Recently I was working on updating a legacy application to MVC 4 that included free form text input. When I set up the new site my initial approach was to not allow any rich HTML input, only simple text formatting that would respect a few simple HTML commands for bold, lists etc.
and automatically handles line break processing for new lines
and paragraphs. This is typical for what I do with most multi-line text input in my apps
and it works very well with very little development effort involved. Then the client sprung another note: Oh by the way we have a bunch of customers (real estate agents) who need to post complete HTML documents. Oh uh! There goes the simple theory. After some discussion
and pleading on my part (<snicker>) to try
and avoid this type of raw HTML input because of potential XSS issues, the client decided to go ahead
and allow raw HTML input anyway. There has been lots of discussions on this subject on StackOverFlow (and here
and here) but to after reading through some of the solutions I didn't really find anything that would work even closely for what I needed. Specifically we need to be able to allow just about any HTML markup, with the exception of script code. Remote CSS
and Images need to be loaded, links need to work
and so. While the 'legit' HTML posted by these agents is basic in nature it does span most of the full gamut of HTML (4). Most of the solutions XSS prevention/sanitizer solutions I found were way to aggressive
and rendered the posted output unusable mostly because they tend to strip any externally loaded content. In short I needed a custom solution. I thought the best solution to this would be to use an HTML parser - in this case the Html Agility Pack -
and then to run through all the HTML markup provided
and remove any of the blacklisted tags
and a number of attributes that are prone to JavaScript injection. There's much discussion on whether to use blacklists vs. whitelists in the discussions mentioned above, but I found that whitelists can make sense in simple scenarios where you might allow manual HTML input, but when you need to allow a larger array of HTML functionality a blacklist is probably easier to manage as the vast majority of elements
and attributes could be allowed. Also white listing gets a bit more complex with HTML5
and the new proliferation of new HTML tags
and most new tags generally don't affect XSS issues directly. Pure whitelisting based on elements
and attributes also doesn't capture many edge cases (see some of the XSS cheat sheets listed below) so even with a white list, custom logic is still required to handle many of those edge cases. The Microsoft Web Protection Library (AntiXSS) My first thought was to check out the Microsoft AntiXSS library. Microsoft has an HTML Encoding
and Sanitation library in the Microsoft Web Protection Library (formerly AntiXSS Library) on CodePlex, which provides stricter functions for whitelist encoding
and sanitation. Initially I thought the Sanitation class
and its static members would do the trick for me,but I found that this library is way too restrictive for my needs. Specifically the Sanitation class strips out images
and links which rendered the full HTML from our real estate clients completely useless. I didn't spend much time with it, but apparently I'm not alone if feeling this library is not really useful without some way to configure operation. To give you an example of what didn't work for me with the library here's a small
and simple HTML fragment that includes script, img
and anchor tags. I would expect the script to be stripped
and everything else to be left intact. Here's the original HTML:var value = "<b>Here</b> <script>alert('hello')</script> we go. Visit the " + "<a href='http://west-wind.com'>West Wind</a> site. " + "<img src='http://west-wind.com/images/new.gif' /> " ;
and the code to sanitize it with the AntiXSS Sanitize class:@Html.Raw(Microsoft.Security.Application.Sanitizer.GetSafeHtmlFragment(value))
This produced a not so useful sanitized string:
Here we go. Visit the <a>West Wind</a> site.
While it removed the <script> tag (good) it also removed the href from the link
and the image tag altogether (bad). In some situations this might be useful, but for most tasks I doubt this is the desired behavior. While links can contain javascript: references
and images can 'broadcast' information to a server, without configuration to tell the library what to restrict this becomes useless to me. I couldn't find any way to customize the white list, nor is there code available in this 'open source' library on CodePlex.
Using Html Agility Pack for HTML Parsing
The WPL library wasn't going to cut it. After doing a bit of research I decided the best approach for a custom solution would be to use an HTML parser
and inspect the HTML fragment/document I'm trying to import. I've used the HTML Agility Pack before for a number of apps where I needed an HTML parser without requiring an instance of a full browser like the Internet Explorer Application object which is inadequate in Web apps. In case you haven't checked out the Html Agility Pack before, it's a powerful HTML parser library that you can use from your .NET code. It provides a simple, parsable HTML DOM model to full HTML documents or HTML fragments that let you walk through each of the elements in your document. If you've used the HTML or XML DOM in a browser before you'll feel right at home with the Agility Pack.
Blacklist based HTML Parsing to strip XSS Code
For my purposes of HTML sanitation, the process involved is to walk the HTML document one element at a time
and then check each element
and attribute against a blacklist. There's quite a bit of argument of what's better: A whitelist of allowed items or a blacklist of denied items. While whitelists tend to be more secure, they also require a lot more configuration. In the case of HTML5 a whitelist could be very extensive. For what I need, I only want to ensure that no JavaScript is executed, so a blacklist includes the obvious <script> tag plus any tag that allows loading of external content including <iframe>, <object>, <embed>
and <link> etc. <form> is also excluded to avoid posting content to a different location. I also disallow <head>
and <meta> tags in particular for my case, since I'm only allowing posting of HTML fragments. There is also some internal logic to exclude some attributes or attributes that include references to JavaScript or CSS expressions.
The default tag blacklist reflects my use case, but is customizable
and can be added to.
Here's my HtmlSanitizer implementation:using System.Collections.Generic;
using System.IO;
using System.Xml;
using HtmlAgilityPack;
namespace Westwind.Web.Utilities
{
public class HtmlSanitizer
{
public HashSet<string> BlackList = new HashSet<string>()
{
{ "script" },
{ "iframe" },
{ "form" },
{ "object" },
{ "embed" },
{ "link" },
{ "head" },
{ "meta" }
};
/// <summary>
/// Cleans up an HTML string
and removes HTML tags in blacklist
/// </summary>
/// <param name="html"></param>
/// <returns></returns>
public static string SanitizeHtml(string html, params string[] blackList)
{
var sanitizer = new HtmlSanitizer();
if (blackList != null && blackList.Length > 0)
{
sanitizer.BlackList.Clear();
foreach (string item in blackList)
sanitizer.BlackList.Add(item);
}
return sanitizer.Sanitize(html);
}
/// <summary>
/// Cleans up an HTML string by removing elements
/// on the blacklist
and all elements that start
/// with onXXX .
/// </summary>
/// <param name="html"></param>
/// <returns></returns>
public string Sanitize(string html)
{
var doc = new HtmlDocument();
doc.LoadHtml(html);
SanitizeHtmlNode(doc.DocumentNode);
//return doc.DocumentNode.WriteTo();
string output = null;
// Use an XmlTextWriter to create self-closing tags
using (StringWriter sw = new StringWriter())
{
XmlWriter writer = new XmlTextWriter(sw);
doc.DocumentNode.WriteTo(writer);
output = sw.ToString();
// strip off XML doc header
if (!string.IsNullOrEmpty(output))
{
int at = output.IndexOf("?>");
output = output.Substring(at + 2);
}
writer.Close();
}
doc = null;
return output;
}
private void SanitizeHtmlNode(HtmlNode node)
{
if (node.NodeType == HtmlNodeType.Element)
{
// check for blacklist items
and remove
if (BlackList.Contains(node.Name))
{
node.Remove();
return;
}
// remove CSS Expressions
and embedded script links
if (node.Name == "style")
{
if (string.IsNullOrEmpty(node.InnerText))
{
if (node.InnerHtml.Contains("expression") || node.InnerHtml.Contains("javascript:"))
node.ParentNode.RemoveChild(node);
}
}
// remove script attributes
if (node.HasAttributes)
{
for (int i = node.Attributes.Count - 1; i >= 0; i--)
{
HtmlAttribute currentAttribute = node.Attributes[i];
var attr = currentAttribute.Name.ToLower();
var val = currentAttribute.Value.ToLower();
span style="background: white; color: green">// remove event handlers
if (attr.StartsWith("on"))
node.Attributes.Remove(currentAttribute);
// remove script links
else if (
//(attr == "href" || attr== "src" || attr == "dynsrc" || attr == "lowsrc") &&
val != null &&
val.Contains("javascript:"))
node.Attributes.Remove(currentAttribute);
// Remove CSS Expressions
else if (attr == "style" &&
val != null &&
val.Contains("expression") || val.Contains("javascript:") || val.Contains("vbscript:"))
node.Attributes.Remove(currentAttribute);
}
}
}
// Look through child nodes recursively
if (node.HasChildNodes)
{
for (int i = node.ChildNodes.Count - 1; i >= 0; i--)
{
SanitizeHtmlNode(node.ChildNodes[i]);
}
}
}
}
}
Please note: Use this as a starting point only for your own parsing
and review the code for your specific use case! If your needs are less lenient than mine were you can you can make this much stricter by not allowing src
and href attributes or CSS links if your HTML doesn't allow it. You can also check links for external URLs
and disallow those - lots of options. The code is simple enough to make it easy to extend to fit your use cases more specifically. It's also quite easy to make this code work using a WhiteList approach if you want to go that
route. The code above is semi-generic for allowing full featured HTML fragments that only disallow script related content.
The Sanitize method walks through each node of the document
and then recursively drills into all of its children until the entire document has been traversed. Note that the code here uses an XmlTextWriter to write output - this is done to preserve XHTML style self-closing tags which are otherwise left as non-self-closing tags.
The sanitizer code scans for blacklist elements
and removes those elements not allowed. Note that the blacklist is configurable either in the instance class as a property or in the static method via the string parameter list. Additionally the code goes through each element's attributes
and looks for a host of rules gleaned from some of the XSS cheat sheets listed at the end of the post. Clearly there are a lot more XSS vulnerabilities, but a lot of them apply to ancient browsers (IE6
and versions of Netscape) - many of these glaring holes (like CSS expressions - WTF IE?) have been removed in modern browsers.
What a Pain
To be honest this is NOT a piece of code that I wanted to write. I think building anything related to XSS is better left to people who have far more knowledge of the topic than I do. Unfortunately, I was unable to find a tool that worked even closely for me, or even provided a working base. For the project I was working on I had no choice
and I'm sharing the code here merely as a base line to start with
and potentially expand on for specific needs. It's sad that Microsoft Web Protection Library is currently such a train wreck - this is really something that should come from Microsoft as the systems vendor or possibly a third party that provides security tools.
Luckily for my application we are dealing with a authenticated
and validated users so the user base is fairly well known,
and relatively small - this is not a wide open Internet application that's directly public facing. As I mentioned earlier in the post, if I had my way I would simply not allow this type of raw HTML input in the first
place,
and instead rely on a more controlled HTML input mechanism like MarkDown or even a good HTML Edit control that can provide some limits on what types of input are allowed. Alas in this case I was overridden
and we had to go forward
and allow *any* raw HTML posted.
Sometimes I really feel sad that it's come this far - how many good applications
and tools have been thwarted by fear of XSS (or worse) attacks? So many things that could be done *if* we had a more secure browser experience
and didn't have to deal with every little script twerp trying to hack into Web pages
and obscure browser bugs. So much time wasted building secure apps, so much time wasted by others trying to hack apps… We're a funny species - no other species manages to waste as much time, effort
and resources as we humans do :-)
Resources
Code on GitHub
Html Agility Pack
XSS Cheat Sheet
XSS Prevention Cheat Sheet
Microsoft Web Protection Library (AntiXss)
StackOverflow Links:
http://stackoverflow.com/questions/341872/html-sanitizer-for-net
http://blog.stackoverflow.com/2008/06/safe-html-and-xss/
http://code.google.com/p/subsonicforums/source/browse/trunk/SubSonic.Forums.Data/HtmlScrubber.cs?r=61© Rick Strahl, West Wind Technologies, 2005-2012Posted in Security HTML ASP.NET JavaScript
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