By Eric Auchard HAMBURG (Reuters) – Major travel booking systems lack a proper way to authenticate air travelers, making it easy to hack the short code used on many boarding passes to alter flight details or steal sensitive personal data, security researchers warned on Tuesday. Passenger Name Records (PNR) are used to store reservations with links to a traveler's name, travel dates, itinerary, ticket details, phone and email contacts, travel agent, credit card numbers, seat number and baggage information. The world's three major global distribution systems (GDS) – Amadeus, Sabre and Travelport – manage a majority of travel reservations but face growing competition from airlines and corporate travel and online booking sites.