diff --git a/src/java/nginx/unit/websocket/WsFrameBase.java b/src/java/nginx/unit/websocket/WsFrameBase.java
index 06d20bf48..f07a89624 100644
--- a/src/java/nginx/unit/websocket/WsFrameBase.java
+++ b/src/java/nginx/unit/websocket/WsFrameBase.java
@@ -260,6 +260,13 @@ private boolean processRemainingHeader() throws IOException {
         } else if (payloadLength == 127) {
             payloadLength = byteArrayToLong(inputBuffer.array(),
                     inputBuffer.arrayOffset() + inputBuffer.position(), 8);
+            // The most significant bit of those 8 bytes is required to be zero
+            // (see RFC 6455, section 5.2). If the most significant bit is set,
+            // the resulting payload length will be negative so test for that.
+            if (payloadLength < 0) {
+                throw new WsIOException(
+                        new CloseReason(CloseCodes.PROTOCOL_ERROR, sm.getString("wsFrame.payloadMsbInvalid")));
+            }
             inputBuffer.position(inputBuffer.position() + 8);
         }
         if (Util.isControl(opCode)) {
@@ -670,7 +677,7 @@ protected static long byteArrayToLong(byte[] b, int start, int len) throws IOExc
         int shift = 0;
         long result = 0;
         for (int i = start + len - 1; i >= start; i--) {
-            result = result + ((b[i] & 0xFF) << shift);
+            result = result + ((b[i] & 0xFFL) << shift);
             shift += 8;
         }
         return result;