diff --git a/include/pico_socket.h b/include/pico_socket.h
index 1bd7a1f8..d167bbf5 100644
--- a/include/pico_socket.h
+++ b/include/pico_socket.h
@@ -234,6 +234,10 @@ int pico_socket_getpeername(struct pico_socket *s, void *remote_addr, uint16_t *
 
 int pico_socket_connect(struct pico_socket *s, const void *remote_addr, uint16_t remote_port);
 int pico_socket_listen(struct pico_socket *s, const int backlog);
+/* `orig` must point to a buffer large enough for the peer address of the
+ * socket's address family: sizeof(struct pico_ip4) for IPv4, sizeof(struct
+ * pico_ip6) for IPv6. Use `union pico_address` to be safe for either family;
+ * a smaller buffer is overflown by the family-sized copy (FSCT-2026-0018). */
 struct pico_socket *pico_socket_accept(struct pico_socket *s, void *orig, uint16_t *port);
 int8_t pico_socket_del(struct pico_socket *s);
 
diff --git a/modules/pico_dns_client.c b/modules/pico_dns_client.c
index 5e518ade..6a7600b5 100644
--- a/modules/pico_dns_client.c
+++ b/modules/pico_dns_client.c
@@ -237,14 +237,18 @@ static struct pico_dns_query *pico_dns_client_find_query(struct pico_stack *S, u
         return NULL;
 }
 
-/* seek end of string */
-static char *pico_dns_client_seek(char *ptr)
+/* seek end of string, without reading past the end of the response buffer */
+static char *pico_dns_client_seek(char *ptr, const char *end)
 {
-    if (!ptr)
+    if (!ptr || !end)
         return NULL;
 
-    while (*ptr != 0)
+    while ((ptr < end) && (*ptr != 0))
         ptr++;
+    /* No terminator within the response: reject instead of walking past it. */
+    if (ptr >= end)
+        return NULL;
+
     return ptr + 1;
 }
 
@@ -358,7 +362,7 @@ static char *pico_dns_client_seek_suffix(char *suf, struct pico_dns_header *pre,
 
         case PICO_DNS_LABEL:
             dns_dbg("DNS: label\n");
-            suf = pico_dns_client_seek(suf);
+            suf = pico_dns_client_seek(suf, (const char *)pre + PICO_IP_MRU);
             break;
 
         default:
@@ -602,7 +606,7 @@ static void pico_dns_client_callback(uint16_t ev, struct pico_socket *s)
 
     /* FIX: what if the query is not a PTR query? */
     domain = (char *)header + sizeof(struct pico_dns_header);
-    qsuffix = (struct pico_dns_question_suffix *)pico_dns_client_seek(domain);
+    qsuffix = (struct pico_dns_question_suffix *)pico_dns_client_seek(domain, dns_response + sizeof(dns_response));
     /* valid asuffix is determined dynamically later on */
 
     if (pico_dns_client_check_qsuffix(qsuffix, q) < 0)
diff --git a/modules/pico_fragments.c b/modules/pico_fragments.c
index 704ffa7e..be4f4cd0 100644
--- a/modules/pico_fragments.c
+++ b/modules/pico_fragments.c
@@ -329,6 +329,14 @@ static int pico_fragments_reassemble(struct pico_tree *tree, unsigned int len, u
         return -3;
     }
 
+    /* A reassembled datagram must fit the 16-bit frame length. Without this
+     * check the (uint16_t) cast below truncates the allocation while the copy
+     * loop writes the full length, overflowing the heap buffer. The fragments
+     * are left for the expiration timer, as on allocation failure below. */
+    if (len > (65535U - header_length)) {
+        return -4;
+    }
+
     full = pico_frame_alloc((uint16_t)(header_length + len));
     if (full) {
         full->net_hdr = full->buffer;
diff --git a/modules/pico_ipv4.c b/modules/pico_ipv4.c
index aa46871b..d8f3683a 100644
--- a/modules/pico_ipv4.c
+++ b/modules/pico_ipv4.c
@@ -675,7 +675,11 @@ static int pico_ipv4_process_in(struct pico_stack *S, struct pico_protocol *self
     f->frag = short_be(hdr->frag);
 #endif
 
-    if (f->transport_len > max_allowed) {
+    /* max_allowed measures the space after the fixed IP header, but the transport
+     * area starts option_len bytes further in; subtract the options so the check
+     * cannot admit a transport region running past the frame (FSCT-2026-0022).
+     * The net_len fit-check above guarantees max_allowed >= option_len. */
+    if (f->transport_len > (uint16_t)(max_allowed - option_len)) {
         pico_frame_discard(f);
         return 0; /* Packet is discarded due to unfeasible length */
     }
diff --git a/modules/pico_udp.c b/modules/pico_udp.c
index f00a421e..006de3c6 100644
--- a/modules/pico_udp.c
+++ b/modules/pico_udp.c
@@ -199,7 +199,15 @@ uint16_t pico_udp_recv(struct pico_socket *s, void *buf, uint16_t len, void *src
     if (f) {
         if (!f->payload_len) {
             f->payload = f->transport_hdr + sizeof(struct pico_udp_hdr);
-            f->payload_len = (uint16_t)(f->transport_len - sizeof(struct pico_udp_hdr));
+            /* A datagram whose transport length is smaller than the UDP header
+             * carries no payload; without this guard the subtraction underflows
+             * to ~65528 and the copy below reads far past the frame
+             * (FSCT-2026-0019). */
+            if (f->transport_len < sizeof(struct pico_udp_hdr)) {
+                f->payload_len = 0;
+            } else {
+                f->payload_len = (uint16_t)(f->transport_len - sizeof(struct pico_udp_hdr));
+            }
         }
 
         udp_dbg("expected: %d, got: %d\n", len, f->payload_len);
diff --git a/test/examples/tcpecho.c b/test/examples/tcpecho.c
index 7a71c7c3..745054ce 100644
--- a/test/examples/tcpecho.c
+++ b/test/examples/tcpecho.c
@@ -58,7 +58,11 @@ void cb_tcpecho(uint16_t ev, struct pico_socket *s)
         struct pico_socket *sock_a = {
             0
         };
-        struct pico_ip4 orig = {
+        /* Family-agnostic buffer: pico_socket_accept() copies the peer address
+         * sized by the socket's family (16 bytes for IPv6), so a bare
+         * struct pico_ip4 here would be overflowed by an IPv6 peer
+         * (FSCT-2026-0018). */
+        union pico_address orig = {
             0
         };
         uint16_t port = 0;
@@ -68,7 +72,7 @@ void cb_tcpecho(uint16_t ev, struct pico_socket *s)
         int yes = 1;
 
         sock_a = pico_socket_accept(s, &orig, &port);
-        pico_ipv4_to_string(peer, orig.addr);
+        pico_ipv4_to_string(peer, orig.ip4.addr);
         printf("Connection established with %s:%d.\n", peer, short_be(port));
         pico_socket_setoption(sock_a, PICO_TCP_NODELAY, &yes);
         /* Set keepalive options */