CNN
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Hundreds of U.S. flights were canceled early Monday as airlines struggle to recover four days after a global technology outage caused massive delays and left travelers stranded at airports across the country.
More than 700 flights into, within and out of the United States had been canceled as of 7:30 a.m. ET Monday, and more than 800 flights were delayed, according to flight tracking site FlightAware.
While most airlines have reduced their number of cancellations, more than 600 of the flights canceled Monday were from Delta Air Lines, according to FlightAware.
Delta declined to comment on the numbers, but in an update to customers Sunday afternoon, Delta Air Lines CEO Ed Bastian said the company was working to restore operations after the outage.
The “largest computer outage in history” – caused by a software update for Microsoft devices on Thursday night and into the early hours of Friday – has caused computers and technical systems to crash around the world, causing airport disruptions, 911 service failures and challenges in health care facilities.
“In particular, one of our crew tracking tools was impacted and unable to effectively process the unprecedented number of changes triggered by the system outage,” Bastian said in the statement.
According to a post on Microsoft’s blog Saturday, the outage affected about 8.5 million Windows devices, or less than 1 percent of all Windows machines. But the effects were pervasive: Airlines, banks, businesses, schools and governments, as well as some health and emergency services, were affected around the world.
The costs of the outage could exceed $1 billion, Patrick Anderson, CEO of Anderson Economic Group, told CNN. But it’s not yet clear if — and how — affected customers will be compensated.
Here’s what we know about the outage and its lingering effects.
The outage is linked to a software update deployed by CrowdStrike, a major American cybersecurity company. The software, used by many Fortune 500 companies, including major global banks, healthcare and energy companies, as well as the US government, detects and blocks hacking threats.
CrowdStrike told customers Friday morning that the outages were caused by “a defect detected in a single content update of its software on Microsoft Windows operating systems,” according to a message posted on X by CEO George Kurtz. The problem has been identified and isolated, and engineers have deployed an update to fix the issue, Kurtz said.
The company’s engineers have taken steps to resolve the issue, according to an advisory seen by CNN, which instructed customers to restart their computers and take other steps if they were still experiencing technical issues. The issue is specific to Falcon, one of CrowdStrike’s core software products, and does not affect Mac or Linux operating systems, according to the advisory. Crowdstrike says Falcon is designed to protect files stored in the cloud.
The company said the outage was not caused by a security incident or cyberattack.
Kurtz later apologized to customers and said the company was “deeply sorry for the inconvenience and disruption.”
In an update posted to LinkedIn Sunday evening, CrowdStrike said that “a significant number” of the 8.5 million devices were back online and operational.
Chaos and confusion for travelers
As of Sunday, more than 1,800 U.S. flights were canceled and more than 9,900 delayed, according to FlightAware. Major hotels, including Marriott International and some Hiltons, have been impacted both in terms of payment processing and delays in check-in procedures.
U.S.-based carriers American Airlines, United Airlines, Delta Air Lines, Allegiant Airlines, SunCountry and Frontier Airlines all reported problems Friday.
American Airlines said Friday morning that it was able to “safely restore operations.” By Friday evening, United said “most of our systems have been restored,” but warned that there could still be flight delays or cancellations due to the issues.
Delta Airlines suspended all flights Friday morning while its systems were offline, and more than 3,500 Delta and Delta Connection flights were canceled through Saturday, according to the airline. More than 1,200 Delta flights were canceled Sunday, after the carrier canceled 1,208 flights Saturday, according to FlightAware.
Delta also announced it would not allow unaccompanied minors to travel until July 24 due to the outage.
Transportation Secretary Pete Buttigieg spoke with Delta Air Lines CEO Ed Bastian on Sunday afternoon as the airline continues to struggle with a high number of flight cancellations, according to the department.
“We continue to receive reports of disruptions and unacceptable customer service conditions at Delta Air Lines, including hundreds of complaints filed with our department. I have made clear to Delta that we expect the airline to promptly refund consumers who choose not to rebook, promptly rebook free of charge and refund meals and hotel accommodations to consumers affected by delays and cancellations, and provide adequate customer service support to all of its passengers,” Buttigieg said in a statement.
Other airlines resumed operations over the weekend, including one of the world’s largest operators, Turkish Airlines, which said on Saturday: “The global technical issue has been resolved. All our operations are back to normal.”
On the same day, airlines Jetstar Japan, Hong Kong Express and Cebu Pacific said their operations were gradually being restored after disruptions caused by the global technology outage.
Even if the faulty computer update has been rolled back, it’s not a quick fix for airlines, which have computers at thousands of gates that will need to be rebooted individually, David Kennedy, co-founder of cybersecurity firm Binary Defense, told CNN on Saturday.
“It’s not as simple as restarting. There are a lot more steps and complexities to consider,” Kennedy said. “There’s just not enough people in these airports, in these locations, to go and do it.”
Amid the travel chaos, Buttigieg said earlier in a social media post Saturday that he had received reports that some airlines were only offering flight credits to passengers for canceled flights.
“Let’s be clear: You have the right to get your money back quickly if your flight is canceled and you don’t rebook,” Buttigieg said.
Hospitals, 911 systems and government agencies affected
The widespread use of CrowdStrike software has led to outages across many industries, including emergency communications, government services and healthcare.
Major hospital systems, including Mass General Brigham in Massachusetts, Penn Medicine in Pennsylvania and Mount Sinai Health System in New York, all reported Friday that they were affected by the outage.
Emory Healthcare in Atlanta said it has delayed “procedures at ambulatory surgery centers and hospitals until our systems are stabilized.”
Some cancer centers, including Dana-Farber Cancer Institute in Boston and Memorial Sloan Kettering Cancer Center in New York, have also suspended some scheduled procedures and appointments.
Several blood banks have also experienced disruptions.
And in some areas, including Arizona and Alaska, 911 services were briefly interrupted before being restored.
Government agencies, including Social Security offices and local Department of Motor Vehicles offices, have also temporarily suspended operations due to the outage.
Public transportation in Washington, D.C., and Pennsylvania was also affected by the outage, but resumed operations Friday.
Postal carriers UPS and FedEx said that while they remained operational, they expected service delays Friday.