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Subverting WCF Security [starting points]. Marc Schönefeld University of Bamberg. The speaker. Marc Schönefeld, Diplom-Wirtschaftsinformatiker For Science: External doctoral student @ Lehrstuhl für praktische Informatik at University of Bamberg, Bavaria, Germany

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subverting wcf security starting points

Subverting WCF Security[starting points]

Marc Schönefeld

University of Bamberg

the speaker
The speaker
  • Marc Schönefeld, Diplom-Wirtschaftsinformatiker
    • For Science: External doctoral student @ Lehrstuhl für praktische Informatik at University of Bamberg, Bavaria, Germany
    • For Living: Security Management for large financial group in Germany
wcf is part of net 3 0
WCF is part of NET 3.0
  • NET 3.0 is
    • is the former WinFX ( Windows Framework Extension)
    • According to Microsoft Arithmetics, it is NET 2.0 plus
      • WPF (Presentation) aka Avalon
      • WCF (Communication) aka Indigo
      • WWF (Workflow)
    • available for Windows XP SP2, Windows Server 2003 and Windows Vista
net 3 0 is not a new net
NET 3.0 is not a new .NET
  • .NET 3.0 is just a set of additional assemblies on top of .NET 2.0
wcf unifies communication handling
WCF unifies Communication handling
  • Communication
    • WCF supports various existing communications and hosting runtime models
      • ASP.NET
      • COM+
      • WSE
      • NET-Remoting
      • Pipes and Queues
  • allows to publish .NET classes as services
  • import from services into .NET classes.
  • includes interoperability code to handle service protocols, type conversion and value marshaling.
  • Allows developers to easily connect their applications to web services.
  • supports non-functional requirements (reliable calls, asynchronisity, transactionality, security)
  • comes with predefined adapters
  • can be extended with custom handlers for non-standard scenarios.
  • classes reside mostly in the System.ServiceModel namespace
  • is physically located in the System.ServiceModel.dll assembly.
wcf terms
WCF terms
  • SOA (Service oriented architectures) help the infrastructure to host local or remote services that can be used with clients.
  • Clients do not need to know the language, platform or version of the service.
  • Services provide functionality, can be newly written code or a decorated COM+ service
  • Clients use functionality provided by services, can be an ASP.NET page, Windows Form
  • Proxies decouple Services from Clients.
wcf authentication and authentication
WCF authentication and authentication
  • Authentication is about to determine the identity of the service caller:
    • Noauth Anonymous access is allowed
    • Windows Kerberos for Windows Domain or NTLM for workgroup (this is default)
    • UID+PWD Checks credentials against Windows accounts or database entries
    • X509 cert In a PKI environment the client authenticates with a known certificate
    • Secure Token Support a federated trust environment (WS-Trust)
    • Custom authentication Roll your authentication, in most cases not a good idea
  • WCF Authorization determines permissions of service caller by using
    • Windows Groups
    • ASP.NET provider (such as a databases)
transports securing the wire
Transports: Securing the wire
  • The goal is to maintain integrity and privacy of the message while in transit. Staged options available:
  • Transport Security
    • uses secure transport layer over TCP, HTTP, IPC, MSMQ
  • Message Security
    • encrypts the message
  • Mixed Transport and Message ,
    • transport security secures privacy and integrity, message security secures client credentials, near to overkill
  • Both
    • First the message content secured by message security mechanisms additionally a secure transport is used. Almost always overkill
wcf separates where what and how









Address (Where)



Binding (How)

Contract (What)

WCF separates Where, What and How






service endpoints
Service Endpoints
  • Clients and Servers use Service Endpoints for WCF communication
    • Client communicates to 1-n server endpoints
    • Client may open an endpoint for duplex message exchange (DualHttpBinding)
  • Service Endpoints specify
    • Endpoint address: Where is the service
    • Binding: How to attach to the service
    • Contract: What is exchanged between client and server
    • Behavior: Extension of binding behavior
endpoint address where







Endpoint address (WHERE)
  • The Physical address of the Endpoint (like an URI)
  • Headers (Optional), needed when multiple servers share an URI
  • Identity information
binding settings how
Binding settings (HOW)
  • Name and namespace identify a service
  • Binding elements adjust communication details (reliability, SOAP message security, TCP layer protection)
  • Many presets




Binding Elements

Bindings Element

wcf contracts what
WCF Contracts (WHAT)


  • Name and namespace again identify a service
  • Operation Description determine the type (send/forget, request/reply)
  • Duplex operations talk back to the client



Operation Description Set

Operation Descr.

Message Descr. Set

Msg. Description

Contract Behavior Set

Contract Behavior

a service description is the server config
A Service description is the server config

Service Description

  • A service is typed and has several behavior settings,
  • It can be reached on multiple endpoints (one on plain TCP, another one over HTTP, …)


Service Behavior Set

Service Behavior

Set of Service endpoints

Service Endpoints

Endpoint address



a channel description is the client config
A Channel description is the client config

Channel Description

  • A channel is used by the client and has several behavior settings,
  • It has one endpoint (one on plain TCP, another one over HTTP, …)

Channel Behavior Set

Channel Behavior

Service Endpoints

Endpoint address



wcf runtime environment concepts
WCF runtime environment concepts
  • Messages
    • Represented as a SOAP infoset, the message is the core part of information in WCF
    • its serialization format can be XML text, binary (for WCF2WCF) or a customizable format (JSON, XML-RPC)
  • Channels
    • Path to send a message send from Endpoint to endpoint, channels are layered
    • A Transport channel defines the media to send the physical data (UDP, MQ, Pipe, TCP)
    • A Protocol channels defines how to manipulate the data (like the SOAP protocol)
    • A security channel defines how to protect the message
wcf startup procedure
WCF startup procedure
  • The server initializes a ServiceHost object and opens the endpoints (XML config  or hardcoded )
  • The client initializes a ChannelFactory object (also in config or In code)
intranet bindings
Intranet bindings
  • BasicHttpBinding
    • For ASMX-based Web services and clients and other services that conform to the WS-I Basic Profile 1.1
    • Security is turned off by default
    • Text encoding, UTF-8
  • NetTcpBinding
    • For secure for cross-machine communication
    • transport security
    • binary message encoding
intranet bindings1
Intranet bindings
  • NetPeerTcpBinding
    • Peer-2-Peer communication over TCP
    • Input, Output and Duplex channels
    • Transport security by default
  • NetNamedPipe
    • for on-machine communication
    • Transport security on by default
    • Pipename is Base64 encoded servicename
internet bindings
Internet bindings
  • WSHttpBinding
    • http binding for Web services features
    • message security
    • transactions, reliable messaging, and WS-Addressing secure and reliable binding
  • WSFederationHttp
    • Like WSHttpBinding but with SSO capabilities
    • Ideal choice for secure tokens (
internet bindings1
Internet bindings
  • WSDualHttpBinding
    • Usable for service contracts in duplex mode allowing both services and clients to send and receive messages
    • supports only SOAP security and requires reliable messaging
  • NetMsmqBinding
    • support for transport on top of Microsoft Message Queuing (MSMQ)
    • For designing loosely coupled applications, failure isolation, load leveling and disconnected operations
    • In early versions there were DOS problems with msgs larger than 64K
threat modeling integrity wcf owasp
Threat modeling: Integrity WCF & OWASP

A1 Unvalidated Parameters

A2 Broken Access Control

A3 Broken Account and Session Management

A4 Cross-Site Scripting (XSS) Flaws

A5 Buffer Overflows

A6 Command Injection Flaws

A7 Error Handling Problems

A8 Insecure Use of Cryptography

A9 Remote Administration Flaws

A10 Server Misconfiguration

wcf owasp
  • A1: Unvalidated parameters :
    • There is no protection against unvalidated parameters, there is no tainting model in .NET 3.0
  • A4: XSS:
    • Therefore no support against XSS
  • A5: Buffer overflows
    • COM+ legacy code (in C/C++) can be overflowed, although connected via WCF
  • A6: Command injection:
    • Depends on how the application handles and propagates data flow of untrusted values into SQL parms
  • A10: Server Misconfiguration
    • Hard coded configurations in applications may lead to flaws, deployer personnel may be confused by the many options to choose from (where to set ciphersuite?)
threat modeling wcf com
Threat modeling: WCF & COM+
  • WCF allows to integrate COM+ with the ComSvcConfig tool
  • Two hosting modes available for COM+
    • COM+ default: The Component runs within DLLHOST
    • Web based: The component runs within IIS, activation with request
  • Beware that you don’t resurrect some old vulnerable components
  • Before deploying check the component!
threat modeling confidentiality
Threat modeling: Confidentiality
  • In a threat modeling setup about communication we have
    • Alice wants to communicate with Bob,
    • Mallory wants to interfere
  • WCF protects us very good against wiretapping
    • Mallory sitting on the transport wire
  • WCF does not protect against interfering
    • Mallory sitting between Alice and her endpoint or Bob and his endpoint (like a patched CLR, application)
  • WCF does not protect against stolen credentials
    • Once Mallory has stolen/leaked the key and code from Alice she has full access
threat modeling code security
Threat modeling: Code security
  • WCF protects us against type confusion
    • by the use of strong contracts
    • Big advantage over Java serialisation
  • .NET protects against integer overflows
    • In contrast to java it alarms the flipping sign
  • XML parser seems to be quite stable
    • Against large message attacks
    • Against several XML anomalies attacks
secure coding in wcf
Secure coding in WCF
  • Static code checking helps to find problem areas in own and third party code
  • Not all code is immediately accessible
    • Third party libraries may be encrypted or otherwise protected, so often a free tool from salamander helps to get the code out
  • No Sourcecode is no Prob: MSIL binary has it all!
    • Despite a lot of false positives static MSIL checkers may help
    • FXCop is a good starting point, is free at
    • Comes with a set of predefined security rules !
secure coding in wcf1
Secure coding in WCF
  • Some Relevant FXCop secure coding guidelines, which are helpful when coding with WCF
    • Review imperative security
    • Review sql queries for security vulnerabilities
    • Pointers should not be visible
    • Do not indirectly expose methods with link demands
    • Do not declare read only mutable reference types
    • Aptca methods should only call aptca methods
    • Aptca types should only extend aptca base types
  • Code your own rules and alter existing ones:
the wishlist
The wishlist
  • Better performance
    • WCF over XML is slow
    • WCF over XML + security is d**n slow (even on 3Ghz machine)
    • Remember: Availability is a security aspect !
  • Home phoning issues
    • WCF wants to validate certificates and sends HTTP request to Microsoft
  • Better usage of Secure string implementation
    • Credentials are stored insecure in normal strings
    • It would be helpful to advise .NET to store directly data directly in SecureString fields after reception from a service
  • No patterns available, Developers are left alone
    • with finding the relevant mix of options
    • With adjusting protection level and performance
  • WCF delivers a new baseline security model.
  • Various deployment and configuration features allow subverting the default security model
  • Easiness of legacy (COM+) integration allows vulnerable code to be called in the backend
  • Secure coding is still a must, despite a secure transport framework


Domo arigato

marc.schoenefeld ät gmx dot org.ilasm