JSON Web Token

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JSON Web Token (JWT, sometimes pronounced /ɒt/[1]) is a JSON-based open standard (RFC 7519) for creating access tokens that assert some number of claims. For example, a server could generate a token that has the claim "logged in as admin" and provide that to a client. The client could then use that token to prove that it is logged in as admin. The tokens are signed by one party's private key (usually the server's), so that both parties (the other already being, by some suitable and trustworthy means, in possession of the corresponding public key) are able to verify that the token is legitimate. The tokens are designed to be compact,[2] URL-safe,[3] and usable especially in a web-browser single-sign-on (SSO) context. JWT claims can be typically used to pass identity of authenticated users between an identity provider and a service provider, or any other type of claims as required by business processes.[4][5]

JWT relies on other JSON-based standards: JWS (JSON Web Signature) RFC 7515 and JWE (JSON Web Encryption) RFC 7516.[6][7][8]


 "alg" : "HS256",
 "typ" : "JWT"
Identifies which algorithm is used to generate the signature

HS256 indicates that this token is signed using HMAC-SHA256.

Typical cryptographic algorithms used are HMAC with SHA-256 (HS256) and RSA signature with SHA-256 (RS256). JWA (JSON Web Algorithms) RFC 7518 introduces many more for both authentication and encryption.[9]

 "loggedInAs" : "admin",
 "iat" : 1422779638
Contains a set of claims. The JWT specification defines seven Registered Claim Names which are the standard fields commonly included in tokens[1]. Custom claims are usually also included, depending on the purpose of the token.

This example has the standard Issued At Claim (iat) and a custom claim (loggedInAs).

 base64urlEncoding(header) + '.' +
Securely validates the token. The signature is calculated by encoding the header and payload using Base64url Encoding and concatenating the two together with a period separator. That string is then run through the cryptographic algorithm specified in the header, in this case HMAC-SHA256. The Base64url Encoding is similar to base64, but uses different non-alphanumeric characters and omits padding.

The three parts are encoded separately using Base64url Encoding, and concatenated using periods to produce the JWT:

const token = base64urlEncoding(header) + '.' + base64urlEncoding(payload) + '.' + base64urlEncoding(signature)

The above data and the secret of "secretkey" creates the token:


This resulting token can be easily passed into HTML and HTTP.[3]


In authentication, when the user successfully logs in using their credentials, a JSON Web Token will be returned and must be saved locally (typically in local or session storage, but cookies can also be used), instead of the traditional approach of creating a session in the server and returning a cookie.

Whenever the user wants to access a protected route or resource, the user agent should send the JWT, typically in the Authorization header using the Bearer schema. The content of the header might look like the following:

Authorization: Bearer eyJhbGci...<snip>...yu5CSpyHI

This is a stateless authentication mechanism as the user state is never saved in server memory. The server's protected routes will check for a valid JWT in the Authorization header, and if it is present, the user will be allowed to access protected resources. As JWTs are self-contained, all the necessary information is there, reducing the need to query the database multiple times.

Standard fields[edit]

The Internet drafts define the following standard fields ("claims") that can be used inside a JWT claim set:

code name description
iss Issuer Identifies principal that issued the JWT.
sub Subject Identifies the subject of the JWT.
aud Audience Identifies the recipients that the JWT is intended for. Each principal intended to process the JWT must identify itself with a value in the audience claim. If the principal processing the claim does not identify itself with a value in the aud claim when this claim is present, then the JWT must be rejected.
exp Expiration Time Identifies the expiration time on and after which the JWT must not be accepted for processing. The value must be a NumericDate[10]: either an integer or decimal, representing seconds past 1970-01-01 00:00:00Z.
nbf Not Before Identifies the time on which the JWT will start to be accepted for processing. The value must be a NumericDate.
iat Issued at Identifies the time at which the JWT was issued. The value must be a NumericDate.
jti JWT ID Case sensitive unique identifier of the token even among different issuers.

The following fields can be used in authentication headers:

code name description
typ Token type If present, it is recommended to set this to JWT.
cty Content type If nested signing or encryption is employed, it is recommended to set this to JWT; otherwise, omit this field[1].
alg Message authentication code algorithm The issuer can freely set an algorithm to verify the signature on the token. However, some supported algorithms are insecure[11].
All other headers introduced by JWS and JWE[7][8].


JWT implementations exist for:

Vulnerabilities and criticism[edit]

Because JSON web tokens are stateless, there is no way to invalidate them without storing server state, thus defeating the advantage of stateless tokens.[36]

Security consultant Tim McLean reported vulnerabilities in some JWT libraries that used the alg field to incorrectly validate tokens. While these vulnerabilities were patched, McLean suggested deprecating the alg field altogether to prevent similar implementation confusion.[11]

With proper design, developers can address algorithm vulnerabilities by taking precautions:[37]

  1. Never let the JWT header alone drive verification
  2. Know the algorithms
  3. Use an appropriate key size


  1. ^ a b c John, Bradley; Nat, Sakimura; Michael, Jones. "JSON Web Token (JWT)". tools.ietf.org. Retrieved 2016-11-14.
  2. ^ Nickel, Jochen (2016). Mastering Identity and Access Management with Microsoft Azure. p. 84. ISBN 9781785887888. Retrieved 20 July 2018.
  3. ^ a b "JWT.IO - JSON Web Tokens Introduction". jwt.io. Retrieved 20 July 2018.
  4. ^ Sevilleja, Chris. "The Anatomy of a JSON Web Token". Retrieved 2015-05-08.
  5. ^ "Atlassian Connect Documentation". developer.atlassian.com. Retrieved 2015-05-08.
  6. ^ "draft-ietf-oauth-json-web-token-32 - JSON Web Token (JWT)". tools.ietf.org. Retrieved 2015-05-08.
  7. ^ a b "draft-ietf-jose-json-web-signature-41 - JSON Web Signature (JWS)". tools.ietf.org. Retrieved 2015-05-08.
  8. ^ a b "draft-ietf-jose-json-web-encryption-40 - JSON Web Encryption (JWE)". tools.ietf.org. Retrieved 2015-05-08.
  9. ^ "draft-ietf-jose-json-web-algorithms-40 - JSON Web Algorithms (JWA)". tools.ietf.org. Retrieved 2015-05-08.
  10. ^ John, Bradley,; Nat, Sakimura,; Michael, Jones,. "JSON Web Token (JWT)". tools.ietf.org. Retrieved 2017-12-20.
  11. ^ a b McLean, Tim (March 31, 2015). "Critical vulnerabilities in JSON Web Token libraries". Auth0. Retrieved 2016-03-29.
  12. ^ jwt-dotnet on github.com
  13. ^ libjwt on github.com
  14. ^ "liquidz/clj-jwt". GitHub. Retrieved 2018-05-07.
  15. ^ cljwt on github.com
  16. ^ [1] on github.com
  17. ^ "bryanjos/joken". GitHub. Retrieved 2018-05-07.
  18. ^ "dgrijalva/jwt-go". GitHub. Retrieved 2018-01-08.
  19. ^ "jwt: JSON Web Token (JWT) decoding and encoding". Hackage. Retrieved 2018-05-07.
  20. ^ auth0/java-jwt on github.com
  21. ^ "kjur/jsrsasign". GitHub. Retrieved 2018-05-07.
  22. ^ "SkyLothar/lua-resty-jwt". GitHub. Retrieved 2018-05-07.
  23. ^ "jsonwebtoken". npm. Retrieved 2018-05-07.
  24. ^ ocaml-jwt on github.com
  25. ^ Crypt::JWT on cpan.org
  26. ^ JSON-WebToken on github.com
  27. ^ lcobucci/jwt on github.com
  28. ^ Egan, Morten (2019-02-07), PLSQL Implementation of JSON Web Tokens. Contribute to morten-egan/jwt_ninja development by creating an account on GitHub, retrieved 2019-03-14
  29. ^ "SP3269/posh-jwt". GitHub. Retrieved 2018-08-01.
  30. ^ "jpadilla/pyjwt". GitHub. Retrieved 2017-03-21.
  31. ^ ruby-jwt on github.com
  32. ^ frank_jwt on github.com
  33. ^ [2] on github.com
  34. ^ jwt-scala on github.com
  35. ^ [3] on github.com
  36. ^ Slootweg, Sven. "Stop using JWT for sessions". joepie91 Ramblings. Retrieved 1 August 2018.
  37. ^ "Common JWT security vulnerabilities and how to avoid them". Retrieved 2018-05-14.

External links[edit]