JSON Web Token Best Current Practices
RFC 8725

From: draft-ietf-oauth-jwt-bcp-07                  Best Current Practice
Internet Engineering Task Force (IETF)                        Y. Sheffer
Request for Comments: 8725                                        Intuit
BCP: 225                                                        D. Hardt
Updates: 7519
Category: Best Current Practice                                 M. Jones
ISSN: 2070-1721                                                Microsoft
                                                           February 2020


                 JSON Web Token Best Current Practices

Abstract

   JSON Web Tokens, also known as JWTs, 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.  This
   Best Current Practices document updates RFC 7519 to provide
   actionable guidance leading to secure implementation and deployment
   of JWTs.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8725.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   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
     1.1.  Target Audience
     1.2.  Conventions Used in this Document
   2.  Threats and Vulnerabilities
     2.1.  Weak Signatures and Insufficient Signature Validation
     2.2.  Weak Symmetric Keys
     2.3.  Incorrect Composition of Encryption and Signature
     2.4.  Plaintext Leakage through Analysis of Ciphertext Length
     2.5.  Insecure Use of Elliptic Curve Encryption
     2.6.  Multiplicity of JSON Encodings
     2.7.  Substitution Attacks
     2.8.  Cross-JWT Confusion
     2.9.  Indirect Attacks on the Server
   3.  Best Practices
     3.1.  Perform Algorithm Verification
     3.2.  Use Appropriate Algorithms
     3.3.  Validate All Cryptographic Operations
     3.4.  Validate Cryptographic Inputs
     3.5.  Ensure Cryptographic Keys Have Sufficient Entropy
     3.6.  Avoid Compression of Encryption Inputs
     3.7.  Use UTF-8
     3.8.  Validate Issuer and Subject
     3.9.  Use and Validate Audience
     3.10. Do Not Trust Received Claims
     3.11. Use Explicit Typing
     3.12. Use Mutually Exclusive Validation Rules for Different Kinds
            of JWTs
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgements
   Authors' Addresses

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
   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 are
   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 are 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 intended audiences of this document are:

   *  Implementers of JWT libraries (and the JWS and JWE libraries used
      by those libraries),

   *  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.

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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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, has 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 (see [McLean] and [CVE-2015-9235]).

   For mitigations, see Sections 3.1 and 3.2.

2.2.  Weak Symmetric Keys

   In addition, some applications use a keyed Message Authentication
   Code (MAC) algorithm, such as "HS256", to sign tokens but supply a
   weak symmetric key with insufficient entropy (such as a human-
   memorable password).  Such keys are vulnerable to offline brute-force
   or dictionary attacks once an attacker gets hold of such a token
   [Langkemper].

   For mitigations, see Section 3.5.

2.3.  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.

2.4.  Plaintext Leakage through Analysis of Ciphertext Length

   Many encryption algorithms leak information about the length of the
   plaintext, with a varying amount of leakage depending on the
   algorithm and mode of operation.  This problem is exacerbated when
   the plaintext is initially compressed, because the length of the
   compressed plaintext and, thus, the ciphertext depends not only on
   the length of the original plaintext but also on its content.
   Compression attacks are particularly powerful when there is attacker-
   controlled data in the same compression space as secret data, which
   is the case for some attacks on HTTPS.

   See [Kelsey] for general background on compression and encryption and
   [Alawatugoda] for a specific example of attacks on HTTP cookies.

   For mitigations, see Section 3.6.

2.5.  Insecure Use of Elliptic Curve Encryption

   Per [Sanso], several Javascript Object Signing and Encryption (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.  Multiplicity of JSON Encodings

   Previous versions of the JSON format, such as the obsoleted
   [RFC7159], allowed several different character encodings: UTF-8, UTF-
   16, and UTF-32.  This is not the case anymore, with the latest
   standard [RFC8259] only allowing UTF-8 except for internal use within
   a "closed ecosystem".  This ambiguity, where older implementations
   and those used within closed environments may generate non-standard
   encodings, may result in the JWT being misinterpreted by its
   recipient.  This, in turn, could be used by a malicious sender to
   bypass the recipient's validation checks.

   For mitigations, see Section 3.7.

2.7.  Substitution Attacks

   There are attacks in which one recipient will be given a JWT that was
   intended for it and will attempt to use it at a different recipient
   for which that JWT was not intended.  For instance, if an OAuth 2.0
   [RFC6749] access token is legitimately presented to an OAuth 2.0
   protected resource for which it is intended, that protected resource
   might then present that same access token to a different protected
   resource for which the access token is not intended, in an attempt to
   gain access.  If such situations are not caught, this can result in
   the attacker gaining access to resources that it is not entitled to
   access.

   For mitigations, see Sections 3.8 and 3.9.

2.8.  Cross-JWT Confusion

   As JWTs are being used by more different protocols in diverse
   application areas, 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 attack.  If the JWT could be used in an application
   context in which it could be confused with other kinds of JWTs, then
   mitigations MUST be employed to prevent these substitution attacks.

   For mitigations, see Sections 3.8, 3.9, 3.11, and 3.12.

2.9.  Indirect Attacks on the Server

   Various JWT claims are used by the recipient to perform lookup
   operations, such as database and Lightweight Directory Access
   Protocol (LDAP) searches.  Others include URLs that are similarly
   looked up by the server.  Any of these claims can be used by an
   attacker as vectors for injection attacks or server-side request
   forgery (SSRF) attacks.

   For mitigations, see Section 3.10.

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
   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 end-to-end 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.  The "none" algorithm
   should only be used when the JWT is cryptographically protected by
   other means.  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.  JWT libraries SHOULD NOT generate JWTs
   using "none" unless explicitly requested to do so by the caller.
   Similarly, JWT libraries SHOULD NOT consume JWTs using "none" unless
   explicitly requested by the caller.

   Applications SHOULD follow these algorithm-specific recommendations:

   *  Avoid all RSA-PKCS1 v1.5 encryption algorithms ([RFC8017],
      Section 7.2), preferring RSAES-OAEP ([RFC8017], Section 7.1).

   *  Elliptic Curve Digital Signature Algorithm (ECDSA) signatures
      [ANSI-X962-2005] require a unique random value for every message
      that is signed.  If even just a few bits of the random value are
      predictable across multiple messages, then the security of the
      signature scheme may be compromised.  In the worst case, the
      private key may be recoverable by an attacker.  To counter these
      attacks, JWT libraries SHOULD implement ECDSA using the
      deterministic approach defined in [RFC6979].  This approach is
      completely compatible with existing ECDSA verifiers and so can be
      implemented without new algorithm identifiers being required.

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.  This includes points not on the specified elliptic curve or
   other invalid points (e.g., [Valenta], Section 7.1).  The JWS/JWE
   library itself must validate these inputs before using them, or it
   must use underlying cryptographic libraries that do so (or both!).

   Elliptic Curve Diffie-Hellman Ephemeral Static (ECDH-ES) ephemeral
   public key (epk) inputs should be validated according to the
   recipient's chosen elliptic curve.  For the NIST prime-order curves
   P-256, P-384, and P-521, validation MUST be performed according to
   Section 5.6.2.3.4 (ECC Partial Public-Key Validation Routine) of
   "Recommendation for Pair-Wise Key-Establishment Schemes Using
   Discrete Logarithm Cryptography" [nist-sp-800-56a-r3].  If the
   "X25519" or "X448" [RFC8037] algorithms are used, then the security
   considerations in [RFC8037] apply.

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".
   Moreover, passwords should only be used to perform key encryption,
   rather than content encryption, as described in Section 4.8 of
   [RFC7518].  Note that even when used for key encryption, password-
   based encryption is still subject to brute-force attacks.

3.6.  Avoid Compression of Encryption Inputs

   Compression of data SHOULD NOT be done before encryption, because
   such compressed data often reveals information about the plaintext.

3.7.  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.  This is also in line with the latest JSON specification
   [RFC8259].  Implementations and applications MUST do this and not use
   or admit the use of other Unicode encodings for these purposes.

3.8.  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 [RFC8414].  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.

3.9.  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.

   In such cases, the relying party or application MUST validate the
   audience value, and if the audience value is not present or not
   associated with the recipient, it MUST reject the JWT.

3.10.  Do Not Trust Received Claims

   The "kid" (key ID) header is used by the relying application to
   perform key lookup.  Applications should ensure that this does not
   create SQL or LDAP injection vulnerabilities by validating and/or
   sanitizing the received value.

   Similarly, blindly following a "jku" (JWK set URL) or "x5u" (X.509
   URL) header, which may contain an arbitrary URL, could result in
   server-side request forgery (SSRF) attacks.  Applications SHOULD
   protect against such attacks, e.g., by matching the URL to a
   whitelist of allowed locations and ensuring no cookies are sent in
   the GET request.

3.11.  Use Explicit Typing

   Sometimes, one kind of JWT can be confused for another.  If a
   particular kind of JWT is subject to such confusion, that JWT can
   include an explicit JWT type value, and the validation rules can
   specify checking the type.  This mechanism can prevent such
   confusion.  Explicit JWT typing is accomplished by using the "typ"
   Header Parameter.  For instance, the [RFC8417] specification uses the
   "application/secevent+jwt" media type to perform explicit typing of
   Security Event Tokens (SETs).

   Per the definition of "typ" in Section 4.1.9 of [RFC7515], it is
   RECOMMENDED that the "application/" prefix be omitted from the "typ"
   value.  Therefore, for example, the "typ" value used to explicitly
   include a type for a SET SHOULD be "secevent+jwt".  When explicit
   typing is employed for a JWT, it is RECOMMENDED that a media type
   name of the format "application/example+jwt" be used, where "example"
   is replaced by the identifier for the specific kind of JWT.

   When applying explicit typing to a Nested JWT, the "typ" Header
   Parameter containing the explicit type value MUST be present in the
   inner JWT of the Nested JWT (the JWT whose payload is the JWT Claims
   Set).  In some cases, the same "typ" Header Parameter value will be
   present in the outer JWT as well, to explicitly type the entire
   Nested JWT.

   Note that the use of explicit typing may not achieve disambiguation
   from existing kinds of JWTs, as the validation rules for existing
   kinds of JWTs often do not use the "typ" Header Parameter value.
   Explicit typing is RECOMMENDED for new uses of JWTs.

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

   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, application
   developers may employ a number of strategies:

   *  Use explicit typing for different kinds of JWTs.  Then the
      distinct "typ" values can be used to differentiate between the
      different kinds of JWTs.

   *  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 types, required claims, values, Header Parameters, key
   usages, and issuers to differentiate among different kinds of JWTs
   will, in general, be application-specific.  As discussed in
   Section 3.11, for new JWT applications, the use of explicit typing is
   RECOMMENDED.

4.  Security Considerations

   This entire document is about security considerations when
   implementing and deploying JSON Web Tokens.

5.  IANA Considerations

   This document has no IANA actions.

6.  References

6.1.  Normative References

   [nist-sp-800-56a-r3]
              Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
              Davis, "Recommendation for Pair-Wise Key-Establishment
              Schemes Using Discrete Logarithm Cryptography", NIST
              Special Publication 800-56A Revision 3,
              DOI 10.6028/NIST.SP.800-56Ar3, April 2018,
              <https://doi.org/10.6028/NIST.SP.800-56Ar3>.

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

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <https://www.rfc-editor.org/info/rfc6979>.

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

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

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,
              <https://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,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [RFC8037]  Liusvaara, I., "CFRG Elliptic Curve Diffie-Hellman (ECDH)
              and Signatures in JSON Object Signing and Encryption
              (JOSE)", RFC 8037, DOI 10.17487/RFC8037, January 2017,
              <https://www.rfc-editor.org/info/rfc8037>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

6.2.  Informative References

   [Alawatugoda]
              Alawatugoda, J., Stebila, D., and C. Boyd, "Protecting
              Encrypted Cookies from Compression Side-Channel Attacks",
              Financial Cryptography and Data Security, pp. 86-106,
              DOI 10.1007/978-3-662-47854-7_6, July 2015,
              <https://doi.org/10.1007/978-3-662-47854-7_6>.

   [ANSI-X962-2005]
              American National Standards Institute, "Public Key
              Cryptography for the Financial Services Industry: the
              Elliptic Curve Digital Signature Algorithm (ECDSA)",
              ANSI X9.62-2005, November 2005.

   [CVE-2015-9235]
              NIST, "CVE-2015-9235 Detail", National Vulnerability
              Database, May 2018,
              <https://nvd.nist.gov/vuln/detail/CVE-2015-9235>.

   [Kelsey]   Kelsey, J., "Compression and Information Leakage of
              Plaintext", Fast Software Encryption, pp. 263-276,
              DOI 10.1007/3-540-45661-9_21, July 2002,
              <https://doi.org/10.1007/3-540-45661-9_21>.

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

   [McLean]   McLean, T., "Critical vulnerabilities in JSON Web Token
              libraries", March 2015, <https://auth0.com/blog/critical-
              vulnerabilities-in-json-web-token-libraries/>.

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", November 2014,
              <https://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,
              <https://www.rfc-editor.org/info/rfc6749>.

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

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

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/info/rfc8414>.

   [RFC8417]  Hunt, P., Ed., Jones, M., Denniss, W., and M. Ansari,
              "Security Event Token (SET)", RFC 8417,
              DOI 10.17487/RFC8417, July 2018,
              <https://www.rfc-editor.org/info/rfc8417>.

   [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>.

   [Valenta]  Valenta, L., Sullivan, N., Sanso, A., and N. Heninger, "In
              search of CurveSwap: Measuring elliptic curve
              implementations in the wild", March 2018,
              <https://ia.cr/2018/298>.

Acknowledgements

   Thanks to Antonio Sanso for bringing the "ECDH-ES" invalid point
   attack to the attention of JWE and JWT implementers.  Tim McLean
   published the RSA/HMAC confusion attack [McLean].  Thanks to Nat
   Sakimura for advocating the use of explicit typing.  Thanks to Neil
   Madden for his numerous comments, and to Carsten Bormann, Brian
   Campbell, Brian Carpenter, Alissa Cooper, Roman Danyliw, Ben Kaduk,
   Mirja Kühlewind, Barry Leiba, Eric Rescorla, Adam Roach, Martin
   Vigoureux, and Éric Vyncke for their reviews.

Authors' Addresses

   Yaron Sheffer
   Intuit

   Email: yaronf.ietf@gmail.com


   Dick Hardt

   Email: dick.hardt@gmail.com


   Michael B. Jones
   Microsoft

   Email: mbj@microsoft.com
   URI:   https://self-issued.info/