diff --git a/src/lib/gen/io.c b/src/lib/gen/io.c
index 389cfc88..bf77d0f2 100644
--- a/src/lib/gen/io.c
+++ b/src/lib/gen/io.c
@@ -62,6 +62,7 @@
 #include "ioqueue.h"
 #include "empio.h"
 #include "gen.h"		/* getfdtablesize */
+#include "server.h"
 
 #include "empthread.h"
 
@@ -337,7 +338,11 @@ io_output_all(struct iop *iop)
 {
     int n;
 
-    while ((n = io_output(iop, IO_NOWAIT)) > 0) {
+    /*
+     * Mustn't block a player thread while update is pending, or else
+     * a malicous player could delay the update indefinitely
+     */
+    while (((n = io_output(iop, IO_NOWAIT)) > 0) && !update_pending) {
 	empth_select(iop->fd, EMPTH_FD_WRITE);
     }
     return n;
diff --git a/src/lib/update/main.c b/src/lib/update/main.c
index 0c265e2d..c1ebf886 100644
--- a/src/lib/update/main.c
+++ b/src/lib/update/main.c
@@ -74,6 +74,9 @@ update_main(void *unused)
     struct natstr *cnp;
     struct natstr *np;
 
+    if (CANT_HAPPEN(!update_pending))
+        update_pending = 1;
+
     /* First, make sure all mobility is updated correctly. */
     if (opt_MOB_ACCESS) {
 	mob_ship(etu);
@@ -81,7 +84,6 @@ update_main(void *unused)
 	mob_plane(etu);
 	mob_land(etu);
     }
-    update_pending = 1;
     player->proc = empth_self();
     player->cnum = 0;
     player->god = 1;
diff --git a/src/server/update.c b/src/server/update.c
index 015747f6..f4026bd4 100644
--- a/src/server/update.c
+++ b/src/server/update.c
@@ -120,6 +120,7 @@ update_wait(void *unused)
 
     while (1) {
 	empth_sem_wait(update_sem);
+	update_pending = 1;
 	running = 0;
 	for (p = player_next(0); p != 0; p = player_next(p)) {
 	    if (p->state != PS_PLAYING)
@@ -137,8 +138,10 @@ update_wait(void *unused)
 	    empth_sleep(now + 2);
 	}
 	if (*pre_update_hook) {
-	    if (run_hook(pre_update_hook, "pre-update"))
+	    if (run_hook(pre_update_hook, "pre-update")) {
+		update_pending = 0;
 		continue;
+	    }
 	}
 	/* 
 	 * we rely on the fact that update's priority is the highest
@@ -147,6 +150,7 @@ update_wait(void *unused)
 	dp = player_new(0, 0);
 	if (!dp) {
 	    logerror("can't create dummy player for update");
+	    update_pending = 0;
 	    continue;
 	}
 	stacksize = 100000 +