Handle invalid algorithm when decoding JWT

jwt · Apr 26, 2023 · 6a4fb55 · 6a4fb55
1 parent cd75890
commit 6a4fb55
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -12,6 +12,7 @@
 
 **Fixes and enhancements:**
 
+- Handle invalid algorithm when decoding JWT [#559](https://github.com/jwt/ruby-jwt/pull/559) - [@nataliastanko](https://github.com/nataliastanko)
 - Your contribution here
 
 ## [v2.7.0](https://github.com/jwt/ruby-jwt/tree/v2.7.0) (2023-02-01)

diff --git a/lib/jwt/algos/algo_wrapper.rb b/lib/jwt/algos/algo_wrapper.rb
@@ -11,7 +11,7 @@ def initialize(alg, cls)
       end
 
       def valid_alg?(alg_to_check)
-        alg.casecmp(alg_to_check)&.zero? == true
+        alg&.casecmp(alg_to_check)&.zero? == true
       end
 
       def sign(data:, signing_key:)

diff --git a/spec/jwt_spec.rb b/spec/jwt_spec.rb
@@ -838,6 +838,14 @@
     end
   end
 
+  context 'when the alg is invalid' do
+    let(:token) { 'eyJhbGciOiJIUzI1NiJ9.eyJwYXkiOiJsb2FkIn0.ZpAhTTtuo-CmbgT6-95NaM_wFckKeyI157baZ29H41o' }
+
+    it 'raises JWT::IncorrectAlgorithm error' do
+      expect { JWT.decode(token, 'secret', true, algorithm: 'invalid-HS256') }.to raise_error(JWT::IncorrectAlgorithm, 'Expected a different algorithm')
+    end
+  end
+
   context 'when algorithm is a custom class' do
     let(:custom_algorithm) do
       Class.new do