Not as hard as you might think.
In any major international airport, it's not uncommon to have your passport checked four times or more between check-in and boarding the aircraft. But if passenger documents aren't checked against Interpol's database of Stolen and Lost Travel Documents, travelers using those documents can slip through layers of security.
Reports that two passengers on missing Malaysia Airlines Flight 370 were traveling on stolen Austrian and Italian passports have highlighted security concerns that have troubled Interpol for years, the international law enforcement agency said Sunday. The flight, carrying over 200 passengers, disappeared from radar on Saturday and hasn't been seen or heard from since.
"Interpol is asking why only a handful of countries worldwide are taking care to make sure that persons possessing stolen passports are not boarding international flights," said Interpol Secretary General Ronald K. Noble, in a statement.
What happened to Flight 370?
Before the departure of Flight 370, no country had checked the stolen passports against Interpol's list since they were added to the lost-documents database in 2012 and 2013, Interpol said.
It is countries, not airlines, that have access to Interpol's data, and many governments don't routinely check passports against the database.
In 2013, passengers were able to board planes more than a billion times without having their travel documents checked against Interpol's data, the agency said. Airlines carried more than 3.1 billion passengers globally in 2013, according to estimates from the International Air Transport Association.
Are stolen passports related to plane's disappearance?
Investigators don't yet know if the travelers with stolen passports had anything to do with the plane's disappearance. On any given day, many people travel using stolen or fake passports for reasons that have nothing to with terrorism, aviation security expert Richard Bloom told CNN.
They might be trying to immigrate illegally to another country, or they might be smuggling stolen goods, people, drugs or weapons or trying to import otherwise legal goods without paying taxes, said Bloom, director of terrorism, intelligence and security studies at Embry-Riddle Aeronautical University.
"For all of those reasons, the very notion that passports might be important in this particular situation may be a red herring," Bloom said.