THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
Summary
| Excerpt |
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Showcase app vulnerability allows remote command execution |
Who should read this | All Struts 2 developers |
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Impact of vulnerability | Remote command execution |
Maximum security rating | Critical |
Recommendation | Developers should immediately upgrade to Struts 2.3.14.1 |
Affected Software | Struts 2.0.0 - Struts 2.3.14 |
Reporter | Xgc Kxlzx, Alibaba Security Team |
Original Description | Reported directly to security@a.o |
Problem
OGNL provides, among other features, extensive expression evaluation capabilities. The vulnerability allows a malicious user to inject OGNL code into a property, then a further assignment of the property cause a further evaluation.
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| Wiki Markup |
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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 protections. |
Proof of concept
| Code Block | ||
|---|---|---|
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public class FooAction {
private String foo;
public String execute() {
return "success";
}
public String getFoo() {
return foo;
}
public void setFoo(String foo) {
this.foo = foo;
}
}
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...
| Code Block | ||
|---|---|---|
| ||
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
The 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.
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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:
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