JSON Web Tokens (JWT)


JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed. JWTs can be signed using a secret (with the HMAC algorithm) or a public/private key pair using RSA or ECDSA.

Attacking JWTs


One of the most common and thorough tools to attack JWT is jwt_tool.

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As the JWT header can be tampered with client-side, a malicious user could change the "alg" header to "none", then remove the signature and check whether the server still accepts the token.

If it does, they could exploit this vulnerability by supplying an arbitrary claim in the JWT payload to escalate their privileges or impersonate other users. For example, if the token contains a "username": "joe" claim, they could change this to "username": "admin".

Brute-force Secret Keys

Some signing algorithms, such as HS256 (HMAC + SHA-256), use an arbitrary, standalone string as the secret key. Just like a password, it's crucial that this secret can't be easily guessed or brute-forced by an attacker. Otherwise, they may be able to create JWTs with any header and payload values they like, then use the key to re-sign the token with a valid signature.

JWT Header Parameter Injections


The JSON Web Signature (JWS) specification describes an optional jwk header parameter, which servers can use to embed their public key directly within the token itself in JWK format.

If the server does not whitelist specific public keys, we can supply an arbitrary key.


Servers may use several cryptographic keys for signing different kinds of data, not just JWTs. For this reason, the header of a JWT may contain a kid (Key ID) parameter, which helps the server identify which key to use when verifying the signature.

If this parameter is also vulnerable to directory traversal, an attacker could potentially force the server to use an arbitrary file from its filesystem as the verification key.

If the server stores its verification keys in a database, the kid header parameter is also a potential vector for SQL injection attacks.


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