(Reuters) - Under Armour Inc <UAA.N> <UA.N> said on Thursday some personal details of about 150 million user accounts of its food and nutrition app and website, MyFitnessPal, were breached last month.
The announcement of the breach, which could be among the largest cyberattacks, sent the company's shares down 2.4 percent in aftermarket trade.
The data affected includes usernames, emails and encrypted passwords, the company said.
The breach did not affect Social Security numbers, driver license numbers and payment card data, the athletic apparel maker said.
RELATED: Notable data breaches in the US
Notable data breaches in the US
Notable data breaches in the US
Credit reporting company Equifax Inc. corporate offices are pictured in Atlanta, Georgia, U.S., September 8, 2017. REUTERS/Tami Chappell
LONDON, ENGLAND - AUGUST 19: A detail of the Ashley Madison website on August 19, 2015 in London, England. Hackers who stole customer information from the cheating site AshleyMadison.com dumped 9.7 gigabytes of data to the dark web on Tuesday fulfilling a threat to release sensitive information including account details, log-ins and credit card details, if Avid Life Media, the owner of the website didn't take Ashley Madison.com offline permanently. (Photo by Carl Court/Getty Images)
Katherine Archuleta, director of the U.S. Office of Personnel Management (OPM), speaks during a House Oversight and Government Reform Committee hearing on the OPM data breach in Washington, D.C., U.S., on Wednesday, June 24, 2015. U.S. senators said yesterday they doubt the government's personnel office understands the breadth of a computer hack that exposed the records of more than 4 million federal workers, or that the agency can stop another breach. Photographer: Andrew Harrer/Bloomberg via Getty Images
WASHINGTON, DC - JUNE 05: The entrance to the Theodore Roosevelt Federal Building that houses the Office of Personnel Management headquarters is shown June 5, 2015 in Washington, DC. U.S. investigators have said that at least four million current and former federal employees might have had their personal information stolen by Chinese hackers. (Photo by Mark Wilson/Getty Images)
SCHAUMBURG, IL - AUGUST 04: A statue of a horse stands at the entrance to a P.F. Chang's restaurant on August 4, 2014 in Schaumburg, Illinois. P.F. Chang's China Bistro Ltd. said today that the company experienced a data breach involving customers' credit and debit card information which affected 33 restaurants in 16 states, including the Schaumburg, Illinois location. (Photo by Scott Olson/Getty Images)
PORTLAND, ME - AUGUST 15: Shaws on Congress Street on Friday, July 15, 2014. Shaws parent company is investigating a possible data breach. (Photo by Logan Werlinger/Portland Press Herald via Getty Images)
COLMA, CA - APRIL 18: Customers enter a Michaels art and crafts store on April 18, 2014 in Colma, California. Michaels, the largest arts and crafts chain in the U.S., announced that an estimated 2.6 million cards used at its stores across the country may have been affected by a security breach. Aaron Brothers, a subsidiary of Michaels, was also affected by the breach. (Photo by Justin Sullivan/Getty Images)
CORAL GABLES, FL - FEBRUARY 28: A checkout keypad is seen at a Sears store on February 28, 2014 in Coral Gables, Florida.
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A couple of shoppers leave a Target store on a rainy afternoon in Alhambra, California on December19, 2013, as the US retail giant said some 40 million customers may have had bank card data compromised by hackers who broke into its database as holiday shopping got underway. Target said there had been 'unauthorized access' to its payment system in US stores affecting credit and debit cards with approximately 40 million credit and debit cards possibly affected by the breach between November 27 and December 15, the company said in a statement. AFP PHOTO / Frederic J. Brown (Photo credit should read FREDERIC J. BROWN/AFP/Getty Images)
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The company said it became aware of the breach on March 25 and have started notifying users.
Under Armour is working with data security firms to assist in its investigation and also coordinating with law enforcement authorities.
MyFitnessPal is one of the most positively reviewed apps in the Apple and Android app stores and allows users to track calories, set exercise goals, integrate data from other devices and share achievements on social media networks such as Facebook.
Under Armour bought MyFitnessPal in 2015 for $475 million.