A lot of websites allow users to input text and submit it to the site.
This could be forums, blogs, content management systems etc.
Imaging if the user writes HTML into these form fields?
It could be perfectly harmless when used for styling, but it could also be used the wrong way.
A typical scenario would be when a user enters JavaScript that does harmful things or embedding a style sheet that ruins the websites layout.
This is normally referred to as Cross-Site Scripting (XSS).
We have to mitigate that risk, and that’s when regular expression comes to the rescue.
Here is a very simple method that strips all HTML tags from a string or just the harmful tags – you decide.
The method takes two parameters: the string that needs tag removal and a boolean flag that determines if harmless tags are allowed or not.
public static string StripHtml(string html, bool allowHarmlessTags)
{
if (html == null || html == string.Empty)
return string.Empty;
if (allowHarmlessTags)
return System.Text.RegularExpressions.Regex.Replace(html, "?(?i:script|embed|object|frameset|frame|iframe|meta|link|style)(.|\\n)*?>", string.Empty);
return System.Text.RegularExpressions.Regex.Replace(html, "]*>", string.Empty);
}
You can add more harmful tags to the regular expression string if you’d like.
Enjoy.
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Mads Kristensen currently works as a Senior Developer at Traceworks located
in Copenhagen, Denmark. Mads graduated from Copenhagen Technical Academy with a multimedia degree in
2003, but has been a professional developer since 2000. His main focus is on ASP.NET but is responsible for Winforms, Windows- and
web services in his daily work as well. A true .NET developer with great passion for the simple solution.
