Struts2 漏洞分析系列 - S2-007/类型转换到RCE

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0x00 漏洞概述

S2-007的漏洞原理是在处理类型转换的错误时会存入错误到内存中,在后续调用流程中触发OGNL表达式注入。

影响版本:2.0.0 – 2.2.3

复现版本:2.2.3

官方issue地址:https://cwiki.apache.org/confluence/display/WW/S2-007

 

0x01 环境搭建

由于本漏洞发生在Bean中某个Property类型转换时,因此需要我们编写一个Action类:

import com.opensymphony.xwork2.ActionSupport;

public class LoginAction extends ActionSupport {
    private String username;
    private String password;
    private int age;

    public String getUsername() {
        return username;
    }

    public String getPassword() {
        return password;
    }

    public int getAge() {
        return age;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String execute() throws Exception{
        if (this.username == null || this.password == null) {
            return "failed";
        }

        if (this.username.equals("admin") && this.password.equals("admin")) {
            return "success";
        }

        return "failed";
    }
}

接着分别编写两个JSP文件,index.jsp用于模拟用户登陆后的界面,login.jsp用户模拟登陆界面。

index.jsp:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>Admin Console</title>
  </head>
  <body>
  Hello admin
  </body>
</html>

login.jsp:

<%@ page language="java" contentType="text/html; charset=UTF-8"
         pageEncoding="UTF-8"%>
<%@ taglib prefix="s" uri="/struts-tags" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>S2-007</title>
</head>
<body>
<h2>S2-007 Demo</h2>
<p>link: <a href="https://struts.apache.org/docs/s2-007.html">https://struts.apache.org/docs/s2-007.html</a></p>

<s:form action="login">
    <s:textfield name="username" label="username" />
    <s:textfield name="password" label="password" />
    <s:textfield name="age" label="age" />
    <s:submit></s:submit>
</s:form>
</body>
</html>

接着编写struts.xml用于定义路由以及成功、失败、错误时的渲染页面:

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE struts PUBLIC
        "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
        "http://struts.apache.org/dtds/struts-2.0.dtd">

<struts>
    <package name="st2-demo" extends="struts-default">
        <action name="login" class="LoginAction">
            <result name="success">index.jsp</result>
            <result name="failed">login.jsp</result>
            <result name="input">login.jsp</result>
        </action>
    </package>
</struts>

随后配置一下WEB服务器,部署后通过'+(#application)+'这个Payload测试是否回显相关信息:

 

0x02 漏洞分析

2.0 上报错误

之前介绍过Struts2存在着许多拦截器用于在请求真正执行到execute方法前的封装,其中就有一个ParametersInterceptor,在这里会将请求中的参数赋到当前请求对应的Bean上,请求中的参数一定是String类型的,但是Bean中的数据类型并不一定是String类型的,这个时候会自动进行一个类型转换,当转换发生错误时,会将错误进行记录。

比如aaa必然不可能被转为int类型,但如果是123(String)还是可以被转为int类型的,具体转换的逻辑我就不在这展开记录了,本文的重点是错误记录部分。

相关代码:

com.opensymphony.xwork2.conversion.impl.XWorkConverter#handleConversionException
protected void handleConversionException(Map<String, Object> context, String property, Object value, Object object) {
        if (context != null && Boolean.TRUE.equals(context.get("report.conversion.errors"))) {
            String realProperty = property;
            String fullName = (String)context.get("conversion.property.fullName");
            if (fullName != null) {
                realProperty = fullName;
            }

            Map<String, Object> conversionErrors = (Map)context.get("com.opensymphony.xwork2.ActionContext.conversionErrors");
            if (conversionErrors == null) {
                conversionErrors = new HashMap();
                context.put("com.opensymphony.xwork2.ActionContext.conversionErrors", conversionErrors);
            }

            ((Map)conversionErrors).put(realProperty, value);
        }

    }

在类型转换过程中触发的异常会通过handleConversionException方法进行搜集,但前提是report.conversion.errors这个键必须为true,默认情况下这个键是为true的。

随后会获取转换出错的property名称以及com.opensymphony.xwork2.ActionContext.conversionErrors这个Map并对其进行初始化,最终将property名称以及其对应的值放入这个Map中。

相关调用栈:

handleConversionException:438, XWorkConverter (com.opensymphony.xwork2.conversion.impl)
convertValue:338, XWorkConverter (com.opensymphony.xwork2.conversion.impl)
convertValue:39, OgnlTypeConverterWrapper (com.opensymphony.xwork2.ognl)
getConvertedType:1060, OgnlRuntime (ognl)
getConvertedTypes:1077, OgnlRuntime (ognl)
getConvertedMethodAndArgs:1103, OgnlRuntime (ognl)
getAppropriateMethod:1189, OgnlRuntime (ognl)
callAppropriateMethod:1203, OgnlRuntime (ognl)
setMethodValue:1474, OgnlRuntime (ognl)
setPossibleProperty:85, ObjectPropertyAccessor (ognl)
setProperty:162, ObjectPropertyAccessor (ognl)
setProperty:27, ObjectAccessor (com.opensymphony.xwork2.ognl.accessor)
setProperty:2245, OgnlRuntime (ognl)
setProperty:77, CompoundRootAccessor (com.opensymphony.xwork2.ognl.accessor)
setProperty:2245, OgnlRuntime (ognl)
setValueBody:127, ASTProperty (ognl)
evaluateSetValueBody:220, SimpleNode (ognl)
setValue:301, SimpleNode (ognl)
setValue:737, Ognl (ognl)
setValue:209, OgnlUtil (com.opensymphony.xwork2.ognl)
trySetValue:173, OgnlValueStack (com.opensymphony.xwork2.ognl)
setValue:160, OgnlValueStack (com.opensymphony.xwork2.ognl)
setValue:151, OgnlValueStack (com.opensymphony.xwork2.ognl)
setParameters:288, ParametersInterceptor (com.opensymphony.xwork2.interceptor)
doIntercept:199, ParametersInterceptor (com.opensymphony.xwork2.interceptor)
intercept:98, MethodFilterInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
doIntercept:207, ParametersInterceptor (com.opensymphony.xwork2.interceptor)
intercept:98, MethodFilterInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:190, StaticParametersInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:75, MultiselectInterceptor (org.apache.struts2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:94, CheckboxInterceptor (org.apache.struts2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:243, FileUploadInterceptor (org.apache.struts2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:100, ModelDrivenInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:141, ScopedModelDrivenInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:270, DebuggingInterceptor (org.apache.struts2.interceptor.debugging)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:145, ChainingInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
doIntercept:171, PrepareInterceptor (com.opensymphony.xwork2.interceptor)
intercept:98, MethodFilterInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:176, I18nInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:164, ServletConfigInterceptor (org.apache.struts2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:190, AliasInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:187, ExceptionMappingInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
execute:52, StrutsActionProxy (org.apache.struts2.impl)
serviceAction:498, Dispatcher (org.apache.struts2.dispatcher)
doFilter:434, FilterDispatcher (org.apache.struts2.dispatcher)
internalDoFilter:239, ApplicationFilterChain (org.apache.catalina.core)
doFilter:206, ApplicationFilterChain (org.apache.catalina.core)
invoke:219, StandardWrapperValve (org.apache.catalina.core)
invoke:106, StandardContextValve (org.apache.catalina.core)
invoke:501, AuthenticatorBase (org.apache.catalina.authenticator)
invoke:142, StandardHostValve (org.apache.catalina.core)
invoke:79, ErrorReportValve (org.apache.catalina.valves)
invoke:610, AbstractAccessLogValve (org.apache.catalina.valves)
invoke:88, StandardEngineValve (org.apache.catalina.core)
service:516, CoyoteAdapter (org.apache.catalina.connector)
process:1086, AbstractHttp11Processor (org.apache.coyote.http11)
process:659, AbstractProtocol$AbstractConnectionHandler (org.apache.coyote)
process:223, Http11NioProtocol$Http11ConnectionHandler (org.apache.coyote.http11)
doRun:1558, NioEndpoint$SocketProcessor (org.apache.tomcat.util.net)
run:1515, NioEndpoint$SocketProcessor (org.apache.tomcat.util.net)
runWorker:1149, ThreadPoolExecutor (java.util.concurrent)
run:624, ThreadPoolExecutor$Worker (java.util.concurrent)
run:61, TaskThread$WrappingRunnable (org.apache.tomcat.util.threads)
run:748, Thread (java.lang)

2.1 错误处理

前面说到,在ParametersInterceptor进行类型转换时产生的错误会被存入com.opensymphony.xwork2.ActionContext.conversionErrors中。

在后续流程中,会使用ConversionErrorInterceptor对前面产生的错误进行处理:

com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor#intercept
public String intercept(ActionInvocation invocation) throws Exception {
        ActionContext invocationContext = invocation.getInvocationContext();
        Map<String, Object> conversionErrors = invocationContext.getConversionErrors();
        ValueStack stack = invocationContext.getValueStack();
        HashMap<Object, Object> fakie = null;
        Iterator i$ = conversionErrors.entrySet().iterator();

        while(i$.hasNext()) {
            Entry<String, Object> entry = (Entry)i$.next();
            String propertyName = (String)entry.getKey();
            Object value = entry.getValue();
            if (this.shouldAddError(propertyName, value)) {
                String message = XWorkConverter.getConversionErrorMessage(propertyName, stack);
                Object action = invocation.getAction();
                if (action instanceof ValidationAware) {
                    ValidationAware va = (ValidationAware)action;
                    va.addFieldError(propertyName, message);
                }

                if (fakie == null) {
                    fakie = new HashMap();
                }

                fakie.put(propertyName, this.getOverrideExpr(invocation, value));
            }
        }

        if (fakie != null) {
            stack.getContext().put("original.property.override", fakie);
            invocation.addPreResultListener(new PreResultListener() {
                public void beforeResult(ActionInvocation invocation, String resultCode) {
                    Map<Object, Object> fakie = (Map)invocation.getInvocationContext().get("original.property.override");
                    if (fakie != null) {
                        invocation.getStack().setExprOverrides(fakie);
                    }

                }
            });
        }

        return invocation.invoke();
    }

这里首先通过getConversionErrors获取到所有的错误,随后通过while循环对错误进行处理,首先获取其key&value,接着通过shouldAddError判断value或propertyName是否为空,如果不为空则继续进入if的代码块中。

在if的代码块中会获取到这个错误对应的message以及当前请求对应的Action,并在Action实现了ValidationAware类的情况下调用其addFielderror方法,但是这都不是重点,重点在下面的fakie.put。

注意看,这里调用了getOverrideExpr方法对value进行处理:

protected Object getOverrideExpr(ActionInvocation invocation, Object value) {
        ValueStack stack = invocation.getStack();

        String var4;
        try {
            stack.push(value);
            var4 = "'" + stack.findValue("top", String.class) + "'";
        } finally {
            stack.pop();
        }

        return var4;
    }

这里首先将value压入栈中,随后通过top语法取出value并在左右两侧进行一个'的拼接,最后通过pop方法取出前面压入栈中的value,并且将上面拼接完毕的value返回,此时我们的Payload由'+(#application)+'转为了''+(#application)+'',可以发现左右两侧的单引号被我们人为闭合了,这里为后面的漏洞利用埋下了一个伏笔。

在value被拼接完后会被放入fakie中,最后在invocation中添加了一个PreResultListener,这里取出了前面的fakie,并且会调用setExprOverrides对其进行处理:

public void setExprOverrides(Map<Object, Object> overrides) {
        if (this.overrides == null) {
            this.overrides = overrides;
        } else {
            this.overrides.putAll(overrides);
        }

    }

setExprOverrides会将传入的overrides赋到this.overrides这个属性上,最后会在模板渲染时,通过lookupForOverrides方法从this.overrides中取出某个键对应的值。

private String lookupForOverrides(String expr) {
        if (this.overrides != null && this.overrides.containsKey(expr)) {
            expr = (String)this.overrides.get(expr);
        }

        return expr;
    }

最终会调用getValue方法处理取出来的值,getValue的过程中会触发OGNL表达式解析,这个在S2-001中有过相关介绍,这里就不重复记录了。

完整调用栈:

tryFindValue:331, OgnlValueStack (com.opensymphony.xwork2.ognl)
tryFindValueWhenExpressionIsNotNull:307, OgnlValueStack (com.opensymphony.xwork2.ognl)
findValue:293, OgnlValueStack (com.opensymphony.xwork2.ognl)
findValue:350, OgnlValueStack (com.opensymphony.xwork2.ognl)
translateVariables:196, TextParseUtil (com.opensymphony.xwork2.util)
translateVariables:115, TextParseUtil (com.opensymphony.xwork2.util)
translateVariables:88, TextParseUtil (com.opensymphony.xwork2.util)
findValue:378, Component (org.apache.struts2.components)
evaluateParams:769, UIBean (org.apache.struts2.components)
end:510, UIBean (org.apache.struts2.components)
doEndTag:42, ComponentTagSupport (org.apache.struts2.views.jsp)
_jspx_meth_s_005ftextfield_005f2:18, login_jsp (org.apache.jsp)
_jspx_meth_s_005fform_005f0:18, login_jsp (org.apache.jsp)
_jspService:14, login_jsp (org.apache.jsp)
service:70, HttpJspBase (org.apache.jasper.runtime)
service:725, HttpServlet (javax.servlet.http)
service:431, JspServletWrapper (org.apache.jasper.servlet)
serviceJspFile:396, JspServlet (org.apache.jasper.servlet)
service:340, JspServlet (org.apache.jasper.servlet)
service:725, HttpServlet (javax.servlet.http)
internalDoFilter:291, ApplicationFilterChain (org.apache.catalina.core)
doFilter:206, ApplicationFilterChain (org.apache.catalina.core)
doFilter:52, WsFilter (org.apache.tomcat.websocket.server)
internalDoFilter:239, ApplicationFilterChain (org.apache.catalina.core)
doFilter:206, ApplicationFilterChain (org.apache.catalina.core)
invoke:721, ApplicationDispatcher (org.apache.catalina.core)
processRequest:466, ApplicationDispatcher (org.apache.catalina.core)
doForward:391, ApplicationDispatcher (org.apache.catalina.core)
forward:318, ApplicationDispatcher (org.apache.catalina.core)
doExecute:157, ServletDispatcherResult (org.apache.struts2.dispatcher)
execute:186, StrutsResultSupport (org.apache.struts2.dispatcher)
executeResult:373, DefaultActionInvocation (com.opensymphony.xwork2)
invoke:277, DefaultActionInvocation (com.opensymphony.xwork2)
doIntercept:263, ValidationInterceptor (com.opensymphony.xwork2.validator)
doIntercept:68, AnnotationValidationInterceptor (org.apache.struts2.interceptor.validation)
intercept:98, MethodFilterInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:133, ConversionErrorInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
doIntercept:207, ParametersInterceptor (com.opensymphony.xwork2.interceptor)
intercept:98, MethodFilterInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
doIntercept:207, ParametersInterceptor (com.opensymphony.xwork2.interceptor)
intercept:98, MethodFilterInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:190, StaticParametersInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:75, MultiselectInterceptor (org.apache.struts2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:94, CheckboxInterceptor (org.apache.struts2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:243, FileUploadInterceptor (org.apache.struts2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:100, ModelDrivenInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:141, ScopedModelDrivenInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:270, DebuggingInterceptor (org.apache.struts2.interceptor.debugging)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:145, ChainingInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
doIntercept:171, PrepareInterceptor (com.opensymphony.xwork2.interceptor)
intercept:98, MethodFilterInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:176, I18nInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:164, ServletConfigInterceptor (org.apache.struts2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:190, AliasInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
intercept:187, ExceptionMappingInterceptor (com.opensymphony.xwork2.interceptor)
invoke:248, DefaultActionInvocation (com.opensymphony.xwork2)
execute:52, StrutsActionProxy (org.apache.struts2.impl)
serviceAction:498, Dispatcher (org.apache.struts2.dispatcher)
doFilter:434, FilterDispatcher (org.apache.struts2.dispatcher)
internalDoFilter:239, ApplicationFilterChain (org.apache.catalina.core)
doFilter:206, ApplicationFilterChain (org.apache.catalina.core)
invoke:219, StandardWrapperValve (org.apache.catalina.core)
invoke:106, StandardContextValve (org.apache.catalina.core)
invoke:501, AuthenticatorBase (org.apache.catalina.authenticator)
invoke:142, StandardHostValve (org.apache.catalina.core)
invoke:79, ErrorReportValve (org.apache.catalina.valves)
invoke:610, AbstractAccessLogValve (org.apache.catalina.valves)
invoke:88, StandardEngineValve (org.apache.catalina.core)
service:516, CoyoteAdapter (org.apache.catalina.connector)
process:1086, AbstractHttp11Processor (org.apache.coyote.http11)
process:659, AbstractProtocol$AbstractConnectionHandler (org.apache.coyote)
process:223, Http11NioProtocol$Http11ConnectionHandler (org.apache.coyote.http11)
doRun:1558, NioEndpoint$SocketProcessor (org.apache.tomcat.util.net)
run:1515, NioEndpoint$SocketProcessor (org.apache.tomcat.util.net)
runWorker:1149, ThreadPoolExecutor (java.util.concurrent)
run:624, ThreadPoolExecutor$Worker (java.util.concurrent)
run:61, TaskThread$WrappingRunnable (org.apache.tomcat.util.threads)
run:748, Thread (java.lang)

分析到这里,其实可以发现一个问题,首先网上流传的Payload,也就是’+(#application)+’,这里可以换种容易理解的写法,即' + #{application} + ',这样也许大家会更容易分清楚其中OGNL表达式的部分。

 

0x03 修复方案

S2-007的修复方式也是十分的简单粗暴,废话不多说,直接看DIFF:

可以发现,修复方式十分粗暴,就是通过StringEscapeUtils.escapeJava对其进行了一个转义,并且将原先左右两侧的单引号换为了双引号,此时我们则没有办法逃逸这左右两侧的引号了,因为"会被转义为\"

 

0x04 一个小问题

在最开始的时候写了,我测试的时候发现某些低版本用不了,这是因为在某些低版本中把XWorkConverter这个类给去掉了,因此没有上报错误的这么一个流程了,自然也就无法产生后续错误处理时的漏洞。所以如果想稳定复现,最好还是使用2.2.3版本进行复现。

 

0x05 吐槽

在分析这个漏洞时自然是参考了网上的一些文章,比如:

其中Github中的参考链接是第二个链接,然而实际上第二篇文章的漏洞分析部分有着些许问题,比如这个漏洞压根就不需要编写什么validate.xml,并且实际上触发OGNL表达式的也不是invoke这里,而是后续的调用中。

但这样错误的文章,也还是被抄来抄去的,这里只作吐槽,希望大家复现漏洞时能够在真正了解漏洞之后再去COPY,否则很容易被没复现过这个漏洞的人误解。

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