Sanitizing HTML using Jeff Atwood's example

Question

I'm working on sanitizing my Html using Jeff Atwood's code found here

But the problem I'm running into is when I input Markdown links into the form (they get removed)

<http://www.example.com&gt;

Here's the code I'm using.

private static Regex _tags = new Regex("<[^>]*(>|$)",
    RegexOptions.Singleline | RegexOptions.ExplicitCapture | RegexOptions.Compiled);
private static Regex _whitelist = new Regex(@"
    ^</?(b(lockquote)?|code|d(d|t|l|el)|em|h(1|2|3)|i|kbd|li|ol|p(re)?|s(ub|up|trong|trike)?|ul)>$|
    ^<(b|h)r\s?/?>$",
    RegexOptions.Singleline | RegexOptions.ExplicitCapture | RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);
private static Regex _whitelist_a = new Regex(@"
    ^<a\s
    href=""(\#\d+|(https?|ftp)://[-a-z0-9+&@#/%?=~_|!:,.;\(\)]+)""
    (\stitle=""[^""<>]+"")?\s?>$|
    ^</a>$",
    RegexOptions.Singleline | RegexOptions.ExplicitCapture | RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);
private static Regex _whitelist_img = new Regex(@"
    ^<img\s
    src=""https?://[-a-z0-9+&@#/%?=~_|!:,.;\(\)]+""
    (\swidth=""\d{1,3}"")?
    (\sheight=""\d{1,3}"")?
    (\salt=""[^""<>]*"")?
    (\stitle=""[^""<>]*"")?
    \s?/?>$",
    RegexOptions.Singleline | RegexOptions.ExplicitCapture | RegexOptions.Compiled | RegexOptions.IgnorePatternWhitespace);


/// <summary>
/// sanitize any potentially dangerous tags from the provided raw HTML input using 
/// a whitelist based approach, leaving the "safe" HTML tags
/// CODESNIPPET:4100A61A-1711-4366-B0B0-144D1179A937
/// </summary>
public static string Sanitize(string html)
{
    if (String.IsNullOrEmpty(html)) return html;

    string tagname;
    Match tag;

    // match every HTML tag in the input
    MatchCollection tags = _tags.Matches(html);
    for (int i = tags.Count - 1; i > -1; i--)
    {
        tag = tags[i];
        tagname = tag.Value.ToLowerInvariant();

        if(!(_whitelist.IsMatch(tagname) || _whitelist_a.IsMatch(tagname) || _whitelist_img.IsMatch(tagname)))
        {
            html = html.Remove(tag.Index, tag.Length);
            System.Diagnostics.Debug.WriteLine("tag sanitized: " + tagname);
        }
    }

    return html;
}

Answer 1

Yeah, because that's not valid HTML... So the code is doing what it purports to do.

Since Markdown allows embedded HTML, but HTML does not allow (all forms of) embedded Markdown, I suggest you convert the Markdown to HTML first, and then sanitize it...