diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index d0ccc909692..9ce86416cef 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1978,6 +1978,13 @@ ProcessStartupPacket(Port *port, bool SSLdone)
 
 	if (proto == CANCEL_REQUEST_CODE)
 	{
+		if (len != sizeof(CancelRequestPacket))
+		{
+			ereport(COMMERROR,
+					(errcode(ERRCODE_PROTOCOL_VIOLATION),
+					 errmsg("invalid length of startup packet")));
+			return STATUS_ERROR;
+		}
 		processCancelRequest(port, buf);
 		/* Not really an error, but we don't want to proceed further */
 		return STATUS_ERROR;