Expected a different algorithm (JWT::IncorrectAlgorithm)

ruby-jwt (Ruby) — JWT::IncorrectAlgorithm from JWT.decode

What it means

The token's `alg` header is not among the algorithms you allowed — and if you passed no `algorithm:`/`algorithms:` option at all, ruby-jwt allows none. Since ruby-jwt 2.x the verifier must name the algorithm explicitly, because trusting the token's own header is the root of the well-known algorithm-confusion attacks. The signature was not checked.

Why it happens

• No algorithm passed to JWT.decode

  common

  Code from ruby-jwt 1.x tutorials calls `JWT.decode(token, key, true)`. Current versions reject every token in that case with this error.

• The issuer uses a different algorithm than configured

  common

  A migration from RS256 to ES256 (or from HS256 to an asymmetric algorithm) on the issuer side breaks verifiers that still pin the old value.

• A token from another issuer, or a probe

  occasional

  Tokens with `alg: none` or `HS256` sent to an API that verifies RS256/ES256 are rejected here — as they should be.

How to fix it

1. 1.

   Name the issuer's algorithm explicitly

   Pass `algorithm:` (or `algorithms:` for a short list). With several algorithms, use a JWKS loader so each token is verified with a key of the matching type.

   payload, header = JWT.decode(token, public_key, true, algorithm: "ES256")
   # several allowed algorithms (e.g. during a migration), each matched to the right key type:
   payload, header = JWT.decode(token, nil, true, algorithms: ["ES256", "RS256"], jwks: jwks_loader)

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