Karsten Nohl and Nemanja Nikodijevic's Chaos Communications Congress presentation details their research into becoming a "Secret travel agent": they figured out how to force the various portals to the Global Distribution System to let them know if they've guessed someone's reservation locator code, which they can use to arbitrarily alter your flight plans, sending you to different cities, reseating you, or cancelling your flight.
The GDS has many portals, and many of those are not rate-limited; to make things worse, the space of all possible locator codes is pretty small, since it's non-case-sensitive letters and numbers (excluding 1 and 0, which could be mistaken for the letters I and O). So by sending a lot of guesses to a lot of places very fast, it's not hard to figure out whether any surname has a valid code associated with it.
The codes don't contain ones or zeros to avoid confusion with I or O, Nohl says—they only use upper case letters and no special characters too. On top of that, in two out of the three larger GDSs, the numbers increase sequentially, Nohl explained. This means a hacker can predict when a particular set of numbers are more likely to be used at a certain time of day, or day of the week, in turn making it much more likely that they will successfully match a six digit code with the correct last name, and gain access to flight information.
The codes themselves can also be easily found on people’s' luggage tags or potentially on a boarding pass, as others have previously found.
Armed with these techniques, a hacker might be able to track someone, finding out where they’re flying to and from. Working with the German TV station ARD, the researchers were able to change the flight booking of a reporter, putting him on the same flight as, and in an adjacent seat to, a German politician.
“We were able to try a couple million for a given last name, and that was enough to find this German senator,” Nohl told Motherboard.
It's Incredibly Easy to Tamper with Someone's Flight Plan, Anywhere on the Globe