I been finding that for something that I consider pretty import there is very little information or libraries on how to deal with this problem.
I found this while searching. I really don't know all the million ways that a hacker could try to insert the dangerous tags.
I have a rich html editor so I need to keep non dangerous tags but strip out bad ones.
So is this script missing anything?
It uses html agility pack.
public string ScrubHTML(string html)
{
HtmlDocument doc = new HtmlDocument();
doc.LoadHtml(html);
//Remove potentially harmful elements
HtmlNodeCollection nc = doc.DocumentNode.SelectNodes("//script|//link|//iframe|//frameset|//frame|//applet|//object|//embed");
if (nc != null)
{
foreach (HtmlNode node in nc)
{
node.ParentNode.RemoveChild(node, false);
}
}
//remove hrefs to java/j/vbscript URLs
nc = doc.DocumentNode.SelectNodes("//a[starts-with(translate(@href, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz'), 'javascript')]|//a[starts-with(translate(@href, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz'), 'jscript')]|//a[starts-with(translate(@href, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz'), 'vbscript')]");
if (nc != null)
{
foreach (HtmlNode node in nc)
{
node.SetAttributeValue("href", "#");
}
}
//remove img with refs to java/j/vbscript URLs
nc = doc.DocumentNode.SelectNodes("//img[starts-with(translate(@src, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz'), 'javascript')]|//img[starts-with(translate(@src, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz'), 'jscript')]|//img[starts-with(translate(@src, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz'), 'vbscript')]");
if (nc != null)
{
foreach (HtmlNode node in nc)
{
node.SetAttributeValue("src", "#");
}
}
//remove on<Event> handlers from all tags
nc = doc.DocumentNode.SelectNodes("//*[@onclick or @onmouseover or @onfocus or @onblur or @onmouseout or @ondoubleclick or @onload or @onunload]");
if (nc != null)
{
foreach (HtmlNode node in nc)
{
node.Attributes.Remove("onFocus");
node.Attributes.Remove("onBlur");
node.Attributes.Remove("onClick");
node.Attributes.Remove("onMouseOver");
node.Attributes.Remove("onMouseOut");
node.Attributes.Remove("onDoubleClick");
node.Attributes.Remove("onLoad");
node.Attributes.Remove("onUnload");
}
}
// remove any style attributes that contain the word expression (IE evaluates this as script)
nc = doc.DocumentNode.SelectNodes("//*[contains(translate(@style, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz'), 'expression')]");
if (nc != null)
{
foreach (HtmlNode node in nc)
{
node.Attributes.Remove("stYle");
}
}
return doc.DocumentNode.WriteTo();
}
