[Asp.Net Core] 网站中的XSS跨站脚本攻击和防范

漏洞说明：
跨站脚本攻击(Cross Site Scripting)，为了不和层叠样式表(Cascading Style Sheets, CSS)的缩写混淆，故将跨站脚本攻击缩写为XSS。恶意攻击者往Web页面里插入恶意Web脚本代码（html、javascript、css等），当用户浏览该页面时，嵌入其中的Web脚本代码会被执行，从而达到恶意攻击用户的特殊目的。

测试步骤
访问系统网站，点击基础报告库进行编辑，使用Burp抓包并重新构造数据包

重新访问，成功触发了XSS弹窗

解决方法：

将危险内容过滤去除，用HTML转义字符串（Escape Sequence）表达的则保留
添加脚本过滤类

/// <summary>
/// Html 脚本过滤
/// </summary>
public class NHtmlFilter
{
    protected static readonly RegexOptions REGEX_FLAGS_SI = RegexOptions.IgnoreCase | RegexOptions.Singleline | RegexOptions.Compiled;

    private static string P_COMMENTS = "<!--(.*?)-->";
    private static Regex P_COMMENT = new Regex("^!--(.*)--$", REGEX_FLAGS_SI);
    private static string P_TAGS = "<(.*?)>";
    private static Regex P_END_TAG = new Regex("^/([a-z0-9]+)", REGEX_FLAGS_SI);
    private static Regex P_START_TAG = new Regex("^([a-z0-9]+)(.*?)(/?)$", REGEX_FLAGS_SI);
    private static Regex P_QUOTED_ATTRIBUTES = new Regex("([a-z0-9|(a-z0-9\\-a-z0-9)]+)=([\"'])(.*?)\\2", REGEX_FLAGS_SI);
    private static Regex P_UNQUOTED_ATTRIBUTES = new Regex("([a-z0-9]+)(=)([^\"\\s']+)", REGEX_FLAGS_SI);
    private static Regex P_PROTOCOL = new Regex("^([^:]+):", REGEX_FLAGS_SI);
    private static Regex P_ENTITY = new Regex("&#(\\d+);?");
    private static Regex P_ENTITY_UNICODE = new Regex("&#x([0-9a-f]+);?");
    private static Regex P_ENCODE = new Regex("%([0-9a-f]{2});?");
    private static Regex P_VALID_ENTITIES = new Regex("&([^&;]*)(?=(;|&|$))");
    private static Regex P_VALID_QUOTES = new Regex("(>|^)([^<]+?)(<|$)", RegexOptions.Singleline | RegexOptions.Compiled);
    private static string P_END_ARROW = "^>";
    private static string P_BODY_TO_END = "<([^>]*?)(?=<|$)";
    private static string P_XML_CONTENT = "(^|>)([^<]*?)(?=>)";
    private static string P_STRAY_LEFT_ARROW = "<([^>]*?)(?=<|$)";
    private static string P_STRAY_RIGHT_ARROW = "(^|>)([^<]*?)(?=>)";
    private static string P_AMP = "&";
    private static string P_QUOTE = "\"";
    private static string P_LEFT_ARROW = "<";
    private static string P_RIGHT_ARROW = ">";
    private static string P_BOTH_ARROWS = "<>";

    // @xxx could grow large... maybe use sesat's ReferenceMap
    private static Dictionary<string, string> P_REMOVE_PAIR_BLANKS = new Dictionary<string, string>();
    private static Dictionary<string, string> P_REMOVE_SELF_BLANKS = new Dictionary<string, string>();
    /** 
     * flag determining whether to try to make tags when presented with "unbalanced"
     * angle brackets (e.g. "<b text </b>" becomes "<b> text </b>").  If set to false,
     * unbalanced angle brackets will be html escaped.
     */
    protected static bool alwaysMakeTags = true;

    /**
     * flag determing whether comments are allowed in input String.
     */
    protected static bool stripComment = true;


    /// <summary>
    /// 不允许
    /// </summary>
    private string[] vDisallowed { get; set; }
    /// <summary>
    /// 允许
    /// </summary>
    protected Dictionary<string, List<string>> vAllowed { get; set; }

    /** counts of open tags for each (allowable) html element **/
    protected Dictionary<string, int> vTagCounts;

    /** html elements which must always be self-closing (e.g. "<img />") **/
    protected string[] vSelfClosingTags;

    /** html elements which must always have separate opening and closing tags (e.g. "<b></b>") **/
    protected string[] vNeedClosingTags;

    /** attributes which should be checked for valid protocols **/
    protected string[] vProtocolAtts;

    /** allowed protocols **/
    protected string[] vAllowedProtocols;

    /** tags which should be removed if they contain no content (e.g. "<b></b>" or "<b />") **/
    protected string[] vRemoveBlanks;

    /** entities allowed within html markup **/
    protected string[] vAllowedEntities;


    /// <summary>
    /// 是否为调试
    /// </summary>
    protected bool vDebug;

    public NHtmlFilter() : this(false) { }

    public NHtmlFilter(bool debug)
    {
        //List<Item> vAllowed = new List<Item>();
        vAllowed = new Dictionary<string, List<string>>();
        #region 允许通过数组

        vAllowed.Add("a", new List<string>() { "target", "href", "title", "class", "style" });
        vAllowed.Add("addr", new List<string>() { "title", "class", "style" });
        vAllowed.Add("address", new List<string>() { "class", "style" });
        vAllowed.Add("area", new List<string>() { "shape", "coords", "href", "alt" });
        vAllowed.Add("article", new List<string>() { });
        vAllowed.Add("aside", new List<string>() { });
        vAllowed.Add("audio", new List<string>() { "autoplay", "controls", "loop", "preload", "src", "class", "style" });
        vAllowed.Add("b", new List<string>() { "class", "style" });
        vAllowed.Add("bdi", new List<string>() { "dir" });
        vAllowed.Add("bdo", new List<string>() { "dir" });
        vAllowed.Add("big", new List<string>() { });
        vAllowed.Add("blockquote", new List<string>() { "cite", "class", "style" });
        vAllowed.Add("br", new List<string>() { });
        vAllowed.Add("caption", new List<string>() { "class", "style" });
        vAllowed.Add("center", new List<string>() { });
        vAllowed.Add("cite", new List<string>() { });
        vAllowed.Add("code", new List<string>() { "class", "style" });
        vAllowed.Add("col", new List<string>() { "align", "valign", "span", "width", "class", "style" });
        vAllowed.Add("colgroup", new List<string>() { "align", "valign", "span", "width", "class", "style" });
        vAllowed.Add("dd", new List<string>() { "class", "style" });
        vAllowed.Add("del", new List<string>() { "datetime" });
        vAllowed.Add("details", new List<string>() { "open" });
        vAllowed.Add("div", new List<string>() { "class", "style" });
        vAllowed.Add("dl", new List<string>() { "class", "style" });
        vAllowed.Add("dt", new List<string>() { "class", "style" });
        vAllowed.Add("em", new List<string>() { "class", "style" });
        vAllowed.Add("font", new List<string>() { "color", "size", "face" });
        vAllowed.Add("footer", new List<string>() { });
        vAllowed.Add("h1", new List<string>() { "class", "style" });
        vAllowed.Add("h2", new List<string>() { "class", "style" });
        vAllowed.Add("h3", new List<string>() { "class", "style" });
        vAllowed.Add("h4", new List<string>() { "class", "style" });
        vAllowed.Add("h5", new List<string>() { "class", "style" });
        vAllowed.Add("h6", new List<string>() { "class", "style" });
        vAllowed.Add("header", new List<string>() { });
        vAllowed.Add("hr", new List<string>() { });
        vAllowed.Add("i", new List<string>() { "class", "style" });
        vAllowed.Add("img", new List<string>() { "src", "alt", "title", "style", "width", "height", "id", "_src", "loadingclass", "class", "data-latex", "data-id", "data-type", "data-s" });
        vAllowed.Add("ins", new List<string>() { "datetime" });
        vAllowed.Add("li", new List<string>() { "class", "style" });
        vAllowed.Add("mark", new List<string>() { });
        vAllowed.Add("nav", new List<string>() { });
        vAllowed.Add("ol", new List<string>() { "class", "style" });
        vAllowed.Add("p", new List<string>() { "class", "style" });
        vAllowed.Add("pre", new List<string>() { "class", "style" });
        vAllowed.Add("s", new List<string>() { });
        vAllowed.Add("section", new List<string>() { });
        vAllowed.Add("small", new List<string>() { });
        vAllowed.Add("span", new List<string>() { "class", "style" });
        vAllowed.Add("sub", new List<string>() { "class", "style" });
        vAllowed.Add("sup", new List<string>() { "class", "style" });
        vAllowed.Add("strong", new List<string>() { "class", "style" });
        vAllowed.Add("table", new List<string>() { "width", "border", "align", "valign", "class", "style" });
        vAllowed.Add("tbody", new List<string>() { "align", "valign", "class", "style" });
        vAllowed.Add("td", new List<string>() { "width", "rowspan", "colspan", "align", "valign", "class", "style" });
        vAllowed.Add("tfoot", new List<string>() { "align", "valign", "class", "style" });
        vAllowed.Add("th", new List<string>() { "width", "rowspan", "colspan", "align", "valign", "class", "style" });
        vAllowed.Add("thead", new List<string>() { "align", "valign", "class", "style" });
        vAllowed.Add("tr", new List<string>() { "rowspan", "align", "valign", "class", "style" });
        vAllowed.Add("tt", new List<string>() { });
        vAllowed.Add("u", new List<string>() { });
        vAllowed.Add("ul", new List<string>() { "class", "style" });
        vAllowed.Add("video", new List<string>() { "autoplay", "controls", "loop", "preload", "src", "height", "width", "class", "style" });
        #endregion


        vDebug = debug;
        vTagCounts = new Dictionary<string, int>();

        vSelfClosingTags = new string[] { "img" };
        vNeedClosingTags = new string[] { "a", "b", "strong", "i", "em" };
        vDisallowed = new string[] { "script" };
        vAllowedProtocols = new string[] { "http", "mailto" }; // no ftp.
        vProtocolAtts = new string[] { "src", "href" };
        vRemoveBlanks = new string[] { "a", "b", "strong", "i", "em" };
        vAllowedEntities = new string[] { "amp", "gt", "lt", "quot" };
        stripComment = true;
        alwaysMakeTags = true;
    }


    protected void reset()
    {
        vTagCounts = new Dictionary<string, int>();
    }

    protected void debug(string msg)
    {
        if (vDebug)
            System.Diagnostics.Debug.WriteLine(msg);
    }

    //---------------------------------------------------------------
    // my versions of some PHP library functions

    public static string chr(int dec)
    {
        return "" + (char)dec;
    }

    /// <summary>
    /// 转换成实体字符
    /// </summary>
    /// <param name="str"></param>
    /// <returns></returns>
    public static string htmlSpecialChars(string str)
    {
        str = str.Replace(P_QUOTE, "\"");

        str = str.Replace(P_LEFT_ARROW, "<");
        str = str.Replace(P_RIGHT_ARROW, ">");
        str = str.Replace("\n", "<br>");
        return str;
    }

    //---------------------------------------------------------------

    /**
     * given a user submitted input String, filter out any invalid or restricted
     * html.
     * 
     * @param input text (i.e. submitted by a user) than may contain html
     * @return "clean" version of input, with only valid, whitelisted html elements allowed
     */
    public string filter(string input)
    {
        reset();
        string s = input;

        debug("************************************************");
        debug("              INPUT: " + input);

        s = escapeComments(s);
        debug("     escapeComments: " + s);

        s = balanceHTML(s);
        debug("        balanceHTML: " + s);

        s = checkTags(s);
        debug("          checkTags: " + s);

        s = processRemoveBlanks(s);
        debug("processRemoveBlanks: " + s);

        s = validateEntities(s);
        debug("    validateEntites: " + s);

        debug("************************************************\n\n");
        return s;
    }

    protected string escapeComments(string s)
    {
        return Regex.Replace(s, P_COMMENTS, new MatchEvaluator(ConverMatchComments), RegexOptions.Singleline);
    }

    protected string regexReplace(string regex_pattern, string replacement, string s)
    {
        return Regex.Replace(s, regex_pattern, replacement);
    }

    protected string balanceHTML(string s)
    {
        if (alwaysMakeTags)
        {
            //
            // try and form html
            //
            s = regexReplace(P_END_ARROW, "", s);
            s = regexReplace(P_BODY_TO_END, "<$1>", s);
            s = regexReplace(P_XML_CONTENT, "$1<$2", s);

        }
        else
        {
            //
            // escape stray brackets
            //
            s = regexReplace(P_STRAY_LEFT_ARROW, "<$1", s);
            s = regexReplace(P_STRAY_RIGHT_ARROW, "$1$2><", s);

            //
            // the last regexp causes '<>' entities to appear
            // (we need to do a lookahead assertion so that the last bracket can
            // be used in the next pass of the regexp)
            //
            s = s.Replace(P_BOTH_ARROWS, "");
        }
        return s;
    }

    protected string checkTags(string s)
    {
        //替换不允许标签
        foreach (var item in vDisallowed)
        {
            s = Regex.Replace(s, string.Format(@"<{0}\b(.)*?>(.)+?</{0}>", item), "");
        }
        s = Regex.Replace(s, P_TAGS, new MatchEvaluator(ConverMatchTags), RegexOptions.Singleline);

        // these get tallied in processTag
        // (remember to reset before subsequent calls to filter method)
        foreach (string key in vTagCounts.Keys)
        {
            for (int ii = 0; ii < vTagCounts[key]; ii++)
            {
                s += "</" + key + ">";
            }
        }

        return s;
    }

    protected string processRemoveBlanks(string s)
    {
        foreach (string tag in vRemoveBlanks)
        {
            s = regexReplace("<" + tag + "(\\s[^>]*)?></" + tag + ">", "", s);
            s = regexReplace("<" + tag + "(\\s[^>]*)?/>", "", s);
        }
        return s;
    }

    private string processTag(string s)
    {
        // ending tags
        Match m = P_END_TAG.Match(s);
        if (m.Success)
        {
            string name = m.Groups[1].Value.ToLower();
            if (allowed(name))
            {
                if (!inArray(name, vSelfClosingTags))
                {
                    if (vTagCounts.ContainsKey(name))
                    {
                        vTagCounts[name] = vTagCounts[name] - 1;
                        return "</" + name + ">";
                    }
                }
            }
        }


        // starting tags
        m = P_START_TAG.Match(s);
        if (m.Success)
        {
            string name = m.Groups[1].Value.ToLower();
            string body = m.Groups[2].Value;
            string ending = m.Groups[3].Value;

            //debug( "in a starting tag, name='" + name + "'; body='" + body + "'; ending='" + ending + "'" );
            if (allowed(name))
            {
                string params1 = "";

                MatchCollection m2 = P_QUOTED_ATTRIBUTES.Matches(body);
                MatchCollection m3 = P_UNQUOTED_ATTRIBUTES.Matches(body);
                List<string> paramNames = new List<string>();
                List<string> paramValues = new List<string>();
                foreach (Match match in m2)
                {
                    paramNames.Add(match.Groups[1].Value); //([a-z0-9]+)
                    paramValues.Add(match.Groups[3].Value); //(.*?)
                }
                foreach (Match match in m3)
                {
                    paramNames.Add(match.Groups[1].Value); //([a-z0-9]+)
                    paramValues.Add(match.Groups[3].Value); //([^\"\\s']+)
                }

                string paramName, paramValue;
                for (int ii = 0; ii < paramNames.Count; ii++)
                {
                    paramName = paramNames[ii].ToLower();
                    paramValue = paramValues[ii];

                    if (allowedAttribute(name, paramName))
                    {
                        if (inArray(paramName, vProtocolAtts))
                        {
                            paramValue = processParamProtocol(paramValue);
                        }
                        params1 += " " + paramName + "=\"" + paramValue + "\"";
                    }
                }

                if (inArray(name, vSelfClosingTags))
                {
                    ending = " /";
                }

                if (inArray(name, vNeedClosingTags))
                {
                    ending = "";
                }

                if (ending == null || ending.Length < 1)
                {
                    if (vTagCounts.ContainsKey(name))
                    {
                        vTagCounts[name] = vTagCounts[name] + 1;
                    }
                    else
                    {
                        vTagCounts.Add(name, 1);
                    }
                }
                else
                {
                    ending = " /";
                }
                return "<" + name + params1 + ending + ">";
            }
            else
            {
                return "";
            }
        }

        // comments
        m = P_COMMENT.Match(s);
        if (!stripComment && m.Success)
        {
            return "<" + m.Value + ">";
        }

        return "";
    }

    private string processParamProtocol(string s)
    {
        s = decodeEntities(s);
        Match m = P_PROTOCOL.Match(s);
        if (m.Success)
        {
            string protocol = m.Groups[1].Value;
            if (!inArray(protocol, vAllowedProtocols))
            {
                // bad protocol, turn into local anchor link instead
                s = "#" + s.Substring(protocol.Length + 1, s.Length - protocol.Length - 1);
                if (s.StartsWith("#//"))
                {
                    s = "#" + s.Substring(3, s.Length - 3);
                }
            }
        }
        return s;
    }

    private string decodeEntities(string s)
    {

        s = P_ENTITY.Replace(s, new MatchEvaluator(ConverMatchEntity));

        s = P_ENTITY_UNICODE.Replace(s, new MatchEvaluator(ConverMatchEntityUnicode));

        s = P_ENCODE.Replace(s, new MatchEvaluator(ConverMatchEntityUnicode));

        s = validateEntities(s);
        return s;
    }

    private string validateEntities(string s)
    {
        s = P_VALID_ENTITIES.Replace(s, new MatchEvaluator(ConverMatchValidEntities));
        s = P_VALID_QUOTES.Replace(s, new MatchEvaluator(ConverMatchValidQuotes));
        return s;
    }

    private static bool inArray(string s, string[] array)
    {
        foreach (string item in array)
        {
            if (item != null && item.Equals(s))
            {
                return true;
            }
        }
        return false;
    }

    private bool allowed(string name)
    {
        return (vAllowed.Count == 0 || vAllowed.ContainsKey(name)) && !inArray(name, vDisallowed);
    }

    private bool allowedAttribute(string name, string paramName)
    {
        return allowed(name) && (vAllowed.Count == 0 || vAllowed[name].Contains(paramName));
    }

    private string checkEntity(string preamble, string term)
    {

        return ";".Equals(term) && isValidEntity(preamble)
                ? '&' + preamble
                : "&" + preamble;
    }
    private bool isValidEntity(string entity)
    {
        return inArray(entity, vAllowedEntities);
    }
    private static string ConverMatchComments(Match match)
    {
        string matchValue = "<!--" + htmlSpecialChars(match.Groups[1].Value) + "-->";
        return matchValue;
    }

    private string ConverMatchTags(Match match)
    {
        string matchValue = processTag(match.Groups[1].Value);
        return matchValue;
    }

    private string ConverMatchEntity(Match match)
    {
        string v = match.Groups[1].Value;
        int decimal1 = int.Parse(v);
        return chr(decimal1);
    }

    private string ConverMatchEntityUnicode(Match match)
    {
        string v = match.Groups[1].Value;
        int decimal1 = Convert.ToInt32("0x" + v, 16);
        return chr(decimal1);
    }

    private string ConverMatchValidEntities(Match match)
    {
        string one = match.Groups[1].Value; //([^&;]*)
        string two = match.Groups[2].Value; //(?=(;|&|$))
        return checkEntity(one, two);
    }
    private string ConverMatchValidQuotes(Match match)
    {
        string one = match.Groups[1].Value; //(>|^)
        string two = match.Groups[2].Value; //([^<]+?)
        string three = match.Groups[3].Value;//(<|$)
        return one + regexReplace(P_QUOTE, "\"", two) + three;
    }

    public bool isAlwaysMakeTags()
    {
        return alwaysMakeTags;
    }

    public bool isStripComments()
    {
        return stripComment;
    }

    class Item
    {
        public string name { get; set; }
        public List<string> parameter { get; set; }
    }

}

源代码出自：https://www.cnblogs.com/OleRookie/p/5970167.html

在请求时对参数的内容进行过滤：

var nHtmlFilter = new NHtmlFilter(false);
surveyPayload.PayloadContent = nHtmlFilter.filter(surveyPayload.PayloadContent);

再次请求时，已将危险代码转成HTML转义字符串的形式了