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From Sally Khudairi ...@apache.org>
Subject [ANNOUNCE] Apache Commons statement to widespread Java object de-serialisation vulnerability
Date Tue, 10 Nov 2015 10:15:28 GMT
>> this statement is available online at http://s.apache.org/bsA

Authors: Bernd Eckenfels, Committer, and Gary Grogory, Vice President of Apache Commons 

In their
[talk](http://frohoff.github.io/appseccali-marshalling-pickles/) "Marshalling Pickles - how
deserializing objects will ruin your day" at
AppSecCali2015 Gabriel Lawrence ([@gebl](https://twitter.com/gebl)) and
Chris Frohoff ([@frohoff](https://twitter.com/frohoff)) presented
various security problems when applications accept serialized objects
from untrusted source. A major finding describes a way to execute
arbitrary Java functions and even inject manipulated bytecode when
using Java Object Serialization (as used in some remote communication
and persistence protocols). 

Building on Frohoff's tool [ysoserial](https://github.com/frohoff/ysoserial), Stephen Breen
([@breenmachine](https://twitter.com/breenmachine)) of Foxglove
Security inspected various products like WebSphere, JBoss, Jenkins, WebLogic, and OpenNMS
and describes
for each of them various attack scenarios. 

Both research works show that developers put too much trust in Java Object Serialization.
Some even de-serialize objects
pre-authentication. When deserializing an Object in Java you typically
cast it to an expected type, and therefore Java's strict type system
will ensure you only get valid object trees. Unfortunately, by the time
the type checking happens, platform code has already created and
executed significant logic. So, before the final type is checked, a lot
of code is executed from the readObject() methods of various objects,
all of which is out of the developer's control. By combining the
readObject() methods of various classes which are available on the
classpath of the vulnerable application an attacker can execute
functions (including calling Runtime.exec() to execute local OS

The best protection against this, is to avoid using a complex
serialization protocol with untrusted peers. It is possible to limit
the impact when using a custom ObjectInputStream which overrides [resolveClass()] http://docs.oracle.com/javase/7/docs/api/java/io/ObjectInputStream.html#resolveClass%28java.io.ObjectStreamClass%29)
to implement a whitelist approach (http://www.ibm.com/developerworks/library/se-lookahead/?).
This might, however, not always be  possible, such as when a framework or application server
provides the endpoint. This is rather bad news, as there is no easy fix and applications need
to revisit their client-server protocols and overall architecture. 

In these rather unfortunate situations, people have looked at the
sample exploits. Frohoff provided "gadget chains" in sample payloads
which combine classes from the Groovy runtime, Spring framework or Apache Commons Collection.
It is quite certain that you can combine more
classes to exploit this weakness, but those are the chains readily
available to attackers today. 

<screenshot https://twitter.com/gebl/status/662786601425080320> 

Even when the classes implementing a certain functionality cannot be
blamed for this vulnerability, and fixing the known cases will also not
make the usage of serialization in an untrusted context safe, there is
still demand to fix at least the known cases, even when this will only
start a Whack-a-Mole game. In fact, it is for this reason the original
team did not think it is necessary to alert the Apache Commons team,
hence work has begun relatively late. The Apache Commons team is using
the ticket
to address the issue in the 3.2 and 4.0 branches of commons-collection by disabling de-serialization
of the class InvokerTransformer. A to-do item being discussed is whether to provide programmatic
enabling of the feature on a per-transformer basis. 

There is some precendence for this, the class com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl
which is part of Oracle and OpenJDK JREs and which allows to inject and run bytecode, does
reject deserialization if a security manager is defined. This can be turned off with the system
property jdk.xml.enableTemplatesImplDeserialization=true. Apache Commons Collection plans
to disable this functionality independent of the existence of a security manager, as this
execution model is less commonly used than it should. 

However, to be clear: this is not the only known and especially not unknown useable gadget.
So replacing your installations with a hardened version of Apache Commons Collections will
not make your application resist this vulnerability.

We want to thank Gabriel Lawrence for reviewing this blog post. 

Apache [Commons Collection](https://commons.apache.org/proper/commons-collections/) is a Java
library offering additional collection classes in addition to the Java Collection framework.
The [InvokerTransformer](https://commons.apache.org/proper/commons-collections/javadocs/api-release/org/apache/commons/collections4/functors/InvokerTransformer.html)
is one specific implementation of the Transformer functional interface which can be used to
transform objects in a collection (specifically by calling a method via reflection invocation).

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