diff --git a/common/rfb/VNCServer.h b/common/rfb/VNCServer.h
index 4e55479..280b68b 100644
--- a/common/rfb/VNCServer.h
+++ b/common/rfb/VNCServer.h
@@ -30,6 +30,13 @@
 
   class VNCServer : public UpdateTracker {
   public:
+    // blockUpdates()/unblockUpdates() tells the server that the pixel buffer
+    // is currently in flux and may not be accessed. The attributes of the
+    // pixel buffer may still be accessed, but not the frame buffer itself.
+    // Note that access must be unblocked the exact same number of times it
+    // was blocked.
+    virtual void blockUpdates() = 0;
+    virtual void unblockUpdates() = 0;
 
     // setPixelBuffer() tells the server to use the given pixel buffer (and
     // optionally a modified screen layout).  If this differs in size from
diff --git a/common/rfb/VNCServerST.cxx b/common/rfb/VNCServerST.cxx
index 697695d..b07f5c3 100644
--- a/common/rfb/VNCServerST.cxx
+++ b/common/rfb/VNCServerST.cxx
@@ -47,6 +47,7 @@
 // otherwise blacklisted connections might be "forgotten".
 
 
+#include <assert.h>
 #include <stdlib.h>
 
 #include <rfb/ServerCore.h>
@@ -77,7 +78,8 @@
 // -=- Constructors/Destructor
 
 VNCServerST::VNCServerST(const char* name_, SDesktop* desktop_)
-  : blHosts(&blacklist), desktop(desktop_), desktopStarted(false), pb(0),
+  : blHosts(&blacklist), desktop(desktop_), desktopStarted(false),
+    blockCounter(0), pb(0),
     name(strDup(name_)), pointerClient(0), comparer(0),
     renderedCursorInvalid(false),
     queryConnectionHandler(0), keyRemapper(&KeyRemapper::defInstance),
@@ -259,6 +261,22 @@
 
 // VNCServer methods
 
+void VNCServerST::blockUpdates()
+{
+  blockCounter++;
+}
+
+void VNCServerST::unblockUpdates()
+{
+  assert(blockCounter > 0);
+
+  blockCounter--;
+
+  // Flush out any updates we might have blocked
+  if (blockCounter == 0)
+    tryUpdate();
+}
+
 void VNCServerST::setPixelBuffer(PixelBuffer* pb_, const ScreenSet& layout)
 {
   pb = pb_;
@@ -545,6 +563,9 @@
 {
   std::list<VNCSConnectionST*>::iterator ci, ci_next;
 
+  if (blockCounter > 0)
+    return;
+
   if (!checkDefer())
     return;
 
@@ -571,6 +592,10 @@
   if (ui.is_empty() && !(renderCursor && renderedCursorInvalid))
     return true;
 
+  // Block clients as the frame buffer cannot be safely accessed
+  if (blockCounter > 0)
+    return false;
+
   // Block client from updating if we are currently deferring updates
   if (!checkDefer())
     return false;
diff --git a/common/rfb/VNCServerST.h b/common/rfb/VNCServerST.h
index 2fed0a8..e75954f 100644
--- a/common/rfb/VNCServerST.h
+++ b/common/rfb/VNCServerST.h
@@ -82,6 +82,8 @@
 
     // Methods overridden from VNCServer
 
+    virtual void blockUpdates();
+    virtual void unblockUpdates();
     virtual void setPixelBuffer(PixelBuffer* pb, const ScreenSet& layout);
     virtual void setPixelBuffer(PixelBuffer* pb);
     virtual void setScreenLayout(const ScreenSet& layout);
@@ -209,6 +211,7 @@
 
     SDesktop* desktop;
     bool desktopStarted;
+    int blockCounter;
     PixelBuffer* pb;
     ScreenSet screenLayout;