diff --git a/guacamole/libguac/configure.in b/guacamole/libguac/configure.in
index 535546f36..e96c739a3 100644
--- a/guacamole/libguac/configure.in
+++ b/guacamole/libguac/configure.in
@@ -13,7 +13,7 @@ AC_CHECK_LIB([png], [png_write_png])
 AC_CHECK_LIB([uuid], [uuid_generate])
 
 # Checks for header files.
-AC_CHECK_HEADERS([fcntl.h stdlib.h string.h sys/socket.h sys/time.h syslog.h unistd.h])
+AC_CHECK_HEADERS([fcntl.h stdlib.h string.h sys/socket.h sys/time.h syslog.h unistd.h semaphore.h])
 
 # Checks for typedefs, structures, and compiler characteristics.
 AC_TYPE_SIZE_T
diff --git a/guacamole/libguac/include/client.h b/guacamole/libguac/include/client.h
index dbd416b37..a997c7ff8 100644
--- a/guacamole/libguac/include/client.h
+++ b/guacamole/libguac/include/client.h
@@ -22,8 +22,9 @@
 
 #include <png.h>
 #include <uuid/uuid.h>
-#include "uuidtree.h"
+#include <semaphore.h>
 
+#include "uuidtree.h"
 #include "guacio.h"
 
 /**
@@ -237,6 +238,11 @@ struct guac_client_registry {
      */
     guac_uuid_tree_node* root;
 
+    /**
+     * Semaphore controlling access to UUID tree.
+     */
+    sem_t tree_lock;
+
 };
 
 guac_client_registry* guac_create_client_registry();
diff --git a/guacamole/libguac/src/clientreg.c b/guacamole/libguac/src/clientreg.c
index fb3e13b20..94766de9b 100644
--- a/guacamole/libguac/src/clientreg.c
+++ b/guacamole/libguac/src/clientreg.c
@@ -22,6 +22,7 @@
 
 #include <stdlib.h>
 #include <uuid/uuid.h>
+#include <semaphore.h>
 
 #include "uuidtree.h"
 #include "client.h"
@@ -31,13 +32,16 @@ guac_client_registry* guac_create_client_registry() {
     guac_client_registry* registry = malloc(sizeof(guac_client_registry));
 
     registry->root = guac_create_uuid_tree();
+    sem_init(&(registry->tree_lock), 0, 1);
 
     return registry;
 
 }
 
 void guac_register_client(guac_client_registry* registry, guac_client* client) {
+    sem_wait(&(registry->tree_lock));
     guac_uuid_tree_put(registry->root, client->uuid, client);
+    sem_post(&(registry->tree_lock));
 }
 
 guac_client* guac_find_client(guac_client_registry* registry, uuid_t uuid) {
@@ -45,11 +49,15 @@ guac_client* guac_find_client(guac_client_registry* registry, uuid_t uuid) {
 }
 
 void guac_remove_client(guac_client_registry* registry, uuid_t uuid) {
+    sem_wait(&(registry->tree_lock));
     guac_uuid_tree_remove(registry->root, uuid);
+    sem_post(&(registry->tree_lock));
 }
 
 void guac_cleanup_registry(guac_client_registry* registry) {
+    sem_wait(&(registry->tree_lock));
     guac_cleanup_uuid_tree(registry->root);
+    sem_destroy(&(registry->tree_lock));
 }
 
 #endif