Signature verification failed (JWT::VerificationError)

Uncaught, Ruby prints the location followed by "Signature verification failed (JWT::VerificationError)"; the text is the same in ruby-jwt 2.10 and 3.3.

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

What it means

ruby-jwt decoded the token and checked its signature with the key you passed, and the check failed. The token is well formed and the algorithm is allowed; the key does not belong to the signer, or the token was altered. `JWT::VerificationError` is a subclass of `JWT::DecodeError`, so `rescue JWT::DecodeError` catches it together with malformed-token errors.

Why it happens

• The configured key is not the signer's

  common

  A public key from another environment, an outdated key after rotation, or a key pair regenerated on every boot in a development setup that leaked into production.

• Verifying with the wrong key of a JWKS

  occasional

  Taking `keys.first` from an issuer's JWKS works until the issuer signs with another key. Let ruby-jwt select by `kid` with the `jwks:` option.

• Different HMAC secrets

  occasional

  Rails apps often derive the secret from `secret_key_base`; a service that uses another value — or the same value with a trailing newline — computes a different MAC.

How to fix it

1. 1.

   Select keys by kid from the issuer's JWKS

   The `jwks:` option accepts a loader; ruby-jwt calls it again with `kid_not_found: true` when the token names an unknown key, so rotations are picked up.

   require "jwt"
   require "net/http"
   
   jwks_loader = ->(options) do
     @jwks = nil if options[:kid_not_found] && @jwks_loaded_at.to_i < Time.now.to_i - 300 # refetch on rotation
     @jwks ||= begin
       @jwks_loaded_at = Time.now.to_i
       JSON.parse(Net::HTTP.get(URI("https://issuer.example.com/.well-known/jwks.json")))
     end
   end
   
   payload, header = JWT.decode(token, nil, true, algorithms: ["ES256"], jwks: jwks_loader,
                                iss: "https://issuer.example.com/", verify_iss: true)

2. 2.

   Prove the keys match

   Identical hashes of the public halves rule out the key; then look for modifications of the token in transit.

   openssl pkey -in issuer-private.pem -pubout | sha256sum
   openssl pkey -pubin -in configured-public.pem | sha256sum   # must be identical

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