server" TextMode="MultiLine" Height="148px" Width="407px">
</asp:TextBox>
<br />
<br />
<asp:Button ID="Button1" runat="server" Text="Submit" CssClass="btn"
    OnClick="Button1_Click" />
<br />
<br />
<h3>You entered</h3>
<asp:Label ID="Label1" runat="server" Text="">
</asp:Label>

And code behind:

Protected Sub Button1_Click(sender As Object, e As EventArgs)

    Label1.Text = TextBox1.Text

End Sub

So, now we have this:

So, the issue is now input by a user is "displayed" and INJECTED into the page. Including that of javaScript I type in.

So, to prevent the above, then make sure on the page, we do NOT have this:

<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="WebForm4.aspx.vb" 
    Inherits="TestWeb.WebForm4"        
    ValidateRequest="false"        
    %>

So, lets remove the above ValidateRequest = false from that page.

So, now when we try this, we get:

So, as a general rule, a page should not allow USER input to be re-displayed. Since if we take user input, and "just display" what the user typed in, then the user can include markup, and VERY much worse, include script code.

Now, while I removed the validate request from the page, you should enable this "system" wide, and not per page in web config.

Thus you should have this in web confige (but, it is enabled by default anyway).

eg this:

<system.web>
  <httpRuntime targetFramework="4.8" requestValidationMode="4.8" />

As a result, then even typing in say

  <b>This is bold text</b>

That will trigger a error, and the page post-back will not be allowed.

In your example, in the url, some JavaScript code is typed in directly, but as a general rule, that script should not run unless the markup on the page is taking that URL parameter and displaying that raw un-touched value into the page for display.