Internet Engineering Task Force (IETF) Y. Sheffer
Request for Comments: 8725
Category: Best Current Practice M. Jones
ISSN: 2070-1721 Microsoft
JSON Web Token Best Current Practices
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
actionable guidance leading to secure implementation and deployment
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 (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
) in effect on the date of
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described in the Simplified BSD License.
Table of Contents 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.
Normative References 6.2.
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
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)
], JSON Web Encryption (JWE) [RFC7516
], and JSON Web
Algorithms (JWA) [RFC7518
]. Others are about use of the JWT claims
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
* 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
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] 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
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
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
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
], allowed several different character encodings: UTF-8, UTF-
16, and UTF-32. This is not the case anymore, with the latest
] 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
] 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
For mitigations, see Sections 3.8
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
be employed to prevent these substitution attacks.
For mitigations, see Sections 3.8
, 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
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
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
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.
follow these algorithm-specific recommendations:
* Avoid all RSA-PKCS1 v1.5 encryption algorithms ([RFC8017
), 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
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 220.127.116.11.4 (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
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
followed. In particular, human-memorizable passwords MUST NOT
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
]. 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
], 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
]. 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
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
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
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
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
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
* Use different keys for different kinds of JWTs. Then the keys
used to validate one kind of JWT will fail to validate other kinds
* 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
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.
6.1. Normative References
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,
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979
, DOI 10.17487/RFC6979
] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515
, DOI 10.17487/RFC7515
] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516
, DOI 10.17487/RFC7516
, May 2015,
] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518
, May 2015,
] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519
, DOI 10.17487/RFC7519
, May 2015,
] 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,
] 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,
] 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
Interchange Format", STD 90, RFC 8259
, December 2017,
6.2. Informative References
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,
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.
NIST, "CVE-2015-9235 Detail", National Vulnerability
Database, May 2018,
[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,
Langkemper, S., "Attacking JWT authentication", September
[McLean] McLean, T., "Critical vulnerabilities in JSON Web Token
libraries", March 2015, <https://auth0.com/blog/critical-
Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0 incorporating
errata set 1", November 2014,
] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749
, DOI 10.17487/RFC6749
, October 2012,
Interchange Format", RFC 7159
, DOI 10.17487/RFC7159
] Jones, M., "JSON Web Key (JWK)", RFC 7517
, May 2015,
] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
Authorization Server Metadata", RFC 8414
, June 2018,
] Hunt, P., Ed., Jones, M., Denniss, W., and M. Ansari,
"Security Event Token (SET)", RFC 8417
, July 2018,
[Sanso] Sanso, A., "Critical Vulnerability Uncovered in JSON
Encryption", March 2017,
[Valenta] Valenta, L., Sullivan, N., Sanso, A., and N. Heninger, "In
search of CurveSwap: Measuring elliptic curve
implementations in the wild", March 2018,
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.
Michael B. Jones