AntiXSS in ASP.Net Core

Question

Microsoft Web Protection Library (AntiXSS) has reached End of Life. The page states "In .NET 4.0 a version of AntiXSS was included in the framework and could be enabled via configuration. In ASP.NET v5 a white list based encoder will be the only encoder."

I have a classic cross site scripting scenario: An ASP.Net Core solution where users can edit text using a WYSIWYG html-editor. The result is displayed for others to see. This means that if users inject a JavaScript into the data they submit when saving the text this code could execute when others visits the page.

I want to be able to whitelist certain HTML-codes (safe ones), but strip out bad codes.

How do I do this? I can't find any methods in ASP.Net Core RC2 to help me. Where is this white list encoder? How do I invoke it? For example I would need to clean output being returned via JSON WebAPI.

Answer 1

The dot.net core community has a wiki on this.

You can inject encoders at a controller level (in the constructor) or reference System.Text.Encodings.Web.

More info can be seen here:

https://docs.microsoft.com/en-us/aspnet/core/security/cross-site-scripting

Answer 2

To execute automatic Xss check, the old MVC used the logic implemented in the System.Web.CrossSiteScriptingValidation class. However this class is not present in ASP.NET CORE 1. So, to reuse it I copied its code:

System.Web.CrossSiteScriptingValidation class

// <copyright file="CrossSiteScriptingValidation.cs" company="Microsoft">
//     Copyright (c) Microsoft Corporation.  All rights reserved.
// </copyright>
public static class CrossSiteScriptingValidation
{
    private static readonly char[] StartingChars = { '<', '&' };

    #region Public methods

    // Only accepts http: and https: protocols, and protocolless urls.
    // Used by web parts to validate import and editor input on Url properties. 
    // Review: is there a way to escape colon that will still be recognized by IE?
    // %3a does not work with IE.
    public static bool IsDangerousUrl(string s)
    {
        if (string.IsNullOrEmpty(s))
        {
            return false;
        }

        // Trim the string inside this method, since a Url starting with whitespace
        // is not necessarily dangerous.  This saves the caller from having to pre-trim 
        // the argument as well.
        s = s.Trim();

        var len = s.Length;

        if ((len > 4) &&
            ((s[0] == 'h') || (s[0] == 'H')) &&
            ((s[1] == 't') || (s[1] == 'T')) &&
            ((s[2] == 't') || (s[2] == 'T')) &&
            ((s[3] == 'p') || (s[3] == 'P')))
        {
            if ((s[4] == ':') || ((len > 5) && ((s[4] == 's') || (s[4] == 'S')) && (s[5] == ':')))
            {
                return false;
            }
        }

        var colonPosition = s.IndexOf(':');
        return colonPosition != -1;
    }

    public static bool IsValidJavascriptId(string id)
    {
        return (string.IsNullOrEmpty(id) || System.CodeDom.Compiler.CodeGenerator.IsValidLanguageIndependentIdentifier(id));
    }

    public static bool IsDangerousString(string s, out int matchIndex)
    {
        //bool inComment = false;
        matchIndex = 0;

        for (var i = 0; ;)
        {

            // Look for the start of one of our patterns 
            var n = s.IndexOfAny(StartingChars, i);

            // If not found, the string is safe
            if (n < 0) return false;

            // If it's the last char, it's safe 
            if (n == s.Length - 1) return false;

            matchIndex = n;

            switch (s[n])
            {
                case '<':
                    // If the < is followed by a letter or '!', it's unsafe (looks like a tag or HTML comment)
                    if (IsAtoZ(s[n + 1]) || s[n + 1] == '!' || s[n + 1] == '/' || s[n + 1] == '?') return true;
                    break;
                case '&':
                    // If the & is followed by a #, it's unsafe (e.g. S) 
                    if (s[n + 1] == '#') return true;
                    break;

            }

            // Continue searching
            i = n + 1;
        }
    }

    #endregion

    #region Private methods

    private static bool IsAtoZ(char c)
    {
        return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z');
    }

    #endregion
}

Than,in order to use the above class for all requests, I created a Middleware that use CrossSiteScriptingValidation class:

AntiXssMiddleware

public class AntiXssMiddleware
{
    private readonly RequestDelegate _next;
    private readonly AntiXssMiddlewareOptions _options;

    public AntiXssMiddleware(RequestDelegate next, AntiXssMiddlewareOptions options)
    {
        if (next == null)
        {
            throw new ArgumentNullException(nameof(next));
        }

        _next = next;
        _options = options;
    }       

    public async Task Invoke(HttpContext context)
    {
        // Check XSS in URL
        if (!string.IsNullOrWhiteSpace(context.Request.Path.Value))
        {
            var url = context.Request.Path.Value;

            int matchIndex;
            if (CrossSiteScriptingValidation.IsDangerousString(url, out matchIndex))
            {
                if (_options.ThrowExceptionIfRequestContainsCrossSiteScripting)
                {
                    throw new CrossSiteScriptingException(_options.ErrorMessage);
                }

                context.Response.Clear();
                await context.Response.WriteAsync(_options.ErrorMessage);
                return;
            }
        }

        // Check XSS in query string
        if (!string.IsNullOrWhiteSpace(context.Request.QueryString.Value))
        {
            var queryString = WebUtility.UrlDecode(context.Request.QueryString.Value);

            int matchIndex;
            if (CrossSiteScriptingValidation.IsDangerousString(queryString, out matchIndex))
            {
                if (_options.ThrowExceptionIfRequestContainsCrossSiteScripting)
                {
                    throw new CrossSiteScriptingException(_options.ErrorMessage);
                }

                context.Response.Clear();
                await context.Response.WriteAsync(_options.ErrorMessage);
                return;
            }
        }

        // Check XSS in request content
        var originalBody = context.Request.Body;
        try
        {                
            var content = await ReadRequestBody(context);

            int matchIndex;
            if (CrossSiteScriptingValidation.IsDangerousString(content, out matchIndex))
            {
                if (_options.ThrowExceptionIfRequestContainsCrossSiteScripting)
                {
                    throw new CrossSiteScriptingException(_options.ErrorMessage);
                }

                context.Response.Clear();
                await context.Response.WriteAsync(_options.ErrorMessage);
                return;
            }

            await _next(context);
        }
        finally
        {
            context.Request.Body = originalBody;
        }            
    }

    private static async Task<string> ReadRequestBody(HttpContext context)
    {
        var buffer = new MemoryStream();
        await context.Request.Body.CopyToAsync(buffer);
        context.Request.Body = buffer;
        buffer.Position = 0;

        var encoding = Encoding.UTF8;
        var contentType = context.Request.GetTypedHeaders().ContentType;
        if (contentType?.Charset != null) encoding = Encoding.GetEncoding(contentType.Charset);

        var requestContent = await new StreamReader(buffer, encoding).ReadToEndAsync();
        context.Request.Body.Position = 0;

        return requestContent;
    }
}