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OAuth Working Group                                           Y. Sheffer
Internet-Draft                                                    Intuit
Intended status: Best Current Practice                          D. Hardt
Expires: December 6, 2017                                         Amazon
                                                                M. Jones
                                                               Microsoft
                                                           June 04, 2017


                 JSON Web Token Best Current Practices
                     draft-sheffer-oauth-jwt-bcp-00

Abstract

   JSON Web Tokens, also known as JWTs [RFC7519], are URL-safe JSON-
   based security tokens that contain a set of claims that can be signed
   and/or encrypted.  JWTs are being widely used and deployed as a
   simple security token format in numerous protocols and applications,
   both in the area of digital identity, and in other application areas.
   The goal of this Best Current Practices document is to provide
   actionable guidance leading to secure implementation and deployment
   of JWTs.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 6, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Target Audience . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Conventions used in this document . . . . . . . . . . . .   4
   2.  Threats and Vulnerabilities . . . . . . . . . . . . . . . . .   4
     2.1.  Weak Signatures and Insufficient Signature Validation . .   4
     2.2.  Weak symmetric keys . . . . . . . . . . . . . . . . . . .   4
     2.3.  Multiplicity of JSON encodings  . . . . . . . . . . . . .   4
     2.4.  Incorrect Composition of Encryption and Signature . . . .   5
     2.5.  Insecure Use of Elliptic Curve Encryption . . . . . . . .   5
     2.6.  Substitution Attacks  . . . . . . . . . . . . . . . . . .   5
     2.7.  Cross-JWT Confusion . . . . . . . . . . . . . . . . . . .   5
   3.  Best Practices  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Perform Algorithm Verification  . . . . . . . . . . . . .   5
     3.2.  Use Appropriate Algorithms  . . . . . . . . . . . . . . .   6
     3.3.  Validate All Cryptographic Operations . . . . . . . . . .   6
     3.4.  Validate Cryptographic Inputs . . . . . . . . . . . . . .   6
     3.5.  Ensure Cryptographic Keys have Sufficient Entropy . . . .   7
     3.6.  Use UTF-8 . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.7.  Validate Issuer and Subject . . . . . . . . . . . . . . .   7
     3.8.  Use and Validate Audience . . . . . . . . . . . . . . . .   7
     3.9.  Use Mutually Exclusive Validation Rules for Different
           Kinds of JWTs . . . . . . . . . . . . . . . . . . . . . .   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  11
     A.1.  draft-sheffer-oauth-jwt-bcp-00  . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   JSON Web Tokens, also known as JWTs [RFC7519], are URL-safe JSON-
   based security tokens that contain a set of claims that can be signed
   and/or encrypted.  The JWT specification has seen rapid adoption
   because it encapsulates security-relevant information in one, easy to
   protect location, and because it is easy to implement using widely-
   available tools.  One application area in which JWTs are commonly



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   used is representing digital identity information, such as OpenID
   Connect ID Tokens [OpenID.Core] and OAuth 2.0 [RFC6749] access tokens
   and refresh tokens, the details of which are deployment-specific.

   Since the JWT specification was published, there have been several
   widely published attacks on implementations and deployments.  Such
   attacks are the result of under-specified security mechanisms, as
   well as incomplete implementations and incorrect usage by
   applications.

   The goal of this document is to facilitate secure implementation and
   deployment of JWTs.  Many of the recommendations in this document
   will actually be about implementation and use of the cryptographic
   mechanisms underlying JWTs that are defined by JSON Web Signature
   (JWS) [RFC7515], JSON Web Encryption (JWE) [RFC7516], and JSON Web
   Algorithms (JWA) [RFC7518].  Others will be about use of the JWT
   claims themselves.

   These are intended to be minimum recommendations for the use of JWTs
   in the vast majority of implementation and deployment scenarios.
   Other specifications that reference this document can have stricter
   requirements related to one or more aspects of the format, based on
   their particular circumstances; when that is the case, implementers
   are advised to adhere to those stricter requirements.  Furthermore,
   this document provides a floor, not a ceiling, so stronger options
   are always allowed (e.g., depending on differing evaluations of the
   importance of cryptographic strength vs. computational load).

   Community knowledge about the strength of various algorithms and
   feasible attacks can change quickly, and experience shows that a Best
   Current Practice (BCP) document about security is a point-in-time
   statement.  Readers are advised to seek out any errata or updates
   that apply to this document.

1.1.  Target Audience

   The targets of this document are:

   -  Implementers of JWT libraries (and the JWS and JWE libraries used
      by them),

   -  Implementers of code that uses such libraries (to the extent that
      some mechanisms may not be provided by libraries, or until they
      are), and

   -  Developers of specifications that rely on JWTs, both inside and
      outside the IETF.




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1.2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

2.  Threats and Vulnerabilities

   This section lists some known and possible problems with JWT
   implementations and deployments.  Each problem description is
   followed by references to one or more mitigations to those problems.

2.1.  Weak Signatures and Insufficient Signature Validation

   Signed JSON Web Tokens carry an explicit indication of the signing
   algorithm, in the form of the "alg" header parameter, to facilitate
   cryptographic agility.  This, in conjunction with design flaws in
   some libraries and applications, have led to several attacks:

   -  The algorithm can be changed to "none" by an attacker, and some
      libraries would trust this value and "validate" the JWT without
      checking any signature.

   -  An "RS256" (RSA, 2048 bit) parameter value can be changed into
      "HS256" (HMAC, SHA-256), and some libraries would try to validate
      the signature using HMAC-SHA256 and using the RSA public key as
      the HMAC shared secret.

   For mitigations, see Section 3.1 and Section 3.2.

2.2.  Weak symmetric keys

   In addition, some applications sign tokens using a weak symmetric key
   and a keyed MAC algorithm such as "HS256".  In most cases, these keys
   are human memorable passwords that are vulnerable to dictionary
   attacks [Langkemper].

   For mitigations, see Section 3.5.

2.3.  Multiplicity of JSON encodings

   Many practitioners are not aware that JSON [RFC7159] allows several
   different character encodings: UTF-8, UTF-16 and UTF-32.  As a
   result, the JWT might be misinterpreted by its recipient.

   For mitigations, see Section 3.6.




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2.4.  Incorrect Composition of Encryption and Signature

   Some libraries that decrypt a JWE-encrypted JWT to obtain a JWS-
   signed object do not always validate the internal signature.

   For mitigations, see Section 3.3.

2.5.  Insecure Use of Elliptic Curve Encryption

   Per [Sanso], several JOSE libraries fail to validate their inputs
   correctly when performing elliptic curve key agreement (the "ECDH-ES"
   algorithm).  An attacker that is able to send JWEs of its choosing
   that use invalid curve points and observe the cleartext outputs
   resulting from decryption with the invalid curve points can use this
   vulnerability to recover the recipient's private key.

   For mitigations, see Section 3.4.

2.6.  Substitution Attacks

   There are attacks in which one recipient will have a JWT intended for
   it and attempt to use it at a different recipient that it was not
   intended for.  If not caught, these attacks can result in the
   attacker gaining access to resources that it is not entitled to
   access.

   For mitigations, see Section 3.7 and Section 3.8.

2.7.  Cross-JWT Confusion

   As JWTs are being used by more and more different protocols, it
   becomes increasingly important to prevent cases of JWT tokens that
   have been issued for one purpose being subverted and used for
   another.  Note that this is a specific type of substitution attacks.

   For mitigations, see Section 3.7, Section 3.8, and Section 3.9.

3.  Best Practices

   The best practices listed below should be applied by practitioners to
   mitigate the threats listed in the preceding section.

3.1.  Perform Algorithm Verification

   Libraries MUST enable the caller to specify a supported set of
   algorithms and MUST NOT use any other algorithms when performing
   cryptographic operations.  The library MUST ensure that the "alg" or
   "enc" header specifies the same algorithm that is used for the



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   cryptographic operation.  Moreover, each key MUST be used with
   exactly one algorithm, and this MUST be checked when the
   cryptographic operation is performed.

3.2.  Use Appropriate Algorithms

   As Section 5.2 of [RFC7515] says, "it is an application decision
   which algorithms may be used in a given context.  Even if a JWS can
   be successfully validated, unless the algorithm(s) used in the JWS
   are acceptable to the application, it SHOULD consider the JWS to be
   invalid."

   Therefore, applications MUST only allow the use of cryptographically
   current algorithms that meet the security requirements of the
   application.  This set will vary over time as new algorithms are
   introduced and existing algorithms are deprecated due to discovered
   cryptographic weaknesses.  Applications must therefore be designed to
   enable cryptographic agility.

   That said, if a JWT is cryptographically protected by a transport
   layer, such as TLS using cryptographically current algorithms, there
   may be no need to apply another layer of cryptographic protections to
   the JWT.  In such cases, the use of the "none" algorithm can be
   perfectly acceptable.  JWTs using "none" are often used in
   application contexts in which the content is optionally signed; then
   the URL-safe claims representation and processing can be the same in
   both the signed and unsigned cases.

3.3.  Validate All Cryptographic Operations

   All cryptographic operations used in the JWT MUST be validated and
   the entire JWT MUST be rejected if any of them fail to validate.
   This is true not only of JWTs with a single set of Header Parameters
   but also for Nested JWTs, in which both outer and inner operations
   MUST be validated using the keys and algorithms supplied by the
   application.

3.4.  Validate Cryptographic Inputs

   Some cryptographic operations, such as Elliptic Curve Diffie-Hellman
   key agreement ("ECDH-ES") take inputs that may contain invalid
   values, such as points not on the specified elliptic curve or other
   invalid points.  Either the JWS/JWE library itself must validate
   these inputs before using them or it must use underlying
   cryptographic libraries that do so (or both!).






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3.5.  Ensure Cryptographic Keys have Sufficient Entropy

   The Key Entropy and Random Values advice in Section 10.1 of [RFC7515]
   and the Password Considerations in Section 8.8 of [RFC7518] MUST be
   followed.  In particular, human-memorizable passwords MUST NOT be
   directly used as the key to a keyed-MAC algorithm such as "HS256".

3.6.  Use UTF-8

   [RFC7515], [RFC7516], and [RFC7519] all specify that UTF-8 be used
   for encoding and decoding JSON used in Header Parameters and JWT
   Claims Sets.  Implementations and applications MUST do this, and not
   use other Unicode encodings for these purposes.

3.7.  Validate Issuer and Subject

   When a JWT contains an "iss" (issuer) claim, the application MUST
   validate that the cryptographic keys used for the cryptographic
   operations in the JWT belong to the issuer.  If they do not, the
   application MUST reject the JWT.

   The means of determining the keys owned by an issuer is application-
   specific.  As one example, OpenID Connect [OpenID.Core] issuer values
   are "https" URLs that reference a JSON metadata document that
   contains a "jwks_uri" value that is an "https" URL from which the
   issuer's keys are retrieved as a JWK Set [RFC7517].  This same
   mechanism is used by [OAuth.Metadata].  Other applications may use
   different means of binding keys to issuers.

   Similarly, when the JWT contains a "sub" (subject) claim, the
   application MUST validate that the subject value corresponds to a
   valid subject and/or issuer/subject pair at the application.  This
   may include confirming that the issuer is trusted by the application.
   If the issuer, subject, or the pair are invalid, the application MUST
   reject the JWT.

3.8.  Use and Validate Audience

   If the same issuer can issue JWTs that are intended for use by more
   than one relying party or application, the JWT MUST contain an "aud"
   (audience) claim that can be used to determine whether the JWT is
   being used by an intended party or was substituted by an attacker at
   an unintended party.  Furthermore, the relying party or application
   MUST validate the audience value and if the audience value is not
   associated with the recipient, it MUST reject the JWT.






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3.9.  Use Mutually Exclusive Validation Rules for Different Kinds of
      JWTs

   NOTE: A goal of this BCP is to recommend specific best practices for
   applications of JWTs to apply.  The strategies listed below are some
   of the options available to these applications.  The authors request
   input from the OAuth working group and other interested parties on
   which of these strategies or which combinations should be considered
   to be best practices in which contexts.  Descriptions of other
   practical strategies not listed below are also solicited.

   Each application of JWTs defines a profile specifying the required
   and optional JWT claims and the validation rules associated with
   them.  If more than one kind of JWT can be issued by the same issuer,
   the validation rules for those JWTs MUST be written such that they
   are mutually exclusive, rejecting JWTs of the wrong kind.  To prevent
   substitution of JWTs from one context into another, a number of
   strategies may be employed:

   -  Use different sets of required claims or different required claim
      values.  Then the validation rules for one kind of JWT will reject
      those with different claims or values.

   -  Use different sets of required header parameters or different
      required header parameter values.  Then the validation rules for
      one kind of JWT will reject those with different header parameters
      or values.

   -  Use different keys for different kinds of JWTs.  Then the keys
      used to validate one kind of JWT will fail to validate other kinds
      of JWTs.

   -  Use different "aud" values for different uses of JWTs from the
      same issuer.  Then audience validation will reject JWTs
      substituted into inappropriate contexts.

   -  Use different issuers for different kinds of JWTs.  Then the
      distinct "iss" values can be used to segregate the different kinds
      of JWTs.

   Given the broad diversity of JWT usage and applications, the best
   combination of required claims, values, header parameters, key
   usages, and issuers to differentiate among different kinds of JWTs
   will, in general, be application specific.







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4.  IANA Considerations

   This document requires no IANA actions.

5.  Acknowledgements

   Thanks to Antonio Sanso for bringing the "ECDH-ES" invalid point
   attack to the attention of JWE and JWT implementers.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <http://www.rfc-editor.org/info/rfc7515>.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <http://www.rfc-editor.org/info/rfc7516>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <http://www.rfc-editor.org/info/rfc7518>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <http://www.rfc-editor.org/info/rfc7519>.

6.2.  Informative References

   [Langkemper]
              Langkemper, S., "Attacking JWT Authentication", September
              2016, <https://www.sjoerdlangkemper.nl/2016/09/28/
              attacking-jwt-authentication/>.







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   [OAuth.Metadata]
              Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", March 2017,
              <http://tools.ietf.org/html/
              draft-ietf-oauth-discovery-06>.

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M., Medeiros, B., and C.
              Mortimore, "OpenID Connect Core 1.0", November 2014,
              <http://openid.net/specs/openid-connect-core-1_0.html>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <http://www.rfc-editor.org/info/rfc6749>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <http://www.rfc-editor.org/info/rfc7517>.

   [Sanso]    Sanso, A., "Critical Vulnerability Uncovered in JSON
              Encryption", March 2017,
              <https://blogs.adobe.com/security/2017/03/critical-
              vulnerability-uncovered-in-json-encryption.html>.




























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Appendix A.  Document History

   [[ to be removed by the RFC editor before publication as an RFC ]]

A.1.  draft-sheffer-oauth-jwt-bcp-00

   -  Initial version.

Authors' Addresses

   Yaron Sheffer
   Intuit

   EMail: yaronf.ietf@gmail.com


   Dick Hardt
   Amazon

   EMail: dick@amazon.com


   Michael B. Jones
   Microsoft

   EMail: mbj@microsoft.com
   URI:   http://self-issued.info/
























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