diff --git a/server/src/main/java/com/cloud/network/security/SecurityGroupManagerImpl.java b/server/src/main/java/com/cloud/network/security/SecurityGroupManagerImpl.java
index f0e3b32ebaa5..8b9387752658 100644
--- a/server/src/main/java/com/cloud/network/security/SecurityGroupManagerImpl.java
+++ b/server/src/main/java/com/cloud/network/security/SecurityGroupManagerImpl.java
@@ -356,7 +356,7 @@ protected Map<PortAndProto, Set<String>> generateRulesForVM(Long userVmId, Secur
                             cidr = cidr + "/32";
                             cidrs.add(cidr);
                             if (defaultNic.getIPv6Address() != null) {
-                                cidrs.add(defaultNic.getIPv6Address() + "/64");
+                                cidrs.add(defaultNic.getIPv6Address() + "/128");
                             }
                         }
                     }
diff --git a/server/src/test/java/com/cloud/network/security/SecurityGroupManagerImplIpv6RuleTest.java b/server/src/test/java/com/cloud/network/security/SecurityGroupManagerImplIpv6RuleTest.java
new file mode 100644
index 000000000000..e2b7e96ff26a
--- /dev/null
+++ b/server/src/test/java/com/cloud/network/security/SecurityGroupManagerImplIpv6RuleTest.java
@@ -0,0 +1,90 @@
+// 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 com.cloud.network.security;
+
+import static org.mockito.Mockito.when;
+
+import java.util.Collections;
+import java.util.Map;
+import java.util.Set;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.Mockito;
+import org.mockito.junit.MockitoJUnitRunner;
+
+import com.cloud.network.NetworkModel;
+import com.cloud.network.security.SecurityGroupManagerImpl.PortAndProto;
+import com.cloud.network.security.SecurityRule.SecurityRuleType;
+import com.cloud.network.security.dao.SecurityGroupRuleDao;
+import com.cloud.network.security.dao.SecurityGroupVMMapDao;
+import com.cloud.vm.Nic;
+import com.cloud.vm.VirtualMachine.State;
+
+@RunWith(MockitoJUnitRunner.Silent.class)
+public class SecurityGroupManagerImplIpv6RuleTest {
+
+    @Mock
+    SecurityGroupRuleDao _securityGroupRuleDao;
+    @Mock
+    SecurityGroupVMMapDao _securityGroupVMMapDao;
+    @Mock
+    NetworkModel _networkModel;
+
+    @InjectMocks
+    SecurityGroupManagerImpl manager = new SecurityGroupManagerImpl();
+
+    @Test
+    public void securityGroupMemberRuleUsesExactIpv6HostCidr() {
+        Long vmId = 1L;
+        SecurityRuleType type = SecurityRuleType.IngressRule;
+
+        // The VM belongs to security group 10, which has one rule referencing another security group (20).
+        SecurityGroupVMMapVO groupMap = Mockito.mock(SecurityGroupVMMapVO.class);
+        when(groupMap.getSecurityGroupId()).thenReturn(10L);
+        when(_securityGroupVMMapDao.listByInstanceId(vmId)).thenReturn(Collections.singletonList(groupMap));
+
+        SecurityGroupRuleVO rule = Mockito.mock(SecurityGroupRuleVO.class);
+        when(rule.getProtocol()).thenReturn("tcp");
+        when(rule.getStartPort()).thenReturn(80);
+        when(rule.getEndPort()).thenReturn(80);
+        when(rule.getAllowedNetworkId()).thenReturn(20L);
+        when(_securityGroupRuleDao.listBySecurityGroupId(10L, type)).thenReturn(Collections.singletonList(rule));
+
+        // Group 20 has one running member VM with both an IPv4 and an IPv6 address.
+        SecurityGroupVMMapVO memberMap = Mockito.mock(SecurityGroupVMMapVO.class);
+        when(memberMap.getInstanceId()).thenReturn(2L);
+        when(_securityGroupVMMapDao.listBySecurityGroup(20L, State.Running)).thenReturn(Collections.singletonList(memberMap));
+
+        Nic nic = Mockito.mock(Nic.class);
+        when(nic.getIPv4Address()).thenReturn("10.1.1.5");
+        when(nic.getIPv6Address()).thenReturn("2001:db8::5");
+        when(_networkModel.getDefaultNic(2L)).thenReturn(nic);
+
+        Map<PortAndProto, Set<String>> allowed = manager.generateRulesForVM(vmId, type);
+
+        Assert.assertEquals(1, allowed.size());
+        Set<String> cidrs = allowed.values().iterator().next();
+        // The member must be authorized as an exact host, matching the IPv4 /32 behaviour.
+        Assert.assertTrue("IPv4 member should be pinned to /32", cidrs.contains("10.1.1.5/32"));
+        Assert.assertTrue("IPv6 member should be pinned to the exact /128 host", cidrs.contains("2001:db8::5/128"));
+        Assert.assertFalse("IPv6 member must not open the whole /64 subnet", cidrs.contains("2001:db8::5/64"));
+    }
+}