Ideas for Improving ASP and ASP.NET Web Application Security - Part 1
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by Brett Burridge
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Clean user input before redisplaying

As described earlier, care should be taken to clean up user input before it is used in SQL queries in order to fend against SQL injection attacks.  Unfortunately, similar techniques can be used by malicious users to perform other actions.  At the very least these can be a nuisance, but there is potential to perform actions that mislead your website's other users, make it easier to launch cross-site scripting attacks or to launch fishing attempts.

Any user input that is gathered by a web application and then redisplayed on a web page should be cleansed before it is displayed on that web page.  In particular, care should be taken to remove all of the HTML tags that a user may have entered or, alternatively, all HTML mark-ups should be removed except for a few "safe" tags, such as bold and italics tags and paragraph formatting tags.  Particular care should be taken to remove <script> tags entered by the user.  Failure to remove script tags will allow a malicious user to run JavaScripts within pages that display their input.  This is a particular problem on websites where one user's input is viewable by many people, such as on bulletin boards and discussion forums.  At the very least they might cause annoying JavaScript alert windows to appear when someone views a page containing their script input.  They could, however, potentially make use of the JavaScript document.location method to cause the user's browser to automatically go to a different URL when they visit your website.

To remove all HTML tags from user input, a regular expression substitution can be used, such as the one shown in the function below.

Function stripHTMLtags(HTMLstring)
Set RegularExpressionObject = New RegExp
With RegularExpressionObject
.Pattern = "<[^>]+>"
.IgnoreCase = True
.Global = True
End With
stripHTMLtags =RegularExpressionObject.Replace(HTMLstring, "")
Set RegularExpressionObject = nothing
End Function

Alternatively, the user's input can be converted using the Server.HTMLEncode method in classic ASP or the HTMLEncode method of the HttpUtility class in the .NET Framework.  This method would prevent any of the user's inputted HTML from being interpreted by a web browser.

See the ASPAlliance articles http://authors.aspalliance.com/brettb/VBScriptRegularExpressions.asp and http://aspalliance.com/555 for further information about using Regular Expressions in ASP.  Regular Expressions are also available in ASP.NET through the System.Text.RegularExpressions namespace.

A safer alternative to allowing users to enter HTML is to permit them to use the Bulletin Board Code (BBCode) text formatting system that is commonly used on many web based bulletin boards.  There are a number of ASP scripts available that will safely convert BBCode formatting to HTML using ASP.


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