diff --git a/src/common/clipboard.c b/src/common/clipboard.c
index 4452bee7..eb2f5480 100644
--- a/src/common/clipboard.c
+++ b/src/common/clipboard.c
@@ -24,6 +24,7 @@
 #include <guacamole/protocol.h>
 #include <guacamole/stream.h>
 #include <guacamole/user.h>
+#include <pthread.h>
 #include <string.h>
 #include <stdlib.h>
 
@@ -37,6 +38,8 @@ guac_common_clipboard* guac_common_clipboard_alloc(int size) {
     clipboard->length = 0;
     clipboard->available = size;
 
+    pthread_mutex_init(&(clipboard->lock), NULL);
+
     return clipboard;
 
 }
@@ -108,18 +111,37 @@ static void* __send_user_clipboard(guac_user* user, void* data) {
 }
 
 void guac_common_clipboard_send(guac_common_clipboard* clipboard, guac_client* client) {
+
+    pthread_mutex_lock(&(clipboard->lock));
+
     guac_client_log(client, GUAC_LOG_DEBUG, "Broadcasting clipboard to all connected users.");
     guac_client_foreach_user(client, __send_user_clipboard, clipboard);
     guac_client_log(client, GUAC_LOG_DEBUG, "Broadcast of clipboard complete.");
+
+    pthread_mutex_unlock(&(clipboard->lock));
+
 }
 
-void guac_common_clipboard_reset(guac_common_clipboard* clipboard, const char* mimetype) {
+void guac_common_clipboard_reset(guac_common_clipboard* clipboard,
+        const char* mimetype) {
+
+    pthread_mutex_lock(&(clipboard->lock));
+
+    /* Clear clipboard contents */
     clipboard->length = 0;
-    strncpy(clipboard->mimetype, mimetype, sizeof(clipboard->mimetype)-1);
+
+    /* Assign given mimetype */
+    strncpy(clipboard->mimetype, mimetype, sizeof(clipboard->mimetype) - 1);
+    clipboard->mimetype[sizeof(clipboard->mimetype) - 1] = '\0';
+
+    pthread_mutex_unlock(&(clipboard->lock));
+
 }
 
 void guac_common_clipboard_append(guac_common_clipboard* clipboard, const char* data, int length) {
 
+    pthread_mutex_lock(&(clipboard->lock));
+
     /* Truncate data to available length */
     int remaining = clipboard->available - clipboard->length;
     if (remaining < length)
@@ -131,5 +153,7 @@ void guac_common_clipboard_append(guac_common_clipboard* clipboard, const char*
     /* Update length */
     clipboard->length += length;
 
+    pthread_mutex_unlock(&(clipboard->lock));
+
 }
 
diff --git a/src/common/common/clipboard.h b/src/common/common/clipboard.h
index 5ebb2617..c129a7c3 100644
--- a/src/common/common/clipboard.h
+++ b/src/common/common/clipboard.h
@@ -23,6 +23,7 @@
 #include "config.h"
 
 #include <guacamole/client.h>
+#include <pthread.h>
 
 /**
  * The maximum number of bytes to send in an individual blob when
@@ -35,6 +36,13 @@
  */
 typedef struct guac_common_clipboard {
 
+    /**
+     * Lock which restricts simultaneous access to the clipboard, guaranteeing
+     * ordered modifications to the clipboard and that changes to the clipboard
+     * are not allowed while the clipboard is being broadcast to all users.
+     */
+    pthread_mutex_t lock;
+
     /**
      * The mimetype of the contained clipboard data.
      */