OAuth 2.0 Proof-of-Possession (PoP) Security Architecture
draft-ietf-oauth-pop-architecture-01.txt

Abstract

The OAuth 2.0 bearer token specification, as defined in RFC 6750, allows any party in possession of a bearer token (a "bearer") to get access to the associated resources (without demonstrating possession of a cryptographic key). To prevent misuse, bearer tokens must to be protected from disclosure in transit and at rest.

Some scenarios demand additional security protection whereby a client needs to demonstrate possession of cryptographic keying material when accessing a protected resource. This document motivates the development of the OAuth 2.0 proof-of-possession security mechanism.

Status of This Memo

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This Internet-Draft will expire on September 4, 2015.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Use Cases
• 3.1. Preventing Access Token Re-Use by the Resource Server
• 3.2. TLS Channel Binding Support
• 3.3. Access to a Non-TLS Protected Resource
• 3.4. Offering Application Layer End-to-End Security
• 4. Security and Privacy Threats
• 5. Threat Mitigation
• 5.1. Confidentiality Protection
• 5.2. Sender Constraint
• 5.3. Key Confirmation
• 5.4. Summary
• 6. Architecture
• 7. Requirements
• 8. Security Considerations
• 9. IANA Considerations
• 10. Acknowledgments
• 11. References
• 11.1. Normative References
• 11.2. Informative References
• Authors' Addresses

1. Introduction

At the time of writing the OAuth 2.0 protocol family ([RFC6749], [RFC6750], and [RFC6819]) offer a single standardized security mechanism to access protected resources, namely the bearer token. RFC 6750 [RFC6750] specifies the bearer token mechanism and defines it as follows:

• "A security token with the property that any party in possession of the token (a "bearer") can use the token in any way that any other party in possession of it can. Using a bearer token does not require a bearer to prove possession of cryptographic key material."

The bearer token meets the security needs of a number of use cases the OAuth 2.0 protocol had originally been designed for. There are, however, other scenarios that require stronger security properties and ask for active participation of the OAuth client in form of cryptographic computations when presenting an access token to a resource server.

This document outlines additional use cases requiring stronger security protection in Section 3, identifies threats in Section 4, proposes different ways to mitigate those threats in Section 5, outlines an architecture for a solution that builds on top of the existing OAuth 2.0 framework in Section 6, and concludes with a requirements list in Section 7.

2. Terminology

The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this specification are to be interpreted as described in [RFC2119], with the important qualification that, unless otherwise stated, these terms apply to the design of the protocol, not its implementation or application.

3. Use Cases

The main use case that motivates better-than-bearer token security is the desire of resource servers to obtain additional assurance that the client is indeed authorized to present an access token. The expectation is that the use of additional credentials (symmetric or asymmetric keying material) will encourage developers to take additional precautions when transferring and storing access token in combination with these credentials.

Additional use cases listed below provide further requirements for the solution development. Note that a single solution does not necessarily need to offer support for all use cases.

3.1. Preventing Access Token Re-Use by the Resource Server

Imagine a scenario where a resource server that receives a valid access token re-uses it with other resource server. The reason for re-use may be malicious or may well be legitimate. In a legitimate use case consider chaining of computations whereby a resource server needs to consult other third party resource servers to complete the requested operation. In both cases it may be assumed that the scope of the access token is sufficiently large that it allows such a re-use. For example, imagine a case where a company operates email services as well as picture sharing services and that company had decided to issue access tokens with a scope that allows access to both services.

With this use case the desire is to prevent such access token re-use. This also implies that the legitimate use cases require additional enhancements for request chaining.

3.2. TLS Channel Binding Support

In this use case we consider the scenario where an OAuth 2.0 request to a protected resource is secured using TLS but the client and the resource server demand that the underlying TLS exchange is bound to additional application layer security to prevent cases where the TLS connection is terminated at a TLS intermediary, which splits the TLS connection into two separate connections.

In this use case additional information is conveyed to the resource server to ensure that no entity entity has tampered with the TLS connection.

3.3. Access to a Non-TLS Protected Resource

This use case is for a web client that needs to access a resource that makes data available (such as videos) without offering integrity and confidentiality protection using TLS. Still, the initial resource request using OAuth, which includes the access token, must be protected against various threats (e.g., token replay, token modification).

While it is possible to utilize bearer tokens in this scenario with TLS protection when the request to the protected resource is made, as described in [RFC6750], there may be the desire to avoid using TLS between the client and the resource server at all. In such a case the bearer token approach is not possible since it relies on TLS for ensuring integrity and confidentiality protection of the access token exchange since otherwise replay attacks are possible: First, an eavesdropper may steal an access token and represent it at a different resource server. Second, an eavesdropper may steal an access token and replay it against the same resource server at a later point in time. In both cases, if the attack is successful, the adversary gets access to the resource owners data or may perform an operation selected by the adversary (e.g., sending a message). Note that the adversary may obtain the access token (if the recommendations in [RFC6749] and [RFC6750] are not followed) using a number of ways, including eavesdropping the communication on the wireless link.

Consequently, the important assumption in this use case is that a resource server does not have TLS support and the security solution should work in such a scenario. Furthermore, it may not be necessary to provide authentication of the resource server towards the client.

3.4. Offering Application Layer End-to-End Security

In Web deployments resource servers are often placed behind load balancers, which are deployed by the same organization that operates the resource servers. These load balancers may terminate the TLS connection setup and HTTP traffic is transmitted in the clear from the load balancer to the resource server. With application layer security in addition to the underlying TLS security it is possible to allow application servers to perform cryptographic verification on an end-to-end basis.

The key aspect in this use case is therefore to offer end-to-end security in the presence of load balancers via application layer security. Enterprise networks also deploy proxies that inspect traffic and thereby break TLS.

4. Security and Privacy Threats

The following list presents several common threats against protocols utilizing some form of tokens. This list of threats is based on NIST Special Publication 800-63 [NIST800-63]. We exclude a discussion of threats related to any form of identity proofing and authentication of the resource owner to the authorization server since these procedures are not part of the OAuth 2.0 protocol specification itself.

Token manufacture/modification:

An attacker may generate a bogus tokens or modify the token content (such as authentication or attribute statements) of an existing token, causing resource server to grant inappropriate access to the client. For example, an attacker may modify the token to extend the validity period. A client may modify the token to have access to information that they should not be able to view.

Token disclosure:

Tokens may contain personal data, such as real name, age or birthday, payment information, etc.

Token redirect:

An attacker uses the token generated for consumption by the resource server to obtain access to another resource server.

Token reuse:

An attacker attempts to use a token that has already been used once with a resource server. The attacker may be an eavesdropper who observes the communication exchange or, worse, one of the communication end points. A client may, for example, leak access tokens because it cannot keep secrets confidential. A client may also re-use access tokens for some other resource servers. Finally, a resource server may use a token it had obtained from a client and use it with another resource server that the client interacts with. A resource server, offering relatively unimportant application services, may attempt to use an access token obtained from a client to access a high-value service, such as a payment service, on behalf of the client using the same access token.

Token repudiation:

Token repudiation refers to a property whereby a resource server is given an assurance that the authorization server cannot deny to have created a token for the client.

5. Threat Mitigation

A large range of threats can be mitigated by protecting the content of the token, for example using a digital signature or a keyed message digest. Alternatively, the content of the token could be passed by reference rather than by value (requiring a separate message exchange to resolve the reference to the token content). To simplify the subsequent description we assume that the token itself is digitally signed by the authorization server and therefore cannot be modified.

To deal with token redirect it is important for the authorization server to include the identifier of the intended recipient - the resource server. A resource server must not be allowed to accept access tokens that are not meant for its consumption.

To provide protection against token disclosure two approaches are possible, namely (a) not to include sensitive information inside the token or (b) to ensure confidentiality protection. The latter approach requires at least the communication interaction between the client and the authorization server as well as the interaction between the client and the resource server to experience confidentiality protection. As an example, TLS with a ciphersuite that offers confidentiality protection has to be applied (which is currently true for all ciphersuites, except for one). Encrypting the token content itself is another alternative. In our scenario the authorization server would, for example, encrypt the token content with a symmetric key shared with the resource server.

To deal with token reuse more choices are available.

5.1. Confidentiality Protection

In this approach confidentiality protection of the exchange is provided on the communication interfaces between the client and the resource server, and between the client and the authorization server. No eavesdropper on the wire is able to observe the token exchange. Consequently, a replay by a third party is not possible. An authorization server wants to ensure that it only hands out tokens to clients it has authenticated first and who are authorized. For this purpose, authentication of the client to the authorization server will be a requirement to ensure adequate protection against a range of attacks. This is, however, true for the description in Section 5.2 and Section 5.3 as well. Furthermore, the client has to make sure it does not distribute (or leak) the access token to entities other than the intended the resource server. For that purpose the client will have to authenticate the resource server before transmitting the access token.

5.2. Sender Constraint

Instead of providing confidentiality protection the authorization server could also put the identifier of the client into the protected token with the following semantic: 'This token is only valid when presented by a client with the following identifier.' When the access token is then presented to the resource server how does it know that it was provided by the client? It has to authenticate the client! There are many choices for authenticating the client to the resource server, for example by using client certificates in TLS [RFC5246], or pre-shared secrets within TLS [RFC4279]. The choice of the preferred authentication mechanism and credential type may depend on a number of factors, including

• security properties
• available infrastructure
• library support
• credential cost (financial)
• performance
• integration into the existing IT infrastructure
• operational overhead for configuration and distribution of credentials

This long list hints to the challenge of selecting at least one mandatory-to-implement client authentication mechanism.

5.3. Key Confirmation

A variation of the mechanism of sender authentication, described in Section 5.2, is to replace authentication with the proof-of-possession of a specific (session) key, i.e., key confirmation. In this model the resource server would not authenticate the client itself but would rather verify whether the client knows the session key associated with a specific access token. Examples of this approach can be found with the OAuth 1.0 MAC token [RFC5849], and Kerberos [RFC4120] when utilizing the AP_REQ/AP_REP exchange (see also [I-D.hardjono-oauth-kerberos] for a comparison between Kerberos and OAuth).

To illustrate key confirmation the first examples borrow from Kerberos and use symmetric key cryptography. Assume that the authorization server shares a long-term secret with the resource server, called K(Authorization Server-Resource Server). This secret would be established between them out-of-band. When the client requests an access token the authorization server creates a fresh and unique session key Ks and places it into the token encrypted with the long term key K(Authorization Server-Resource Server). Additionally, the authorization server attaches Ks to the response message to the client (in addition to the access token itself) over a confidentiality protected channel. When the client sends a request to the resource server it has to use Ks to compute a keyed message digest for the request (in whatever form or whatever layer). The resource server, when receiving the message, retrieves the access token, verifies it and extracts K(Authorization Server-Resource Server) to obtain Ks. This key Ks is then used to verify the keyed message digest of the request message.

Note that in this example one could imagine that the mechanism to protect the token itself is based on a symmetric key based mechanism to avoid any form of public key infrastructure but this aspect is not further elaborated in the scenario.

A similar mechanism can also be designed using asymmetric cryptography. When the client requests an access token the authorization server creates an ephemeral public / privacy key pair (PK/SK) and places the public key PK into the protected token. When the authorization server returns the access token to the client it also provides the PK/SK key pair over a confidentiality protected channel. When the client sends a request to the resource server it has to use the privacy key SK to sign the request. The resource server, when receiving the message, retrieves the access token, verifies it and extracts the public key PK. It uses this ephemeral public key to verify the attached signature.

5.4. Summary

As a high level message, there are various ways how the threats can be mitigated and while the details of each solution is somewhat different they all ultimately accomplish the goal.

The three approaches are:

Confidentiality Protection:

The weak point with this approach, which is briefly described in Section 5.1, is that the client has to be careful to whom it discloses the access token. What can be done with the token entirely depends on what rights the token entitles the presenter and what constraints it contains. A token could encode the identifier of the client but there are scenarios where the client is not authenticated to the resource server or where the identifier of the client rather represents an application class rather than a single application instance. As such, it is possible that certain deployments choose a rather liberal approach to security and that everyone who is in possession of the access token is granted access to the data.

Sender Constraint:

The weak point with this approach, which is briefly described in Section 5.2, is to setup the authentication infrastructure such that clients can be authenticated towards resource servers. Additionally, the authorization server must encode the identifier of the client in the token for later verification by the resource server. Depending on the chosen layer for providing client-side authentication there may be additional challenges due Web server load balancing, lack of API access to identity information, etc.

Key Confirmation:

The weak point with this approach, see Section 5.3, is the increased complexity: a complete key distribution protocol has to be defined.

In all cases above it has to be ensured that the client is able to keep the credentials secret.

6. Architecture

The proof-of-possession security concept assumes that the authorization server acts as a trusted third party that binds keys to access tokens. These keys are then used by the client to demonstrate the possession of the secret to the resource server when accessing the resource. The resource server, when receiving an access token, needs to verify that the key used by the client matches the one included in the access token.

There are slight differences between the use of symmetric keys and asymmetric keys when they are bound to the access token and the subsequent interaction between the client and the authorization server when demonstrating possession of these keys. Figure 1 shows the symmetric key procedure and Figure 2 illustrates how asymmetric keys are used. While symmetric cryptography provides better performance properties the use of asymmetric cryptography allows the client to keep the private key locally and never expose it to any other party.

With the JSON Web Token (JWT) [I-D.ietf-oauth-json-web-token] a standardized format for access tokens is available. The necessary elements to bind symmetric or asymmetric keys to a JWT are described in [I-D.ietf-oauth-proof-of-possession].

Note: The negotiation of cryptographic algorithms between the client and the authorization server is not shown in the examples below and assumed to be present in a protocol solution to meet the requirements for crypto-agility.

						
                     +---------------+
                    ^|               |  
                  // | Authorization |  
                 /   | Server        |          
               //    |               |        
              /      |               |        
       (I)  //      /+---------------+        
Access     /      //                          
Token     /      /                            
Request //     //  (II) Access Token          
+Params /      /        +Symmetric Key       
     //     //                                
    /      v                                  
  +-----------+                       +------------+
  |           |                       |            |
  |           |                       | Resource   |
  | Client    |                       | Server     |
  |           |                       |            |
  |           |                       |            |
  +-----------+                       +------------+

					

						
                     +---------------+
                    ^|               |
Access Token Req. // | Authorization |
+Parameters      /   | Server        |
+[Fingerprint] //    |               |
              /      |               |
    (I)     //      /+---------------+
           /      //
          /      /     (II)
        //     //  Access Token
        /      /   +[ephemeral
     //     //      asymmetric key pair]
    /      v
  +-----------+                       +------------+
  |           |                       |            |
  |           |                       | Resource   |
  | Client    |                       | Server     |
  |           |                       |            |
  |           |                       |            |
  +-----------+                       +------------+

					

						
                   +---------------+
                   |               |
                   | Authorization |
                   | Server        |
                   |               |
                   |               |
                   +---------------+





                 Request
+-----------+  + Signature/MAC (a)  +------------+
|           |---------------------->|            |
|           |  [+Access Token]      | Resource   |
| Client    |                       | Server     |
|           |    Response (b)       |            |
|           |<----------------------|            |
+-----------+  [+ Signature/MAC]    +------------+

     ^                                    ^
     |                                    |
     |                                    |
 Symmetric Key                       Symmetric Key
    or                                   or 
 Asymmetric Key Pair                Public Key (Client)
    +                                     +
  Parameters                          Parameters

					

						
                   +---------------+
     Access       ^|               |
     Token Req. // | Authorization |^
       (I)     /   | Server        | \  (IV) Token
             //    |               |  \ Introspection Req.
            /      |               |   \     +Access
          //      /+---------------+    \     Token
         /      // (II)             \    \\
        /      /   Access            \     \
      //     //    Token              \ (V) \
      /      /                         \Resp.\
   //     //                            \     \
  /      v                               V     \
+-----------+ Request +Signature/MAC+------------+
|           |  (III)  +Access Token |            |
|           |---------------------->| Resource   |
| Client    |   (VI) Success or     | Server     |
|           |        Failure        |            |
|           |<----------------------|            |
+-----------+                       +------------+