diff --git a/easywsclient.cpp b/easywsclient.cpp
index 8f8f4be..0770870 100644
--- a/easywsclient.cpp
+++ b/easywsclient.cpp
@@ -172,8 +172,13 @@ class _RealWebSocket : public easywsclient::WebSocket
     socket_t sockfd;
     readyStateValues readyState;
     bool useMask;
+    bool isRxBad;
 
-    _RealWebSocket(socket_t sockfd, bool useMask) : sockfd(sockfd), readyState(OPEN), useMask(useMask) {
+    _RealWebSocket(socket_t sockfd, bool useMask)
+            : sockfd(sockfd)
+            , readyState(OPEN)
+            , useMask(useMask)
+            , isRxBad(false) {
     }
 
     readyStateValues getReadyState() const {
@@ -264,6 +269,9 @@ class _RealWebSocket : public easywsclient::WebSocket
 
     virtual void _dispatchBinary(BytesCallback_Imp & callable) {
         // TODO: consider acquiring a lock on rxbuf...
+        if (isRxBad) {
+            return;
+        }
         while (true) {
             wsheader_type ws;
             if (rxbuf.size() < 2) { return; /* Need at least 2 */ }
@@ -296,6 +304,20 @@ class _RealWebSocket : public easywsclient::WebSocket
                 ws.N |= ((uint64_t) data[8]) << 8;
                 ws.N |= ((uint64_t) data[9]) << 0;
                 i = 10;
+                if (ws.N & 0x8000000000000000ull) {
+                    // https://tools.ietf.org/html/rfc6455 writes the "the most
+                    // significant bit MUST be 0."
+                    //
+                    // We can't drop the frame, because (1) we don't we don't
+                    // know how much data to skip over to find the next header,
+                    // and (2) this would be an impractically long length, even
+                    // if it were valid. So just close() and return immediately
+                    // for now.
+                    isRxBad = true;
+                    fprintf(stderr, "ERROR: Frame has invalid frame length. Closing.\n");
+                    close();
+                    return;
+                }
             }
             if (ws.mask) {
                 ws.masking_key[0] = ((uint8_t) data[i+0]) << 0;
@@ -309,6 +331,9 @@ class _RealWebSocket : public easywsclient::WebSocket
                 ws.masking_key[2] = 0;
                 ws.masking_key[3] = 0;
             }
+
+            // Note: The checks above should hopefully ensure this addition
+            //       cannot overflow:
             if (rxbuf.size() < ws.header_size+ws.N) { return; /* Need: ws.header_size+ws.N - rxbuf.size() */ }
 
             // We got a whole message, now do something with it: