thFriday the 13 : JSON Attacks
Alvaro Muñoz (@pwntester)
Oleksandr Mirosh
HPE Security

> whoarewe• Alvaro Muñoz• Security Research with HPE• @pwntester• Oleksandr Mirosh• Security Research with HPE

Introduction• 2016 was the year of Java Deserialization apocalypse• Known vector since 2011
• Previous lack of good RCE gadgets in common libraries• Apache Commons-Collections Gadget caught many off-guard.
• Solution?
• Stop using Java serialization• Use a secure JSON/XML serializer instead• Do not let history repeat itself• Is JSON/XML/<Put your favorite format here> any better?
• Raise awareness for .NET deserialization vulnerabilities

Agenda1. Attacking JSON serializers• Affected Libraries• Gadgets• Demo2. Attacking .NET serializers• Affected formatters• Gadgets• Demo3. Generalizing the attack• DemoIs JSON any better?

Introduction• Probably secure when used to transmit data and simple JS objects• Replacing Java/.NET serialization with JSON requires OOP support.
• How do we serialize a java.lang.Object field?
• How do we deal with generics?
• How do we serialize interface fields?
• How do we deal with polymorphism?

Quick recap of Java deser attacks• Attackers can force the execution of any readObject() /readResolve() methods of any class sitting in the classpath• By controlling the deserialized field values attackers may abuse thelogic of these methods to run arbitrary code
• JSON libraries do not (normally) invoke deserialization callbacks ormagic methods
Can we initiate a gadget chain in some other way?

Object Reconstruction• JSON libraries need to reconstruct objects by either:• Calling default constructor and using reflection to set field values• Calling default constructor and calling setters to set field values• Calling “special” constructors, type converters or callbacks• Calling common methods such as:• hashcode(), toString(), equals(), finalize(), …

• Combinations of the previous ones J

Gadgets: .NET Edition• System.Configuration.Install.AssemblyInstaller• set_Path• Execute payload on local assembly load

• System.Activities.Presentation.WorkflowDesigner• set_PropertyInspectorFontAndColorData• Arbitrary XAML load
• Requires Single Threaded Apartment (STA) thread• System.Windows.ResourceDictionary• set_Source• Arbitrary XAML load
• Required to be able to work with setters of types derived from IDictionary• System.Windows.Data.ObjectDataProvider• set_(MethodName | ObjectInstance | ObjectType)• Arbitrary Method Invocation

ObjectDataProvider
set_MethodName()
set_ObjectType()
set_ObjectInstance()

QueryWorker()

Refresh()InvokeMethodOnInstance()

BeginQuery()ObjectDataProvider{"$type": "System.Windows.Data.ObjectDataProvider, PresentationFramework","ObjectInstance":{"$type":"System.Diagnostics.Process, System”},"MethodParameters":{"$type":"System.Collections.ArrayList, mscorlib","$values":["calc"]},"MethodName":"Start"}• Non-default constructor with controlled parameters• ObjectType + ConstructorParameters• Any public instance method of unmarshaled object without parameters• ObjectInstance + MethodName• Any public static/instance method with controlled parameters• ObjectType + ConstructorParameters + MethodName + MethodParametersGadgets: Java Edition• org.hibernate.jmx.StatisticsService• setSessionFactoryJNDIName• JNDI lookup• Presented during our JNDI attacks talk at BlackHat 2016
• com.atomikos.icatch.jta.RemoteClientUserTransaction• toString• JNDI lookup• com.sun.rowset.JdbcRowSetImpl• setAutoCommit• JNDI lookup• Available in Java JRE

JdbcRowSetImpl.setAutoCommit

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8u40-b25/com/sun/rowset/JdbcRowSetImpl.java/

JdbcRowSetImpl.setAutoCommit

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8u40-b25/com/sun/rowset/JdbcRowSetImpl.java/Gadgets: non RCE
Arbitrary Getter call• org.antlr.stringtemplate.StringTemplate (Java)• toString• Can be used to chain to other gadgets such as the infamous
TemplatesImpl.getOutputProperties()• System.Windows.Forms.BindingSource (.NET)• set_DataMember

XXE• System.Xml.XmlDocument/XmlDataDocument (.NET < 4.5.2)• set_InnerXml• System.Data.DataViewManager (.NET < 4.5.2)• set_DataViewSettingCollectionStringAnalyzed Libraries• Arbitrary Code Execution Requirements:
1. Attacker can control type of reconstructed objects• Can specify Type• _type, $type, class, classname, javaClass, …

• Library loads and instantiate Type

2. Library/GC will call methods on reconstructed objects
3. There are gadget chains starting on method executed upon/afterreconstruction

Categorization• Format includes type discriminator
1. Default
2. Configuration setting{"$type": "Newtonsoft.Json.Samples.Stockholder, Newtonsoft.Json.Tests","FullName": "Steve Stockholder","Businesses": {"$type": "System.Collections.Generic.List`1[[Newtonsoft.Json.Samples.Business, Newtonsoft.Json.Tests]], mscorlib","$values": [ {"$type": "Newtonsoft.Json.Samples.Hotel, Newtonsoft.Json.Tests","Stars": 4,"Name": "Hudson Hotel”}]}}• Type control
1. Cast after deserialization
2. Inspection of expected type

Expected Type’s Object Graph Inspection• Inspection of expected type’s object graph• Check assignability from provided type
• In some cases it also create a whitelist of allowed types• Vulnerable if• Expected type is user-controllable• Attacker can find injection member in object graph and no whitelist is applied
Message
IUser

User

Name : String
Items : Dict<String, Object>
Message : Message
Props : Hashtable

Name : String
Items : Dict<String, Object>
Message : Message

Body : Object
Exc: Exception

Exception

ValidationException

Data : IDictionary
Message : String
Source: String
StackTrace: String
InnerException: Exception…

…
Value : Object

Summary
Name

FastJSON
Json.Net
FSPickler

Language Type Name.NET
Default.NET.NET

Type Control
Cast

VectorSetterConfiguration Expected Object Graph Inspection

SetterDefault

Expected Object Graph Inspection

Deser. callbacks
SetterCast

Deser. callbacks
SetterSweet.Jayson
JavascriptSerializer
DataContractJsonSeri
alizerJackson.NET

Default.NET

Configuration Cast

Setter.NET

Default

SetterJava

Configuration Expected Object Graph Inspection

Deser. callbacks
SetterGensonJava

Configuration Expected Object Graph Inspection

Setter

JSON-IO
FlexSON
GSON

Java

Default

Cast

toString

Java

Default

Cast

SetterJava

Configuration Expected Object Graph Inspection

Expected Object Graph Inspection +whitelist-

FastJson• Always includes Type discriminators• There is no Type check controls other than a post-deserialization cast
Var obj = (ExpectedType) JSON.ToObject(untrusted);• Invokes• Setter• Should never be used with untrusted data
• Example:• KalikoCMS• CVE-2017-10712

JavaScriptSerializer• System.Web.Script.Serialization.JavaScriptSerializer• By default, it will not include type discriminator information• Type Resolver can be used to include this information.

JavaScriptSerializer sr = new JavaScriptSerializer(new SimpleTypeResolver());
string reqdInfo = apiService.authenticateRequest();reqdDetails det = (reqdDetails)(sr.Deserialize<reqdDetails>(reqdInfo));• Weak Type control: post-deserialization cast operation• During deserialization, it will call:• Setters• It can be used securely as long as a type resolver is not used or thetype resolver is configured to whitelist valid types.

DataContractJsonSerializer• System.Runtime.Serialization.Json.DataContractJsonSerializer• Performs a strict type graph inspection and whitelist creation.
• However, we found that if the attacker can control the expected type used
to configure the deserializer, they will be able to gain code execution. Eg:
var typename = cookie["typename"];…
var serializer = new DataContractJsonSerializer(Type.GetType(typename));var obj = serializer.ReadObject(ms);• Invokes:• Setters• Serialization Constructors• Can be used securely as long as the expected type cannot be controlled byusers.

Json.Net• It does not include Type discriminators unless TypeNameHandling setting
other than None is used
• Performs an inspection of Expected Type’s Object Graphpublic class Message {[JsonProperty(TypeNameHandling = TypeNameHandling.All)]
public object Body { get; set; }}• Invokes:• Setters• Serialization callbacks• Type Converters• Use SerializationBinder to whitelist Types if TypeNameHandling isrequiredDemo 1: Breeze (CVE-2017-9424)

Fixed in Breeze 1.6.5 onwards

Serializer Settings

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8u40-b25/com/sun/rowset/JdbcRowSetImpl.java/Unsafe Deserialization & Entrypointhttps://github.com/Breeze/breeze.server.net/blob/master/AspNet/Breeze.ContextProvider/ContextProvider.cs

Demo 1: Breeze (CVE-2017-9424)

Similar Research• Java Unmarshaller Security• Author: Moritz Bechler• Parallel research published on May 22, after our research was accepted forBlackHat and abstract was published J.

• Focus exclusively on Java
• Overlaps with our research on:• Jackson and JSON-IO libraries• JdbcRowSetImpl.setAutoCommit gadget• Include other interesting gadgets• https://github.com/mbechler/marshalsec

.NET Formatters

Introduction• Attacks on .NET formatters are notnew• James Forshaw already introduced
them at BlackHat 2012 for• BinaryFormatter• NetDataContractSerializer• Lack of RCE gadget until recently L• Goals:• Raise awareness about perils of .NET
deserialization• Present new vulnerable formattersscenarios• Present new gadgets•

Need new gadgets that works withFormatters other than BinaryFormatter

PSObject Gadget (CVE-2017-8565)• Bridges to custom deserializer

https://github.com/stangelandcl/pash-1/blob/master/System.Management.Automation/System.Management.Automation/PSObject.cs

PSObject Gadget (CVE-2017-8565)…

…

https://github.com/stangelandcl/pash-1/blob/master/System.Management.Automation/System.Management.Automation/InternalDeserializer.cs

PSObject Gadget (CVE-2017-8565)

LanguagePrimitives.FigureConversion() allows to:• Call the constructor of any public Type with one argument (attacker controlled)• Call any setters of public properties for the attacker controlled type
• Call the static public Parse(string) method of the attacker controlled type.

https://github.com/stangelandcl/pash-1/blob/master/System.Management.Automation/System.Management.Automation/LanguagePrimitives.cs

PSObject Gadget (CVE-2017-8565)…

https://github.com/stangelandcl/pash-1/blob/master/System.Management.Automation/System.Management.Automation/LanguagePrimitives.cs

XAML Payload
System.Windows.Markup.XamlReader.Parse() --> Process.Start(“calc”)<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:System="clr-namespace:System;assembly=mscorlib"
xmlns:Diag="clr-namespace:System.Diagnostics;assembly=system"><ObjectDataProvider x:Key="LaunchCalc“
ObjectType="{x:Type Diag:Process}"
MethodName="Start"><ObjectDataProvider.MethodParameters><System:String>calc</System:String></ObjectDataProvider.MethodParameters></ObjectDataProvider></ResourceDictionary>

.NET Native Formatters
Name

FormatAdditional requirements

Comments

BinaryFormatter
SoapFormatter
NetDataContractSerializer
JavaScriptSerializer
DataContractSerializer

BinarySOAP XMLNo
No

ISerializable gadgets
ISerializable gadgets

XML

No

ISerializable gadgets

JSON
XML

Insecure TypeResolver
Control of expected Type

Setters gadgets
Setters gadgets

or knownTypes

Some ISerializable gadgets

JSON

or weak DataContractResolver
Control of expected Type

Setters gadgets

XmlSerializerXMLor knownTypes
Control of expected Type

Some ISerializable gadgets
Quite limited; does not work with interfaces

ObjectStateFormatter

Text, Binary

No

Uses BinaryFormatter internally;

DataContractJsonSerializer

LosFormatter

Text, Binary

No

TypeConverters gadgets
Uses ObjectStateFormatter internallyBinaryMessageFormatter
XmlMessageFormatter

BinaryXML

No
Control of expected Type

Uses BinaryFormatter internally
Uses XmlSerializer internallyDemo 2: NancyFX (CVE-2017-9785)

Fixed in version 1.4.4 / 2.0-dangermouse onwards

NCSRF Cookie• CSRF cookie• Latest stable version used a BinaryFormatter serialized cookie (1.x)• AAEAAAD/////AQAAAAAAAAAMAgAAAD1OYW5jeSwgVmVyc2lvbj0wLjEwLjAuMCwgQ3VsdHVyZT1uZX
V0cmFsLCBQdWJsaWNLZXlUb2tlbj1udWxsBQEAAAAYTmFuY3kuU2VjdXJpdHkuQ3NyZlRva2VuAwAA
ABw8UmFuZG9tQnl0ZXM+a19fQmFja2luZ0ZpZWxkHDxDcmVhdGVkRGF0ZT5rX19CYWNraW5nRmllbG
QVPEhtYWM+a19fQmFja2luZ0ZpZWxkBwAHAg0CAgAAAAkDAAAAspLEeOrO0IgJBAAAAA8DAAAACgAA
AAJ9FN3bma5ztsdODwQAAAAgAAAAAt9dloO6qU2iUAuPUAtsq+Ud0w5Qu1py8YhoCn5hv+PJCwAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=• Pre-released 2.x used a custom JSON parser to make it compatible with .NET Core first versions• Pre-auth Remote Code Execution in both versions

Demo 2: NancyFX (CVE-2017-9785)

Generalizing the
Attacks

Attacking all the deserializers• During unmarshaling, objects will need to be created and populated
which normally mean calling setters or deserialization constructors.
• Arbitrary Code Execution Requirements:
1. Attacker can control type to be instantiated upon deserialization
2. Methods are called on the reconstructed objects
3. Gadget space is big enough to find types we can chain to get RCE• We can use our setter gadgets to attack most formats J

Examples• FsPickler (xml/binary)• A fast, multi-format messaging serializer for .NET
• Includes arbitrary Type discriminators• Invokes setters and ISerializable constructor and callbacks• Object Graph Inspection• SharpSerializer• XML and binary serialization for .NET and Silverlight• Includes arbitrary Type discriminators• Invokes setters• No type control other than post-deserialization cast

• Wire/Hyperion• A high performance polymorphic serializer for the .NET framework used by Akka.NET• JSON.NET with TypeNameHandling = All or custom binary one• Includes Type discriminators and invokes setters and ISerializable constructor andcallbacks

Beware of rolling your own format• NancyFX• Custom JSON parser replacing BinaryFormatter (Pre-released 2.x ) to make itcompatible with .NET Core first versions{"RandomBytes":[60,142,24,76,245,9,202,183,56,252],"CreatedDate":"2017-0403T10:42:16.7481461Z","Hmac":[3,17,70,188,166,30,66,0,63,186,44,2
13,201,164,3,19,56,139,78,159,170,193,192,183,242,187,170,221,140,46,24,197],"TypeObject":"Nancy.Security.CsrfToken, Nancy,
Version=2.0.0.0, Culture=neutral, PublicKeyToken=null”}• DotNetNuke CMS (DNN Platform)• Wraps XmlSerializer around a custom XML format which includes the type
to be used to create the XmlSerializer• This deserves a slide on its own J

Overcoming XmlSerializer constraints• Types with interface members cannot be serialized• System.Windows.Data.ObjectDataProvider is XmlSerializer friendly J• System.Diagnostic.Process has Interface members L … use any other

Type!

•
•
•
•

XamlReader.Load(String) -> RCE
ObjectStateFormatter.Deserialize(String) -> RCE
DotNetNuke.Common.Utilities.FileSystemUtils.PullFile(String) -> WebShell
DotNetNuke.Common.Utilities.FileSystemUtils.WriteFile(String) -> Read files• Runtime Types needs to be known at serializer construction time

• ObjectDataProvider contains an Object member (unknown runtime Type)• Use a parametrized Type to “teach” XmlSerializer about runtime types. Eg:

System.Data.Services.Internal.ExpandedWrapper`2[[PUT_RUNTIME_TYPE_1_HERE],[PUT_RUNTIME_TYPE_2_HERE]], System.Data.Services, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089

Demo 3: DotNetNuke (CVE-2017-9822)

Fixed in DNN Platform 9.1.1 or EVOQ 9.1.1 onwards

Source

Processed, for example, whenaccessing a 404 error pagehttps://github.com/dnnsoftware/Dnn.Platform/blob/a142594a0c18a589cb5fb913a022eebe34549a8f/DNN%20Platform/Library/Services/Personalization/PersonalizationController.cs#L72

Sink

https://github.com/dnnsoftware/Dnn.Platform/blob/a142594a0c18a589cb5fb913a022eebe34549a8f/DNN%20Platform/Library/Common/Utilities/XmlUtils.cs#L201

VideoWrap-UpMain Takeaways• Do not deserialize untrusted data!
• … no, seriously, do not deserialize untrusted data!
• … ok, if you really need to:• Make sure to evaluate the security of the chosen library• Avoid libraries without strict Type control• Type discriminators are necessary but not sufficient condition• Never use user-controlled data to define the deserializer expected Type• Do not roll your own format

Thank you!
Alvaro Muñoz (@pwntester) & Oleksandr Mirosh