Raise an error when JWK::Set::Fetcher fetches something that's not a JWKS

lib/json/jwk/set.rb

@@ -31,4 +31,4 @@ def as_json(options = {})
       end
     end
   end
-end
+end

lib/json/jwk/set/fetcher.rb

@@ -2,6 +2,8 @@ module JSON
   class JWK
     class Set
       module Fetcher
+        class MalformedJWKSet < JWT::Exception; end
+
         class Cache
           def fetch(cache_key, options = {})
             yield
@@ -65,13 +67,15 @@ def self.fetch(jwks_uri, kid:, auto_detect: true, **options)
             kid
           ].collect(&:to_s).join(':')
 
-          jwks = Set.new(
-            JSON.parse(
-              cache.fetch(cache_key, options) do
-                http_client.get(jwks_uri).body
-              end
-            )
+          parsed_jwks = JSON.parse(
+            cache.fetch(cache_key, options) do
+              http_client.get(jwks_uri).body
+            end
           )
+
+          raise MalformedJWKSet, "Malformed JWK Set: #{parsed_jwks.inspect}" unless parsed_jwks.is_a?(Hash) && parsed_jwks['keys'].is_a?(Array)
+
+          jwks = Set.new(parsed_jwks)
           cache.delete(cache_key, options) if jwks[kid].blank?
 
           if auto_detect
@@ -83,4 +87,4 @@ def self.fetch(jwks_uri, kid:, auto_detect: true, **options)
       end
     end
   end
-end
+end

spec/json/jwk/set/fetcher_spec.rb

@@ -4,7 +4,7 @@
   describe JSON::JWK::Set::Fetcher::Cache do
     let(:something) { SecureRandom.hex(32) }
 
-    it 'just execute givne block' do
+    it 'just execute given block' do
       expect(
         subject.fetch('cache_key') do
           something
@@ -111,6 +111,17 @@ def delete(cache_key)
           end.to request_to jwks_uri
         end
 
+        context "when the JWKS uri returns a structure that's not a valid JWK Set" do
+          it "raises a JSON::JWK::Set::Fetcher::MalformedJWKSet error" do
+            stub_request(:get, jwks_uri).to_return(
+              status: 200,
+              body: '"hello there"' # Note that this is valid JSON, but not a valid JWK Set
+            )
+
+            expect { subject }.to raise_error(JSON::JWK::Set::Fetcher::MalformedJWKSet)
+          end
+        end
+
         context 'when unknown' do
           let(:kid) { 'unknown' }
           let(:cache_key) do