JSON Web Token

JSON Web Token (JWT, sometimes pronounced /dʒɒt/^[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 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]

Structure

{
 "alg" : "HS256",
 "typ" : "JWT"
}

{
 "loggedInAs" : "admin",
 "iat" : 1422779638
}

HMAC-SHA256(
 encodeBase64Url(header) + '.' +
 encodeBase64Url(payload),
 secret
)

The three parts are base64url encoded separately, and concatenated using periods:

const token = encodeBase64Url(header) + '.' + encodeBase64Url(payload) + '.' + encodeBase64Url(signature)

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

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJsb2dnZWRJbkFzIjoiYWRtaW4iLCJpYXQiOjE0MjI3Nzk2Mzh9.gzSraSYS8EXBxLN_oWnFSRgCzcmJmMjLiuyu5CSpyHI

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

Use

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

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

The following fields can be used in authentication headers:

Implementations

JWT implementations exist for

• C^[12]
• Common Lisp^[13]
• Go^[14]
• .NET (C# VB.Net etc)^[15]
• OCaml^[16]
• Perl^[17]
• Perl6^[18]
• PHP^[19]
• Python^[20]
• Ruby^[21]
• Rust^{[22] [23]}
• Scala^[24]
• Swift^[25]
• Clojure^[26]
• Elixir^[27]
• Erlang
• Haskell^[28]
• Java^[29]
• JavaScript^[30]
• Lua^[31]
• Node.js^[32]
• PowerShell^[33]

Vulnerabilities and criticism

Because JSON web tokens are stateless, there is no way to invalidate them without storing server state, thus defeating the advantage of stateless tokens.^[34]

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:^[35]

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

References

External links

• jwt.io – specialized website about JWT with tools and documentation, maintained by Auth0
• Spring Boot JWT Auth – Integrating JWT authentication with Spring framework