How To Review Visual Basic WebClasses (IIS Applications) for CSSI Vulnerability (253155)

SUMMARY

This article describes how to identify and correct Visual Basic WebClass (IIS Applications) that are susceptible to cross-site scripting security issues (CSSI). Only input that is not correctly validated or formatted makes your application vulnerable to attack.

This article focuses on Visual Basic 6.0 Webclass functionality (for example, replacement tags). For general guidelines regarding Active Server Pages (ASP) code, please see the following article in the Microsoft Knowledge Base:

253119 How To Review ASP Code for CSSI Vulnerability

MORE INFORMATION

Visual Basic Webclass developers should follow the guidelines for ASP code. These include:

Set the CHARSET for the page through the use of either a <META> tag or Response.Charset.
Use Server.HTMLEncode() to encode any output that is generated from user input.
Use Server.URLEncode() to encode any output that is generated from user input that is included as part of a URL.

The following sample code is an example of how you can use Server.HTMLEncode() and Server.URLEncode() in a Visual Basic 6.0 Webclass application to protect against the cross-site scripting vulnerability. It contains an HTML template (with replacement tags) and the associated Webclass source code. User-generated output that is included in the page source is encoded through the use of Server.HTMLEncode(). User-generated output that is included in the URL is encoded though the use of Server.URLEncode():

Template1.htm

<HTML>
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset= ISO-LATIN-1">
<TITLE>Webclass Sample</TITLE>
</HEAD>
<BODY background=CSSIDSR.ASP?WCI=Template1&amp;WCE=Body1&amp;WCU>
<P>
Here is some user generated output: <WC@UserOutput></WC@UserOutput>
</P>
<P>
Here is a URL that contains user generated output: <a href="<WC@URLFor></WC@URLFor>&userdata=<WC@UserOutputURL></WC@UserOutputURL>">Click here!</a>
</P>
</BODY>
</HTML>

WebClass Code

Option Explicit
Option Compare Text

Private Sub WebClass_Start()
  Set NextItem = Template1
End Sub

Private Sub Template1_Respond()
  Template1.WriteTemplate
End Sub

Private Sub Template1_ProcessTag(ByVal TagName As String, TagContents As String, SendTags As Boolean)
  Select Case TagName
    Case "WC@UserOutput"
      TagContents = Server.HTMLEncode("User Generated Output Here")
    Case "WC@URLFor"
      TagContents = URLFor(Template1)
    Case "WC@UserOutputURL"
      TagContents = Server.URLEncode("User Generated Output for URL Here")
  End Select
End Sub

REFERENCES

For more information, see the following advisory from the Computer Emergency Response Team (CERT) at Carnegie Mellon University:

CERT(r) Advisory CA-2000-02 Malicious HTML Tags Embedded in Client Web Requests

For additional information, click the article numbers below to view the articles in the Microsoft Knowledge Base:

252985 How To Prevent Cross-Site Scripting Security Issues

253119 How To Review ASP Code for CSSI Vulnerability

253121 How To Review MTS/ASP Code for CSSI Vulnerability

253120 How To Review Visual InterDev Generated Code for CSSI Vulnerability

253117 Preventing Internet Explorer and Outlook Express Cross-Site Scripting Security Issues