The worldwide system used to coordinate travel bookings between airlines, travel agents, and price comparison websites is hopelessly insecure, according to researchers.

The lack of modern security features, both in the design of the system itself and of the many sites and services that control access to it, makes it easy for an attacker to harvest personal information from bookings, steal flights by altering ticketing details, or earn millions of air miles by attaching new frequent-flyer numbers to pre-booked flights, according to German security firm SR Labs.



Known as Global Distribution Systems (GDS), the technology dates back to the 1960s, when one of the first companies in the field, Sabre, was founded. To most travellers, the technology is most obviously associated with the six-character Passenger Name Record (PNR) frequently used to enable online check-in and ticket retrieval.

The PNR system was also the route for many of the weaknesses demonstrated by Karsten Nohl and Nemanja Nikodijevic, the researchers who revealed the flaws at this year’s Chaos Communication Congress hacker convention in Hamburg. While it was presented at a hacker convention, “much less hacking was actually needed to exploit” the booking system, Nohl said.

At the core of many of the weaknesses was the standard use of just two pieces of information to authenticate a booking: the six-character PNR, combined with the user’s last name.



“If the PNR is supposed to be a secure password, then it should be treated like one,” Nohl said. “But they don’t keep it secret: it is printed on every piece of luggage. It used to be printed on boarding passes, until it disappeared and they replaced it with a barcode.”



However, the barcode is also easy to read using a number of apps, meaning many of the 80,000 travellers who have posted pictures on the #boardingpass tag on Instagram are at risk of information theft, as Nikodijevic demonstrated.



“This is supposed to be the only way of authenticating users,” Nohl said, “and it’s printed on pieces of paper you just throw away at the end of the journey.”



A bigger problem for most users, though, is that the six-character code is easy to guess. Each GDS provider (there are several, but the biggest two are Sabre, founded in 1960, and Amadeus, founded in 1987) uses a different system for generating them, but all have multiple problems that make them weaker than a simple six-character password.



For instance, some providers iterate the first two characters sequentially, meaning all the PNRs generated in one day will have the same opening characters. Others reserve some codes for specific airlines, again narrowing the range of guesses an attacker has to make.



Many of the portals into the GDS system also have minimal security features – or at least had minimal security features until Kohl and Nikodijevic notified them.

Some websites that have access to the system and allow you to use your PNR and last name to check the status of your flight offer no defences at all against an attacker guessing thousands of combinations a minute. The researchers were able to access multiple records. Looking for bookings under the name “Smith”, for example, and using a thousand randomly generated booking codes, five came back with active bookings.

Attackers could use that access to cancel a flight in exchange for airline credit and then use that to book new tickets. Or they could add your frequent flyer number to hundreds of flights and chalk up the air miles.

Even more damage could be done with the information contained in the booking. There is enough personal and flight data in them to craft convincing phishing emails purporting to report problems with the flights or bookings.



The PNR weaknesses are just scratching the surface of the problems with the GDS in general, the researchers said: there appears to be no good logging for who has accessed data and why, and access controls in general are almost non-existent, allowing anyone from any company involved in your booking to see the whole thing.



One saving grace, they said, was that the whole system might end up being rewritten anyway. As the “Smith” example shows, the namespace for booking codes is slowly filling up. Simply running out of characters for new bookings could force a rewrite of the system long before security fears do.



If not, Nohl suggested that a rise in cybercrime could do the same job. “Airlines sometimes notice this, but only when it becomes excessive,” he said. “I just hope it becomes so excessive that it can’t be ignored so that it gets fixed, because then the privacy issues get fixed as well. Privacy is never enough on its own.”

