Fix ClassCastException

Closes gh-9651

Author: tiborkoch

oauth2/oauth2-jose/src/main/java/org/springframework/security/oauth2/jwt/JwtDecoderProviderConfigurationUtils.java

@@ -90,7 +90,8 @@ static Set<SignatureAlgorithm> getSignatureAlgorithms(JWKSource<SecurityContext>
 			List<? extends JWK> jwks = jwkSource.get(new JWKSelector(jwkMatcher), null);
 			for (JWK jwk : jwks) {
 				if (jwk.getAlgorithm() != null) {
-					jwsAlgorithms.add((JWSAlgorithm) jwk.getAlgorithm());
+					JWSAlgorithm jwsAlgorithm = JWSAlgorithm.parse(jwk.getAlgorithm().getName());
+					jwsAlgorithms.add(jwsAlgorithm);
 				}
 				else {
 					if (jwk.getKeyType() == KeyType.RSA) {

oauth2/oauth2-jose/src/test/java/org/springframework/security/oauth2/jwt/JwtDecoderProviderConfigurationUtilsTests.java

@@ -20,6 +20,7 @@
 import java.util.Collections;
 import java.util.Set;
 
+import com.nimbusds.jose.Algorithm;
 import com.nimbusds.jose.JWSAlgorithm;
 import com.nimbusds.jose.jwk.Curve;
 import com.nimbusds.jose.jwk.ECKey;
@@ -32,6 +33,7 @@
 import org.junit.Test;
 
 import org.springframework.security.oauth2.jose.TestKeys;
+import org.springframework.security.oauth2.jose.jws.JwsAlgorithms;
 import org.springframework.security.oauth2.jose.jws.SignatureAlgorithm;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -73,4 +75,15 @@ public void getSignatureAlgorithmsWhenJwkSetSpecifiesFamilyThenUses() throws Exc
 		assertThat(algorithms).contains(SignatureAlgorithm.ES256, SignatureAlgorithm.ES384, SignatureAlgorithm.ES512);
 	}
 
+	// gh-9651
+	@Test
+	public void getSignatureAlgorithmsWhenAlgorithmThenParses() throws Exception {
+		JWKSource<SecurityContext> jwkSource = mock(JWKSource.class);
+		RSAKey key = new RSAKey.Builder(TestKeys.DEFAULT_PUBLIC_KEY).keyUse(KeyUse.SIGNATURE)
+				.algorithm(new Algorithm(JwsAlgorithms.RS256)).build();
+		given(jwkSource.get(any(JWKSelector.class), isNull())).willReturn(Collections.singletonList(key));
+		Set<SignatureAlgorithm> algorithms = JwtDecoderProviderConfigurationUtils.getSignatureAlgorithms(jwkSource);
+		assertThat(algorithms).containsOnly(SignatureAlgorithm.RS256);
+	}
+
 }