China-tied hackers that hit U.S. said to breach United Airlines

In May, the OPM investigators began drawing up a list of possible victims in the private sector and provided the companies with digital signatures that would indicate their systems had been breached. United Airlines was on that list.

Safety concerns

In contrast to the theft of health records or financial data, the breach of airlines raises concerns of schedule disruptions or transportation gridlock. Mistakes by hackers or defenders could bring down sensitive systems that control the movement of millions of passengers annually in the U.S. and internationally.

Even if their main goal was data theft, state-sponsored hackers might seek to preserve access to airline computers for later use in more disruptive attacks, according to security experts. One of the chief tasks of the investigators in the United breach is ensuring that the hackers have no hidden backdoors that could be used to re-enter the carrier's computer systems later, one of the people familiar with the probe said.

United spokesman Punzenberger said the company remains "vigilant in protecting against unauthorized access" and is focused on protecting its customers' personal information.

There is evidence the hackers were in the carrier's network for months. One web domain apparently set up for the attack — UNITED-AIRLINES.NET — was established in April 2014. The domain was registered by a James Rhodes, who provided an address in American Samoa.

James Rhodes is also the alias of the character War Machine in Marvel Comics' Iron Man. Security companies tracking the OPM hackers say they often use Marvel comic book references as a way to "sign" their attack.

Targeting Pentagon

This isn't the first time such an attack has been documented. Chinese military hackers have repeatedly targeted the U.S. Transportation Command, the Pentagon agency that coordinates defense logistics and travel.

A report last year from the Senate Armed Services Committee documented at least 50 successful hacks of the command's contractors from June 2012 through May 2013. Hacks against the agency's contractors have led to the theft of flight plans, shipping routes and other data from organizations working with the military, according to the report.

See also: 5 ways cybersecurity could cause cyber hives

"The Chinese have been trying to get flight information from the government; now it looks as if they're trying to do the same in the commercial sector," said Tony Lawrence, a former Army sergeant and founder and chief executive officer of VOR Technology, a Columbia, Md.-based cybersecurity firm.

It's unclear whether United is considering notifying customers that data may have been compromised. Punzenberger said United "would abide by notification requirements if a situation warranted" it.

The airline is still trying to determine exactly which data was removed from the network, said two of the people familiar with the probe. That assessment took months in the OPM case, which was discovered in April and made public in June.

M&A Strategy

Besides passenger lists and other flight-related data, the hackers may also have taken information related to United's mergers and acquisitions strategy, one of the people familiar with the investigation said.

Flight manifests usually contain the names and birthdates of passengers, but even if those files were taken, experts say that would be unlikely to trigger disclosure requirements in any of the 47 states with breach-notification laws.

Those disclosure laws are widely seen as outdated. The theft by hackers of corporate secrets usually goes unreported, while reporting the stealing of customer records such as Social Security numbers and credit cards is required in most states.

"In most states, this is not going to trigger a notification," said Srini Subramanian, state government leader for Deloitte cyber risk services.