Fix JWT Vulnerabilities (Weak Signing, None Algo) in Grape

JWT implementation in Grape often falls victim to the 'none' algorithm bypass or weak symmetric secrets. If your middleware doesn't explicitly enforce signing algorithms, an attacker can flip the header to 'none', strip the signature, and escalate privileges. This guide demonstrates how to lock down ruby-jwt within a Grape API context.

The Vulnerable Pattern

class API < Grape::API
  helpers do
    def current_user
      token = headers['Authorization']&.split(' ')&.last
      # VULNERABLE: No algorithm enforcement and 'verify' is false or missing.
      # Attacker can use 'none' algorithm or brute-force the weak secret.
      payload, _header = JWT.decode(token, 'super_secret_123', false)
      User.find(payload['user_id'])
    rescue
      error!('401 Unauthorized', 401)
    end
  end
end

The Secure Implementation

The vulnerability stems from the ruby-jwt library's flexible decoding logic. By setting the third parameter to 'false', the library skips signature verification entirely. Even if set to 'true', failing to provide a restricted 'algorithm' list allows an attacker to change the JWT header to 'alg: none', which some versions treat as a valid unsigned token. The fix requires three steps: 1. Always set verification to 'true'. 2. Explicitly define the accepted algorithm (e.g., HS256) in the options hash to prevent algorithm switching attacks. 3. Ensure the signing key is a high-entropy string loaded from the environment, not hardcoded in the source.

class API < Grape::API
  helpers do
    def current_user
      token = headers['Authorization']&.split(' ')&.last
      # SECURE: Enforce algorithm, verify signature, and use strong ENV-based secret.
      secret = ENV.fetch('JWT_SECRET_KEY')
      opts = { algorithm: 'HS256', verify_iat: true, verify_jti: true }
  begin
    decoded_token = JWT.decode(token, secret, true, opts)
    payload = decoded_token.first
    User.find(payload['user_id'])
  rescue JWT::DecodeError => e
    error!({ error: 'Invalid Token', detail: e.message }, 401)
  end
end
end
end