Airports around the world stepped up security checks after the September 11, 2001 terrorist attacks in the United States, but security lapses still happen.
Beijing Capital International Airport, for example, which has some of the toughest security checks in the world, has specific lanes for flights to Tibet and Xinjiang . There, luggage undergoes closer scrutiny, including the swiping of each bag for traces of explosives. But passengers then wait in a common departure lounge and face no further checks to verify their travel documents. Passengers could, in theory, swap boarding passes and board different planes.
The aviation security loophole received wider attention on Sunday when Interpol, the international police agency, said its computer systems detected that two passengers had used stolen passports to board the missing Malaysia Airlines flight. Few national authorities routinely checked the database, Interpol said.
And that's a worrisome trend, experts say.
"Using a stolen passport is a common tactic by organised crime for human and drug trafficking and a big problem in Western countries," says anti-terrorism expert Li Wei , from the China Institutes of Contemporary International Relations.
"The passport is modified with the user's picture and for customs it is still a valid passport. The tactic is used by smugglers or drug traffickers to cover their identity or traces as an former convict."
A 49-year-old former convict in Guiyang used another woman's identity to apply for a passport and an exit-entry permit for Hong Kong and Macau. She travelled to Macau via Zhuhai eight times last year before being caught, according to state media.
Mainland media also reported the case of a woman with a British fiance who travelled widely. To meet him overseas, she paid a forger to change her sister-in-law's passport to include her own photo. She passed through Chinese customs 34 times in six years until police discovered the scam in 2010.
A woman who travelled from Kuala Lumpur to Guangxi last August told the South China Morning Post that she took a plastic bag with a bottle of water inside but was not asked to put it on a scanner for security and boarded the plane with it.
A Chinese travel agent who specialises in booking international flights told the Post that "in some cases we don't have the passport number of the passenger and we just make one up. [Some] airports usually verify the name only and not the passport number. Airports in Southeast Asia are not very strict with passport checks."
Even airports that have reputations as being thorough and secure, verifying passengers' identities before allowing them to board, such as Hong Kong, can get tripped up.
In 2010, airport ground staff allowed a Fujian man in his 20s who wore a silicone mask to disguise himself as an elderly white US national, to board a Canada-bound plane.
An investigation found the man arrived at Hong Kong airport using genuine travel documents and later met members of a gang that included airport staff in the restricted zone to get fake documents and a boarding pass.