Fix JWT::JWK::Set sharing its key collection when copied

JWT::JWK::Set.new(other_set) returned the source set's `keys` array by
reference, despite the "Simple duplication" comment on that branch. The
class also defined no `initialize_copy`, so `dup` and `clone` shared the
array as well.

Mutating a copy through `add`, `<<`, `delete`, `select!`, `reject!` or
`uniq!` therefore mutated the original too, which matters because key
sets are commonly cached and shared between consumers. `union` was
unaffected, as `merge` rebinds `@keys` with `+=` instead of mutating.

Both paths now copy the array. The JWK objects themselves stay shared.

Fixes #750

Author: anakinj

CHANGELOG.md

@@ -12,6 +12,7 @@
 
 - Fix rejection of unknown algorithms from JWKs for RFC compliance and pquip [#728](https://github.com/jwt/ruby-jwt/pull/728)
 - Fix the `Style/DirectiveScope` RuboCop offense failing the build [#752](https://github.com/jwt/ruby-jwt/pull/752)
+- Fix `JWT::JWK::Set` sharing its key collection with the set it was copied from [#751](https://github.com/jwt/ruby-jwt/pull/751)
 
 ## [v3.2.0](https://github.com/jwt/ruby-jwt/tree/v3.2.0) (2026-05-13)
 

lib/jwt/jwk/set.rb

@@ -17,7 +17,7 @@ def initialize(jwks = nil, options = {}) # rubocop:disable Metrics/CyclomaticCom
 
         @keys = case jwks
                 when JWT::JWK::Set # Simple duplication
-                  jwks.keys
+                  jwks.keys.dup
                 when JWT::JWK::KeyBase # Singleton
                   [jwks]
                 when Hash
@@ -34,6 +34,13 @@ def initialize(jwks = nil, options = {}) # rubocop:disable Metrics/CyclomaticCom
                 end
       end
 
+      # Ensures a duplicated set owns its key collection. The keys themselves are
+      # intentionally shared; only the collection is copied.
+      def initialize_copy(other)
+        super
+        @keys = other.keys.dup
+      end
+
       def export(options = {})
         { keys: @keys.map { |k| k.export(options) } }
       end

spec/jwt/jwk/set_spec.rb

@@ -36,6 +36,61 @@
       end
     end
 
+    context 'when created from an existing JWT::JWK::Set' do
+      let(:jwk) { JWT::JWK.new({ kty: 'oct', k: Base64.strict_encode64('testkey') }) }
+      let(:other) { JWT::JWK.new({ kty: 'oct', k: Base64.strict_encode64('otherkey') }) }
+      let(:original) { described_class.new([jwk]) }
+      let(:copy) { described_class.new(original) }
+
+      it 'does not share the key collection with the original' do
+        expect(copy.keys).not_to be(original.keys)
+      end
+
+      it 'keeps the original intact when keys are added to the copy' do
+        copy.add(other)
+        expect(original.keys).to eql([jwk])
+      end
+
+      it 'keeps the original intact when keys are removed from the copy' do
+        copy.delete(jwk)
+        expect(original.keys).to eql([jwk])
+      end
+
+      it 'keeps the original intact when the copy is filtered' do
+        copy.select! { false }
+        expect(original.keys).to eql([jwk])
+      end
+
+      it 'shares the key objects with the original' do
+        expect(copy.keys.first).to be(original.keys.first)
+      end
+    end
+
+    context 'when duplicated' do
+      let(:jwk) { JWT::JWK.new({ kty: 'oct', k: Base64.strict_encode64('testkey') }) }
+      let(:other) { JWT::JWK.new({ kty: 'oct', k: Base64.strict_encode64('otherkey') }) }
+      let(:original) { described_class.new([jwk]) }
+
+      it 'does not share the key collection with the original' do
+        expect(original.dup.keys).not_to be(original.keys)
+      end
+
+      it 'keeps the original intact when keys are added to the duplicate' do
+        original.dup << other
+        expect(original.keys).to eql([jwk])
+      end
+
+      it 'keeps the original intact when the duplicate is filtered' do
+        original.dup.reject! { true }
+        expect(original.keys).to eql([jwk])
+      end
+
+      it 'keeps the original intact when a union is built from it' do
+        original.union([other])
+        expect(original.keys).to eql([jwk])
+      end
+    end
+
     it 'ignores keys with unsupported kty values (RFC 7517 §5), required for hybrid PQC' do
       jwks = {
         keys: [