Major websites of banks, airlines, and big retailers went down Thursday in what appeared to be a brief but widespread outage. The websites of Airbnb, AT&T, Costco and Delta showed error messages around midday Eastern time. They seemed to be operating normally, however, by 12:45 p.m. Other websites temporarily down included Expedia, American Express, and Home Depot. Downdetector, which tracks outages by collating status reports from a series of sources, said users reported issues with 48 services, mostly within the United States. Akamai, a major behind-the-scenes internet network company, said the disruption lasted about an hour. The disruption was caused by a bug in the domain name system (DNS) service, which allows web addresses to take users to their destinations, that was triggered during a software update, Akamai said. It said there was no cyberattack. “We have implemented a fix for this issue, and based on current observations, the service is resuming …