diff --git a/rt/rs/security/oauth-parent/oauth2/src/main/java/org/apache/cxf/rs/security/oauth2/grants/jwt/AbstractJwtHandler.java b/rt/rs/security/oauth-parent/oauth2/src/main/java/org/apache/cxf/rs/security/oauth2/grants/jwt/AbstractJwtHandler.java
index 1a8c92a6e42..8f962334223 100644
--- a/rt/rs/security/oauth-parent/oauth2/src/main/java/org/apache/cxf/rs/security/oauth2/grants/jwt/AbstractJwtHandler.java
+++ b/rt/rs/security/oauth-parent/oauth2/src/main/java/org/apache/cxf/rs/security/oauth2/grants/jwt/AbstractJwtHandler.java
@@ -82,6 +82,13 @@ protected void validateSubject(Client client, String subject) {
         if (subject == null) {
             throw new OAuthServiceException(OAuthConstants.INVALID_GRANT);
         }
+        // Strict-by-default: require the assertion subject to match the authenticated client id.
+        // Subclasses can override this method to support a different authorization model
+        // (for example, trusted delegation with an explicit client-to-subject policy).
+        if (client != null && client.getClientId() != null
+            && !client.getClientId().equals(subject)) {
+            throw new OAuthServiceException(OAuthConstants.INVALID_GRANT);
+        }
     }
     public void setSupportedIssuers(Set<String> supportedIssuers) {
         this.supportedIssuers = supportedIssuers;
diff --git a/rt/rs/security/oauth-parent/oauth2/src/test/java/org/apache/cxf/rs/security/oauth2/grants/jwt/JwtBearerGrantHandlerTest.java b/rt/rs/security/oauth-parent/oauth2/src/test/java/org/apache/cxf/rs/security/oauth2/grants/jwt/JwtBearerGrantHandlerTest.java
new file mode 100644
index 00000000000..2e0b41c43c5
--- /dev/null
+++ b/rt/rs/security/oauth-parent/oauth2/src/test/java/org/apache/cxf/rs/security/oauth2/grants/jwt/JwtBearerGrantHandlerTest.java
@@ -0,0 +1,134 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.cxf.rs.security.oauth2.grants.jwt;
+
+import java.lang.reflect.Field;
+
+import jakarta.ws.rs.core.MultivaluedMap;
+import org.apache.cxf.jaxrs.impl.MetadataMap;
+import org.apache.cxf.message.Message;
+import org.apache.cxf.message.MessageImpl;
+import org.apache.cxf.phase.PhaseInterceptorChain;
+import org.apache.cxf.rs.security.jose.jwa.SignatureAlgorithm;
+import org.apache.cxf.rs.security.jose.jws.HmacJwsSignatureProvider;
+import org.apache.cxf.rs.security.jose.jws.HmacJwsSignatureVerifier;
+import org.apache.cxf.rs.security.jose.jws.JwsJwtCompactProducer;
+import org.apache.cxf.rs.security.jose.jwt.JwtClaims;
+import org.apache.cxf.rs.security.oauth2.common.AccessTokenRegistration;
+import org.apache.cxf.rs.security.oauth2.common.Client;
+import org.apache.cxf.rs.security.oauth2.common.ServerAccessToken;
+import org.apache.cxf.rs.security.oauth2.grants.OAuthDataProviderImpl;
+import org.apache.cxf.rs.security.oauth2.provider.OAuthServiceException;
+import org.apache.cxf.rs.security.oauth2.tokens.bearer.BearerAccessToken;
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.fail;
+
+public class JwtBearerGrantHandlerTest {
+
+    private static final String SIGNING_KEY = "jwt-grant-test-signing-key";
+
+    @Before
+    public void setUp() throws Exception {
+        setThreadLocalMessage(new MessageImpl());
+    }
+
+    @After
+    public void tearDown() throws Exception {
+        setThreadLocalMessage(null);
+    }
+
+    @Test
+    public void testMismatchedClientAndSubjectRejected() {
+        JwtBearerGrantHandler handler = new JwtBearerGrantHandler();
+        handler.setDataProvider(new SubjectAwareDataProvider());
+        handler.setJwsVerifier(new HmacJwsSignatureVerifier(SIGNING_KEY, SignatureAlgorithm.HS256));
+
+        Client client = new Client("fuzz-client", "secret", true);
+        String assertion = createSignedAssertion("trusted-issuer", "victim-user");
+
+        MultivaluedMap<String, String> params = new MetadataMap<>();
+        params.putSingle(Constants.CLIENT_GRANT_ASSERTION_PARAM, assertion);
+
+        try {
+            handler.createAccessToken(client, params);
+            fail("OAuthServiceException expected");
+        } catch (OAuthServiceException expected) {
+            assertEquals("invalid_grant", expected.getMessage());
+        }
+    }
+
+    @Test
+    public void testMatchingClientAndSubjectAccepted() {
+        JwtBearerGrantHandler handler = new JwtBearerGrantHandler();
+        handler.setDataProvider(new SubjectAwareDataProvider());
+        handler.setJwsVerifier(new HmacJwsSignatureVerifier(SIGNING_KEY, SignatureAlgorithm.HS256));
+
+        Client client = new Client("fuzz-client", "secret", true);
+        String assertion = createSignedAssertion("trusted-issuer", client.getClientId());
+
+        MultivaluedMap<String, String> params = new MetadataMap<>();
+        params.putSingle(Constants.CLIENT_GRANT_ASSERTION_PARAM, assertion);
+
+        ServerAccessToken token = handler.createAccessToken(client, params);
+
+        assertNotNull(token);
+        assertNotNull(token.getSubject());
+        assertEquals(client.getClientId(), token.getSubject().getLogin());
+    }
+
+    private static String createSignedAssertion(String issuer, String subject) {
+        long now = System.currentTimeMillis() / 1000;
+        JwtClaims claims = new JwtClaims();
+        claims.setIssuer(issuer);
+        claims.setSubject(subject);
+        claims.setIssuedAt(now);
+        claims.setExpiryTime(now + 300);
+
+        JwsJwtCompactProducer producer = new JwsJwtCompactProducer(claims);
+        return producer.signWith(new HmacJwsSignatureProvider(SIGNING_KEY, SignatureAlgorithm.HS256));
+    }
+
+    private static final class SubjectAwareDataProvider extends OAuthDataProviderImpl {
+        @Override
+        public ServerAccessToken createAccessToken(AccessTokenRegistration accessToken) {
+            BearerAccessToken token = new BearerAccessToken(accessToken.getClient(), 3600);
+            token.setSubject(accessToken.getSubject());
+            token.setGrantType(accessToken.getGrantType());
+            return token;
+        }
+    }
+
+    private static void setThreadLocalMessage(Message message) throws Exception {
+        Field f = PhaseInterceptorChain.class.getDeclaredField("CURRENT_MESSAGE");
+        f.setAccessible(true);
+        @SuppressWarnings("unchecked")
+        ThreadLocal<Message> tl = (ThreadLocal<Message>) f.get(null);
+        if (message == null) {
+            tl.remove();
+        } else {
+            tl.set(message);
+        }
+    }
+}
\ No newline at end of file