diff --git a/test/e2e/upg_e2e.go b/test/e2e/upg_e2e.go
index 65c9436..29a12b8 100644
--- a/test/e2e/upg_e2e.go
+++ b/test/e2e/upg_e2e.go
@@ -845,6 +845,37 @@ var _ = ginkgo.Describe("UPG Binary API", func() {
 	})
 })
 
+var _ = ginkgo.Describe("Accessing SGi interface", func() {
+	ginkgo.Context("from UE", func() {
+		f := framework.NewDefaultFramework(framework.UPGModeTDF, framework.UPGIPModeV4)
+		ginkgo.It("ping should be ignored", func() {
+			sessionCfg := &framework.SessionConfig{
+				IdBase: 1,
+				UEIP:   f.UEIP(),
+				Mode:   f.Mode,
+			}
+			anotherSessionCfg := &framework.SessionConfig{
+				IdBase: 2,
+				UEIP:   net.ParseIP("10.0.1.2"),
+				Mode:   f.Mode,
+			}
+			_, err := f.PFCP.EstablishSession(f.Context, 0, sessionCfg.SessionIEs()...)
+			framework.ExpectNoError(err)
+			_, err = f.PFCP.EstablishSession(f.Context, 0, anotherSessionCfg.SessionIEs()...)
+			framework.ExpectNoError(err)
+			tg, clientNS, serverNS := newTrafficGen(f, &traffic.ICMPPingConfig{
+				//ServerIP: sgiAddress,
+				ServerIP:    net.ParseIP("10.0.1.2"),
+				PacketCount: 10, // 10s
+				Retry:       true,
+				Delay:       100 * time.Millisecond,
+			}, &traffic.SimpleTrafficRec{})
+			tg.Start(f.Context, clientNS, serverNS)
+			// Expect no crash here
+		})
+	})
+})
+
 var _ = ginkgo.Describe("Clearing message queue", func() {
 	ginkgo.Context("during session deletion", func() {
 		f := framework.NewDefaultFramework(framework.UPGModeTDF, framework.UPGIPModeV4)
diff --git a/upf/upf.h b/upf/upf.h
index 31f9104..9051060 100644
--- a/upf/upf.h
+++ b/upf/upf.h
@@ -78,38 +78,16 @@ STATIC_ASSERT (sizeof (upf_buffer_opaque_t) <=
   ((upf_buffer_opaque_t *)((u8 *)((b)->opaque2) +       \
 STRUCT_OFFSET_OF (vnet_buffer_opaque_t, unused)))
 
-#if CLIB_DEBUG > 0
-
-/*
- * For debug builds, we add a flag to each buffer when we initialize
- * GTPU metadata when the buffer is processed by one of the UPF
- * entry nodes (upf-gtpu[46]-input, upf-ip[46]-session-dpo,
- * upf-ip[46]-proxy-server-output)
- */
 #define UPF_BUFFER_F_GTPU_INITIALIZED VNET_BUFFER_F_AVAIL1
 #define UPF_ENTER_SUBGRAPH(b, sidx, is_ip4)				\
   do {									\
-    ASSERT (!((b)->flags & UPF_BUFFER_F_GTPU_INITIALIZED));		\
     clib_memset(upf_buffer_opaque (b), 0, sizeof(upf_buffer_opaque_t));	\
     b->flags |= UPF_BUFFER_F_GTPU_INITIALIZED;				\
     upf_buffer_opaque (b)->gtpu.session_index = sidx;			\
     upf_buffer_opaque (b)->gtpu.flags =					\
       is_ip4 ? BUFFER_GTP_UDP_IP4 : BUFFER_GTP_UDP_IP6;			\
   } while (0)
-#define UPF_CHECK_INNER_NODE(b) ASSERT (b->flags & UPF_BUFFER_F_GTPU_INITIALIZED)
-
-#else
-
-#define UPF_ENTER_SUBGRAPH(b, sidx, is_ip4)				\
-  do {									\
-    clib_memset(upf_buffer_opaque (b), 0, sizeof(upf_buffer_opaque_t)); \
-    upf_buffer_opaque (b)->gtpu.session_index = sidx;			\
-    upf_buffer_opaque (b)->gtpu.flags =					\
-      is_ip4 ? BUFFER_GTP_UDP_IP4 : BUFFER_GTP_UDP_IP6;			\
-  } while (0)
-#define UPF_CHECK_INNER_NODE(b)
-
-#endif
+#define UPF_CHECK_INNER_NODE(b) (b->flags & UPF_BUFFER_F_GTPU_INITIALIZED)
 
 #define BUFFER_FAR_ONLY     (1<<3)	/* don't include in QER/URR processing */
 #define BUFFER_HAS_GTP_HDR  (1<<4)
diff --git a/upf/upf_session_dpo.c b/upf/upf_session_dpo.c
index 2481c72..da31067 100644
--- a/upf/upf_session_dpo.c
+++ b/upf/upf_session_dpo.c
@@ -259,8 +259,9 @@ VLIB_INIT_FUNCTION (upf_session_dpo_module_init);
 #endif /* CLIB_MARCH_VARIANT */
 
 /* Statistics (not all errors) */
-#define foreach_upf_session_dpo_error    	\
-  _(SESSION_DPO, "good packets session_dpo")	\
+#define foreach_upf_session_dpo_error		  \
+  _(SESSION_DPO, "good packets session_dpo")  	  \
+  _(SUBGRAPH_ENTRY, "packet re-entered subgraph") \
   _(PROXY_LOOP, "proxy output loop detected")
 
 static char *upf_session_dpo_error_strings[] = {
@@ -423,6 +424,19 @@ VLIB_NODE_FN (upf_ip4_session_dpo_node) (vlib_main_t * vm,
 	      goto trace;
 	    }
 
+	  /* Traffic hits UPF Subgraph for second time. This might happen if
+	   * packet was originated from UE to another UE or local SGi interface
+	   * and hit DPO of SGi table. By security reasons this is not allowed.
+	   */
+	  if (UPF_CHECK_INNER_NODE (b))
+	    {
+	      upf_debug ("UPF Subgraph re-entered: %U", format_ip4_header,
+			 ip0, b->current_length);
+	      error0 = UPF_SESSION_DPO_ERROR_SUBGRAPH_ENTRY;
+	      next = UPF_SESSION_DPO_NEXT_DROP;
+	      goto trace;
+	    }
+
 	  UPF_ENTER_SUBGRAPH (b, sidx, 1);
 	  error0 = IP4_ERROR_NONE;
 	  next = UPF_SESSION_DPO_NEXT_FLOW_PROCESS;
@@ -556,6 +570,19 @@ VLIB_NODE_FN (upf_ip6_session_dpo_node) (vlib_main_t * vm,
 	      goto trace;
 	    }
 
+	  /* Traffic hits UPF Subgraph for second time. This might happen if
+	   * packet was originated from UE to another UE or local SGi interface
+	   * and hit DPO of SGi table. By security reasons this is not allowed.
+	   */
+	  if (UPF_CHECK_INNER_NODE (b))
+	    {
+	      upf_debug ("UPF Subgraph re-entered: %U", format_ip4_header,
+			 ip0, b->current_length);
+	      error0 = UPF_SESSION_DPO_ERROR_SUBGRAPH_ENTRY;
+	      next = UPF_SESSION_DPO_NEXT_DROP;
+	      goto trace;
+	    }
+
 	  UPF_ENTER_SUBGRAPH (b, sidx, 0);
 	  error0 = IP6_ERROR_NONE;
 	  next = UPF_SESSION_DPO_NEXT_FLOW_PROCESS;