后端应用经常接收各种信息参数，例如评论，回复等文本内容。除了一些场景下面，可以特定接受的富文本标签和属性之外(如:b,ul,li,h1, h2, h3...),需要过滤掉危险的字符和标签，防止xss攻击。

一、什么是XSS?

看完这个，应该有一个大致的概念。

• XSS攻击常识及常见的XSS攻击脚本汇总
• XSS过滤速查表

二、准则

1. 永远不要相信用户的输入和请求的参数(包括文字、上传等一切内容)
2. 参考第1条

三、实现做法

结合具体业务场景，对相应内容进行过滤，这里使用Jsoup。

jsoup是一款Java的HTML解析器。Jsoup提供的Whitelist(白名单)对文本内容进行过滤，过滤掉字符、属性，但是又保留必要的富文本格式。
如，白名单中允许b标签存在（并且不允许b标签带有其他属性）那么在一段Html内容，在过滤之后，会变成：

过滤前：

<b style="xxx" onclick="<script>alert(0);</script>">abc</>

过滤后：

<b>abc</b>

Whitelist主要方法说明

四、例子

基于springboot

pom.xml依赖

<dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <!-- jsoup -->
        <dependency>
            <groupId>org.jsoup</groupId>
            <artifactId>jsoup</artifactId>
            <version>1.13.1</version>
        </dependency>

        <dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-lang3</artifactId>
        </dependency>

        <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
            <version>2.6</version>
        </dependency>

    </dependencies>

HtmlFilter过滤类

package net.lofish.xpra.xss;

import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.safety.Whitelist;

import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.List;

/**
 * HtmlFilter
 *
 * @author 撸小鱼
 * Created by lofish@foxmail.com on 2020-04-12
 */
public class HtmlFilter {

    /**
     * 默认使用relaxed()
     * 允许的标签: a, b, blockquote, br, caption, cite, code, col, colgroup, dd, dl, dt, em, h1, h2, h3, h4, h5, h6, i, img, li, ol, p, pre, q, small, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, u, ul。结果不包含标签rel=nofollow ，如果需要可以手动添加。
     */
    private Whitelist whiteList;


    /**
     * 配置过滤化参数,不对代码进行格式化
     */
    private Document.OutputSettings outputSettings;


    private HtmlFilter() {
    }

    /**
     * 静态创建HtmlFilter方法
     * @param whiteList 白名单标签
     * @param pretty 是否格式化
     * @return HtmlFilter
     */
    public static HtmlFilter create(Whitelist whiteList, boolean pretty) {
        HtmlFilter filter = new HtmlFilter();
        if (whiteList == null) {
            filter.whiteList = Whitelist.relaxed();
        }
        filter.outputSettings = new Document.OutputSettings().prettyPrint(pretty);
        return filter;
    }

    /**
     * 静态创建HtmlFilter方法
     * @return HtmlFilter
     */
    public static HtmlFilter create() {
        return create(null, false);
    }

    /**
     * 静态创建HtmlFilter方法
     * @param whiteList 白名单标签
     * @return HtmlFilter
     */
    public static HtmlFilter create(Whitelist whiteList) {
        return create(whiteList, false);
    }

    /**
     * 静态创建HtmlFilter方法
     * @param excludeTags 例外的特定标签
     * @param includeTags 需要过滤的特定标签
     * @param pretty      是否格式化
     * @return HtmlFilter
     */
    public static HtmlFilter create( List<String> excludeTags,List<String> includeTags, boolean pretty) {
        HtmlFilter filter = create(null, pretty);
        //要过滤的标签
        if (includeTags != null && !includeTags.isEmpty()) {
            String[] tags = (String[]) includeTags.toArray(new String[0]);
            filter.whiteList.removeTags(tags);
        }
        //例外标签
        if (excludeTags != null && !excludeTags.isEmpty()) {
            String[] tags = (String[]) excludeTags.toArray(new String[0]);
            filter.whiteList.addTags(tags);
        }
        return filter;
    }

    /**
     * 静态创建HtmlFilter方法
     * @param excludeTags 例外的特定标签
     * @param includeTags 需要过滤的特定标签
     * @return HtmlFilter
     */
    public static HtmlFilter create(List<String> excludeTags,List<String> includeTags) {
        return create( includeTags, excludeTags, false );
    }

    /**
     * @param content 需要过滤内容
     * @return 过滤后的String
     */
    public String clean(String content) {
        return Jsoup.clean(content, "", this.whiteList, this.outputSettings);

    }

    public static void main(String[] args) throws FileNotFoundException, IOException {
        String text = "<a href=\"http://www.baidu.com/a\" onclick=\"alert(1);\"></a><script>alert(0);</script><b style=\"xxx\" onclick=\"<script>alert(0);</script>\">abc</>";
        System.out.println(HtmlFilter.create().clean(text));
    }


}

XssFilter过滤器

package net.lofish.xpra.xss;

import org.apache.commons.lang3.StringUtils;

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * XssFilter
 *
 * @author 撸小鱼
 * Created by lofish@foxmail.com on 2020-04-12
 */
public class XssFilter implements Filter {

    /**
     * 例外urls
     */
    private List<String> excludeUrls = new ArrayList<>();

    /**
     * 例外标签
     */
    private List<String> excludeTags = new ArrayList<>();

    /**
     * 需要过滤标签
     */
    private List<String> includeTags = new ArrayList<>();

    /**
     * 开关
     */
    public boolean enabled = false;

    /**
     * 编码
     */
    private String encoding = "UTF-8";

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        String enabledStr = filterConfig.getInitParameter("enabled");
        String excludeUrlStr = filterConfig.getInitParameter("excludeUrls");
        String excludeTagStr = filterConfig.getInitParameter("excludeTagStr");
        String includeTagStr = filterConfig.getInitParameter("excludeTagStr");
        String encodingStr = filterConfig.getInitParameter("encoding");

        if (StringUtils.isNotEmpty(excludeUrlStr)) {
            String[] url = excludeUrlStr.split(",");
            Collections.addAll(this.excludeUrls, url);
        }

        if (StringUtils.isNotEmpty(excludeTagStr)) {
            String[] url = excludeTagStr.split(",");
            Collections.addAll(this.excludeTags, url);
        }

        if (StringUtils.isNotEmpty(excludeTagStr)) {
            String[] url = excludeTagStr.split(",");
            Collections.addAll(this.includeTags, url);
        }

        if (StringUtils.isNotEmpty(enabledStr)) {
            this.enabled = Boolean.parseBoolean(enabledStr);
        }

        if (StringUtils.isNotEmpty(encodingStr)) {
            this.encoding = encodingStr;
        }

    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse resp = (HttpServletResponse) response;
        if (handleExcludeUrls(req, resp)) {
            chain.doFilter(request, response);
            return;
        }

        XssHttpServletRequestWrapper xssRequest = new XssHttpServletRequestWrapper((HttpServletRequest) request, encoding, excludeTags, includeTags );
        chain.doFilter(xssRequest, response);

    }

    private boolean handleExcludeUrls(HttpServletRequest request, HttpServletResponse response) {
        if (!enabled) {
            return true;
        }
        if (excludeUrls == null || excludeUrls.isEmpty()) {
            return false;
        }
        String url = request.getServletPath();
        for (String pattern : excludeUrls) {
            Pattern p = Pattern.compile("^" + pattern);
            Matcher m = p.matcher(url);
            if (m.find()) {
                return true;
            }
        }
        return false;
    }
}

一般情况下，我们都是通过request的parameter来传递参数。
但是，如果在某些场景下面，通过requestBody体(json等)，来传递相应参数应该怎么办？
这就要需要我们对request的inputStream来进行来过滤处理了

有个地方需要注意一下的:
servlet中inputStream只能一次读取，后续不能再次读取inputStream。Xss过滤器中读取了stream之后，后续如果其他逻辑涉及到inputStream读取，会抛出异常。那我们就需要想办法把已经读取的stream，重新放回到请求中。

import org.apache.commons.io.IOUtils;
import org.apache.commons.lang3.StringUtils;

import javax.servlet.ReadListener;
import javax.servlet.ServletInputStream;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;


/**
 * XSS过滤处理
 * @author 撸小鱼
 * Created by lofish@foxmail.com
 */
public class XssHttpServletRequestWrapper extends HttpServletRequestWrapper{

    HttpServletRequest orgRequest;

    String encoding;

    HtmlFilter htmlFilter;

    private final static String JSON_CONTENT_TYPE =  "application/json";

    private final static String CONTENT_TYPE = "Content-Type";


    /**
     * @param request  HttpServletRequest
     * @param encoding 编码
     * @param excludeTags 例外的特定标签
     * @param includeTags 需要过滤的标签
     */
    public XssHttpServletRequestWrapper( HttpServletRequest request, String encoding, List<String> excludeTags, List<String> includeTags ){
        super( request );
        orgRequest = request;
        this.encoding = encoding;
        this.htmlFilter = HtmlFilter.create( excludeTags, includeTags );
    }

    /**
     *
     * @param request HttpServletRequest
     * @param encoding 编码
     */
    public XssHttpServletRequestWrapper( HttpServletRequest request, String encoding ){
        this( request, encoding, null, null  );
    }

    private String xssFilter( String input ){
        return htmlFilter.clean( input );
    }

    @Override
    public ServletInputStream getInputStream() throws IOException{
        // 非json处理
        if( !JSON_CONTENT_TYPE.equalsIgnoreCase( super.getHeader( CONTENT_TYPE ) ) ){
            return super.getInputStream();
        }
        InputStream in = super.getInputStream();
        String body = IOUtils.toString( in, encoding );
        IOUtils.closeQuietly( in );

        //空串处理直接返回
        if( StringUtils.isBlank( body ) ){
            return super.getInputStream();
        }

        // xss过滤
        body = xssFilter( body );
        return new RequestCachingInputStream( body.getBytes( encoding ) );

    }

    @Override
    public String getParameter( String name ){
        String value = super.getParameter( xssFilter( name ) );
        if( StringUtils.isNotBlank( value ) ){
            value = xssFilter( value );
        }
        return value;
    }

    @Override
    public String[] getParameterValues( String name ){
        String[] parameters = super.getParameterValues( name );
        if( parameters == null || parameters.length == 0 ){
            return null;
        }

        for( int i = 0; i < parameters.length; i++ ){
            parameters[i] = xssFilter( parameters[i] );
        }
        return parameters;
    }

    @Override
    public Map<String, String[]> getParameterMap(){
        Map<String, String[]> map = new LinkedHashMap<>();
        Map<String, String[]> parameters = super.getParameterMap();
        for( String key : parameters.keySet() ){
            String[] values = parameters.get( key );
            for( int i = 0; i < values.length; i++ ){
                values[i] = xssFilter( values[i] );
            }
            map.put( key, values );
        }
        return map;
    }

    @Override
    public String getHeader( String name ){
        String value = super.getHeader( xssFilter( name ) );
        if( StringUtils.isNotBlank( value ) ){
            value = xssFilter( value );
        }
        return value;
    }

    /**
     * <b>
     * #获取最原始的request
     * </b>
     */
    public HttpServletRequest getOrgRequest(){
        return orgRequest;
    }

    /**
     * <b>
     * #获取最原始的request
     * </b>
     * @param request HttpServletRequest
     */
    public static HttpServletRequest getOrgRequest( HttpServletRequest request ){
        if( request instanceof XssHttpServletRequestWrapper ){
            return ((XssHttpServletRequestWrapper) request).getOrgRequest();
        }
        return request;
    }

    /**
     * <pre>
     * servlet中inputStream只能一次读取，后续不能再次读取inputStream
     * xss过滤body后，重新把流放入ServletInputStream中
     * </pre>
     */
    private static class RequestCachingInputStream extends ServletInputStream {
        private final ByteArrayInputStream inputStream;
        public RequestCachingInputStream(byte[] bytes) {
            inputStream = new ByteArrayInputStream(bytes);
        }

        @Override
        public int read() throws IOException {
            return inputStream.read();
        }

        @Override
        public boolean isFinished() {
            return inputStream.available() == 0;
        }

        @Override
        public boolean isReady() {
            return true;
        }

        @Override
        public void setReadListener( ReadListener readListener ){
        }

    }

}

springboot2.2.4.RELEASE中注册Filter

@Configuration
public class XssFilterConfig {

    @Value("${xss.enabled:true}")
    private String enabled;

    @Value("${xss.excludes:}")
    private String excludes;

    @Value("${xss.includes$:}")
    private String includes;

    @Value("${xss.urlPatterns:/*}")
    private String urlPatterns;

    @Bean
    public FilterRegistrationBean<XssFilter> xssFilterRegistrationBean() {
        FilterRegistrationBean<XssFilter> registration = new FilterRegistrationBean<>();
        registration.setDispatcherTypes(DispatcherType.REQUEST);
        registration.setFilter(new XssFilter());
        registration.addUrlPatterns(urlPatterns.split(","));
        registration.setName("XssFilter");
        registration.setOrder(Integer.MAX_VALUE);
        Map<String, String> initParameters = new HashMap<String, String>();
        initParameters.put("excludes", excludes);
        initParameters.put("includes", excludes);
        initParameters.put("enabled", enabled);
        registration.setInitParameters(initParameters);
        return registration;
    }
}

测试

http://localhost:8080/demo/th/xss?abc=%3Ca%20href=%22http://www.baidu.com/a%22%20onclick=%22alert(1);%22%3Eabc%3C/a%3E%3Cscript%3Ealert(0);%3C/script%3E&abc=%3Cb%20style=%22xxx%22%20onclick=%22%3Cscript%3Ealert(0);%3C/script%3E%22%3Eabc%3C/%3E

image