diff --git a/lib/tevent/tevent_select.c b/lib/tevent/tevent_select.c
index d974189..890e031 100644
--- a/lib/tevent/tevent_select.c
+++ b/lib/tevent/tevent_select.c
@@ -111,6 +111,11 @@ static struct tevent_fd *select_event_add_fd(struct tevent_context *ev, TALLOC_C
 							   struct select_event_context);
 	struct tevent_fd *fde;
 
+	if (fd < 0 || fd >= FD_SETSIZE) {
+		errno = EBADF;
+		return NULL;
+	}
+
 	fde = tevent_common_add_fd(ev, mem_ctx, fd, flags,
 				   handler, private_data,
 				   handler_name, location);
@@ -143,6 +148,11 @@ static int select_event_loop_select(struct select_event_context *select_ev, stru
 
 	/* setup any fd events */
 	for (fde = select_ev->ev->fd_events; fde; fde = fde->next) {
+		if (fde->fd < 0 || fde->fd >= FD_SETSIZE) {
+			errno = EBADF;
+			return -1;
+		}
+
 		if (fde->flags & TEVENT_FD_READ) {
 			FD_SET(fde->fd, &r_fds);
 		}
diff --git a/lib/tevent/tevent_standard.c b/lib/tevent/tevent_standard.c
index c3f8b36..d4a00cc 100644
--- a/lib/tevent/tevent_standard.c
+++ b/lib/tevent/tevent_standard.c
@@ -457,6 +457,11 @@ static int std_event_loop_select(struct std_event_context *std_ev, struct timeva
 
 	/* setup any fd events */
 	for (fde = std_ev->ev->fd_events; fde; fde = fde->next) {
+		if (fde->fd < 0 || fde->fd >= FD_SETSIZE) {
+			std_ev->exit_code = EBADF;
+			return -1;
+		}
+
 		if (fde->flags & TEVENT_FD_READ) {
 			FD_SET(fde->fd, &r_fds);
 		}
diff --git a/nsswitch/wb_common.c b/nsswitch/wb_common.c
index d0dfcb8..75ce585 100644
--- a/nsswitch/wb_common.c
+++ b/nsswitch/wb_common.c
@@ -240,6 +240,12 @@ static int winbind_named_pipe_sock(const char *dir)
 
 		switch (errno) {
 			case EINPROGRESS:
+
+				if (fd < 0 || fd >= FD_SETSIZE) {
+					errno = EBADF;
+					goto error_out;
+				}
+
 				FD_ZERO(&w_fds);
 				FD_SET(fd, &w_fds);
 				tv.tv_sec = CONNECT_TIMEOUT - wait_time;
@@ -384,6 +390,12 @@ int winbind_write_sock(void *buffer, int count, int recursing, int need_priv)
 		struct timeval tv;
 		fd_set r_fds;
 
+		if (winbindd_fd < 0 || winbindd_fd >= FD_SETSIZE) {
+			errno = EBADF;
+			winbind_close_sock();
+			return -1;
+		}
+
 		/* Catch pipe close on other end by checking if a read()
 		   call would not block by calling select(). */
 
@@ -444,6 +456,11 @@ int winbind_read_sock(void *buffer, int count)
 		struct timeval tv;
 		fd_set r_fds;
 
+		if (winbindd_fd < 0 || winbindd_fd >= FD_SETSIZE) {
+			errno = EBADF;
+			winbind_close_sock();
+			return -1;
+		}
 		/* Catch pipe close on other end by checking if a read()
 		   call would not block by calling select(). */
 
diff --git a/source3/client/client.c b/source3/client/client.c
index b706591..1e5e8e1 100644
--- a/source3/client/client.c
+++ b/source3/client/client.c
@@ -4384,8 +4384,10 @@ static void readline_callback(void)
 
  again:
 
-	if (cli->fd == -1)
+	if (cli->fd < 0 || cli->fd >= FD_SETSIZE) {
+		errno = EBADF;
 		return;
+	}
 
 	FD_ZERO(&fds);
 	FD_SET(cli->fd,&fds);
diff --git a/source3/client/dnsbrowse.c b/source3/client/dnsbrowse.c
index 5e3a4de..a6b9360 100644
--- a/source3/client/dnsbrowse.c
+++ b/source3/client/dnsbrowse.c
@@ -81,6 +81,11 @@ static void do_smb_resolve(struct mdns_smbsrv_result *browsesrv)
 			TALLOC_FREE(fdset);
 		}
 
+		if (mdnsfd < 0 || mdnsfd >= FD_SETSIZE) {
+			errno = EBADF;
+			break;
+		}
+
 		fdsetsz = howmany(mdnsfd + 1, NFDBITS) * sizeof(fd_mask);
 		fdset = TALLOC_ZERO(ctx, fdsetsz);
 		FD_SET(mdnsfd, fdset);
@@ -181,6 +186,12 @@ int do_smb_browse(void)
 			TALLOC_FREE(fdset);
 		}
 
+		if (mdnsfd < 0 || mdnsfd >= FD_SETSIZE) {
+			errno = EBADF;
+			TALLOC_FREE(ctx);
+			return 1;
+		}
+
 		fdsetsz = howmany(mdnsfd + 1, NFDBITS) * sizeof(fd_mask);
 		fdset = TALLOC_ZERO(ctx, fdsetsz);
 		FD_SET(mdnsfd, fdset);
diff --git a/source3/lib/events.c b/source3/lib/events.c
index 90d86c6..3c83f81 100644
--- a/source3/lib/events.c
+++ b/source3/lib/events.c
@@ -55,6 +55,14 @@ bool event_add_to_select_args(struct tevent_context *ev,
 	bool ret = false;
 
 	for (fde = ev->fd_events; fde; fde = fde->next) {
+		if (fde->fd < 0 || fde->fd >= FD_SETSIZE) {
+			/* We ignore here, as it shouldn't be
+			   possible to add an invalid fde->fd
+			   but we don't want FD_SET to see an
+			   invalid fd. */
+			continue;
+		}
+
 		if (fde->flags & EVENT_FD_READ) {
 			FD_SET(fde->fd, read_fds);
 			ret = true;
diff --git a/source3/lib/packet.c b/source3/lib/packet.c
index ef28bf9..c14d1e5 100644
--- a/source3/lib/packet.c
+++ b/source3/lib/packet.c
@@ -106,6 +106,11 @@ NTSTATUS packet_fd_read_sync(struct packet_context *ctx)
 	int res;
 	fd_set r_fds;
 
+	if (ctx->fd < 0 || ctx->fd >= FD_SETSIZE) {
+		errno = EBADF;
+		return map_nt_error_from_unix(errno);
+	}
+
 	FD_ZERO(&r_fds);
 	FD_SET(ctx->fd, &r_fds);
 
diff --git a/source3/lib/readline.c b/source3/lib/readline.c
index 34867aa..70a82f2 100644
--- a/source3/lib/readline.c
+++ b/source3/lib/readline.c
@@ -91,6 +91,11 @@ static char *smb_readline_replacement(const char *prompt, void (*callback)(void)
 		timeout.tv_sec = 5;
 		timeout.tv_usec = 0;
 
+		if (fd < 0 || fd >= FD_SETSIZE) {
+			errno = EBADF;
+			break;
+		}
+
 		FD_ZERO(&fds);
 		FD_SET(fd,&fds);
 
diff --git a/source3/lib/select.c b/source3/lib/select.c
index b5443ff..2230b43 100644
--- a/source3/lib/select.c
+++ b/source3/lib/select.c
@@ -75,6 +75,17 @@ int sys_select(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, s
 			return -1;
 		}
 
+		if (select_pipe[0] < 0 || select_pipe[0] >= FD_SETSIZE) {
+			DEBUG(0, ("sys_select: bad fd\n"));
+			if (readfds != NULL)
+				FD_ZERO(readfds);
+			if (writefds != NULL)
+				FD_ZERO(writefds);
+			if (errorfds != NULL)
+				FD_ZERO(errorfds);
+			errno = EBADF;
+			return -1;
+		}
 		/*
 		 * These next two lines seem to fix a bug with the Linux
 		 * 2.0.x kernel (and probably other UNIXes as well) where
@@ -101,6 +112,7 @@ int sys_select(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, s
 		readfds2 = &readfds_buf;
 		FD_ZERO(readfds2);
 	}
+
 	FD_SET(select_pipe[0], readfds2);
 
 	errno = 0;
diff --git a/source3/lib/util_sock.c b/source3/lib/util_sock.c
index da79aca..238daf4 100644
--- a/source3/lib/util_sock.c
+++ b/source3/lib/util_sock.c
@@ -560,6 +560,11 @@ NTSTATUS read_fd_with_timeout(int fd, char *buf,
 	timeout.tv_usec = (long)(1000 * (time_out % 1000));
 
 	for (nread=0; nread < mincnt; ) {
+		if (fd < 0 || fd >= FD_SETSIZE) {
+			errno = EBADF;
+			return map_nt_error_from_unix(EBADF);
+		}
+
 		FD_ZERO(&fds);
 		FD_SET(fd,&fds);
 
@@ -1294,7 +1299,7 @@ bool open_any_socket_out(struct sockaddr_storage *addrs, int num_addrs,
 
 	for (i=0; i<num_addrs; i++) {
 		sockets[i] = socket(addrs[i].ss_family, SOCK_STREAM, 0);
-		if (sockets[i] < 0)
+		if (sockets[i] < 0 || sockets[i] >= FD_SETSIZE)
 			goto done;
 		set_blocking(sockets[i], false);
 	}
@@ -1343,8 +1348,10 @@ bool open_any_socket_out(struct sockaddr_storage *addrs, int num_addrs,
 	FD_ZERO(&r_fds);
 
 	for (i=0; i<num_addrs; i++) {
-		if (sockets[i] == -1)
+		if (sockets[i] < 0 || sockets[i] >= FD_SETSIZE) {
+			/* This cannot happen - ignore if so. */
 			continue;
+		}
 		FD_SET(sockets[i], &wr_fds);
 		FD_SET(sockets[i], &r_fds);
 		if (sockets[i]>maxfd)
diff --git a/source3/lib/wbclient.c b/source3/lib/wbclient.c
index 3cf992c..9cb250d 100644
--- a/source3/lib/wbclient.c
+++ b/source3/lib/wbclient.c
@@ -120,7 +120,7 @@ static bool winbind_closed_fd(int fd)
 	struct timeval tv;
 	fd_set r_fds;
 
-	if (fd == -1) {
+	if (fd == -1 || fd >= FD_SETSIZE) {
 		return true;
 	}
 
@@ -222,6 +222,13 @@ static struct tevent_req *wb_connect_send(TALLOC_CTX *mem_ctx,
 		wbc_err = map_wbc_err_from_errno(errno);
 		goto post_status;
 	}
+	if (wb_ctx->fd >= FD_SETSIZE) {
+		close(wb_ctx->fd);
+		wb_ctx->fd = -1;
+		errno = EBADF;
+		wbc_err = map_wbc_err_from_errno(errno);
+		goto post_status;
+	}
 
 	subreq = async_connect_send(mem_ctx, ev, wb_ctx->fd,
 				    (struct sockaddr *)&sunaddr,
diff --git a/source3/libaddns/dnssock.c b/source3/libaddns/dnssock.c
index 7c8bd41..bf11fea 100644
--- a/source3/libaddns/dnssock.c
+++ b/source3/libaddns/dnssock.c
@@ -219,6 +219,11 @@ static DNS_ERROR read_all(int fd, uint8 *data, size_t len)
 		ssize_t ret;
 		int fd_ready;
 		
+		if (fd < 0 || fd >= FD_SETSIZE) {
+			/* read timeout */
+			return ERROR_DNS_SOCKET_ERROR;
+		}
+
 		FD_ZERO( &rfds );
 		FD_SET( fd, &rfds );
 
diff --git a/source3/libads/cldap.c b/source3/libads/cldap.c
index ae087d9..ac4e2cb 100644
--- a/source3/libads/cldap.c
+++ b/source3/libads/cldap.c
@@ -134,6 +134,11 @@ static int recv_cldap_netlogon(TALLOC_CTX *mem_ctx,
 		return -1;
 	}
 
+	if (sock < 0 || sock >= FD_SETSIZE) {
+		errno = EBADF;
+		return -1;
+	}
+
 	FD_ZERO(&r_fds);
 	FD_SET(sock, &r_fds);
 
diff --git a/source3/libsmb/nmblib.c b/source3/libsmb/nmblib.c
index 1206d9d..8230c5a 100644
--- a/source3/libsmb/nmblib.c
+++ b/source3/libsmb/nmblib.c
@@ -1089,6 +1089,11 @@ struct packet_struct *receive_packet(int fd,enum packet_type type,int t)
 	struct timeval timeout;
 	int ret;
 
+	if (fd < 0 || fd >= FD_SETSIZE) {
+		errno = EBADF;
+		return NULL;
+	}
+
 	FD_ZERO(&fds);
 	FD_SET(fd,&fds);
 	timeout.tv_sec = t/1000;
diff --git a/source3/nmbd/nmbd_packets.c b/source3/nmbd/nmbd_packets.c
index f69845b..1c570ea 100644
--- a/source3/nmbd/nmbd_packets.c
+++ b/source3/nmbd/nmbd_packets.c
@@ -1683,7 +1683,7 @@ static bool create_listen_fdset(fd_set **ppset, int **psock_array, int *listen_n
 	for (subrec = FIRST_SUBNET; subrec; subrec = NEXT_SUBNET_EXCLUDING_UNICAST(subrec))
 		count++;
 
-	if((count*2) + 2 > FD_SETSIZE) {
+	if((count*2) + 2 >= FD_SETSIZE) {
 		DEBUG(0,("create_listen_fdset: Too many file descriptors needed (%d). We can \
 only use %d.\n", (count*2) + 2, FD_SETSIZE));
 		SAFE_FREE(pset);
@@ -1699,24 +1699,44 @@ only use %d.\n", (count*2) + 2, FD_SETSIZE));
 	FD_ZERO(pset);
 
 	/* Add in the broadcast socket on 137. */
+	if (ClientNMB < 0 || ClientNMB >= FD_SETSIZE) {
+		errno = EBADF;
+		SAFE_FREE(pset);
+		return True;
+	}
+
 	FD_SET(ClientNMB,pset);
 	sock_array[num++] = ClientNMB;
 	*maxfd = MAX( *maxfd, ClientNMB);
 
 	/* Add in the 137 sockets on all the interfaces. */
 	for (subrec = FIRST_SUBNET; subrec; subrec = NEXT_SUBNET_EXCLUDING_UNICAST(subrec)) {
+		if (subrec->nmb_sock < 0 || subrec->nmb_sock >= FD_SETSIZE) {
+			/* We have to ignore sockets outside FD_SETSIZE. */
+			continue;
+		}
 		FD_SET(subrec->nmb_sock,pset);
 		sock_array[num++] = subrec->nmb_sock;
 		*maxfd = MAX( *maxfd, subrec->nmb_sock);
 	}
 
 	/* Add in the broadcast socket on 138. */
+	if (ClientDGRAM < 0 || ClientDGRAM >= FD_SETSIZE) {
+		errno = EBADF;
+		SAFE_FREE(pset);
+		return True;
+	}
+
 	FD_SET(ClientDGRAM,pset);
 	sock_array[num++] = ClientDGRAM;
 	*maxfd = MAX( *maxfd, ClientDGRAM);
 
 	/* Add in the 138 sockets on all the interfaces. */
 	for (subrec = FIRST_SUBNET; subrec; subrec = NEXT_SUBNET_EXCLUDING_UNICAST(subrec)) {
+		if (subrec->dgram_sock < 0 || subrec->dgram_sock >= FD_SETSIZE) {
+			/* We have to ignore sockets outside FD_SETSIZE. */
+			continue;
+		}
 		FD_SET(subrec->dgram_sock,pset);
 		sock_array[num++] = subrec->dgram_sock;
 		*maxfd = MAX( *maxfd, subrec->dgram_sock);
@@ -1767,7 +1787,7 @@ bool listen_for_packets(bool run_election)
 
 #ifndef SYNC_DNS
 	dns_fd = asyncdns_fd();
-	if (dns_fd != -1) {
+	if (dns_fd >= 0 && dns_fd < FD_SETSIZE) {
 		FD_SET(dns_fd, &r_fds);
 		maxfd = MAX( maxfd, dns_fd);
 	}
diff --git a/source3/utils/smbfilter.c b/source3/utils/smbfilter.c
index 39a2640..a3ed657 100644
--- a/source3/utils/smbfilter.c
+++ b/source3/utils/smbfilter.c
@@ -170,8 +170,8 @@ static void filter_child(int c, struct sockaddr_storage *dest_ss)
 		int num;
 		
 		FD_ZERO(&fds);
-		if (s != -1) FD_SET(s, &fds);
-		if (c != -1) FD_SET(c, &fds);
+		if (s >= 0 && s < FD_SETSIZE) FD_SET(s, &fds);
+		if (c >= 0 && c < FD_SETSIZE) FD_SET(c, &fds);
 
 		num = sys_select_intr(MAX(s+1, c+1),&fds,NULL,NULL,NULL);
 		if (num <= 0) continue;
@@ -243,6 +243,10 @@ static void start_filter(char *desthost)
 		struct sockaddr_storage ss;
 		socklen_t in_addrlen = sizeof(ss);
 		
+		if (s < 0 || s >= FD_SETSIZE) {
+			break;
+		}
+
 		FD_ZERO(&fds);
 		FD_SET(s, &fds);
 
diff --git a/source3/winbindd/winbindd.c b/source3/winbindd/winbindd.c
index 0443ebf..b72d42b 100644
--- a/source3/winbindd/winbindd.c
+++ b/source3/winbindd/winbindd.c
@@ -1064,6 +1064,12 @@ static void process_loop(void)
 	}
 
 	for (ev = fd_events; ev; ev = ev->next) {
+		if (ev->fd < 0 || ev->fd >= FD_SETSIZE) {
+			/* Ignore here - event_add_to_select_args
+			   should make this impossible. */
+			continue;
+		}
+
 		if (ev->flags & EVENT_FD_READ) {
 			FD_SET(ev->fd, &r_fds);
 			maxfd = MAX(ev->fd, maxfd);
diff --git a/source3/winbindd/winbindd_dual.c b/source3/winbindd/winbindd_dual.c
index 72e5607..3eef3d3 100644
--- a/source3/winbindd/winbindd_dual.c
+++ b/source3/winbindd/winbindd_dual.c
@@ -1403,6 +1403,13 @@ static bool fork_domain_child(struct winbindd_child *child)
 
 		FD_ZERO(&r_fds);
 		FD_ZERO(&w_fds);
+
+		if (state.sock < 0 || state.sock >= FD_SETSIZE) {
+			TALLOC_FREE(frame);
+			perror("EBADF");
+			_exit(1);
+		}
+
 		FD_SET(state.sock, &r_fds);
 		maxfd = state.sock;
 
