The breach occurred at GatewayCDI, a distributor of promotional products based in St. Louis, Missouri.
The Mozilla Store sells various items of clothing, software, and other merchandise related to Firefox and other Mozilla projects.
Mozilla declined to provide information about the number of customers whose personal data may have been accessed or to elaborate on the circumstances surrounding the breach.
Conrad Franey, chief marketing officer at GatewayCDI, said that some customer names and passwords were exposed, but that at this point, it appears that no credit card information was accessed.
He said the attack came from a hacker who appears to be based in India. He said he wasn't ready to provide an estimate about the number of people affected. He expects further information to be released shortly.
"We've been in this industry for 25 years," Franey said. "We take this very, very seriously."
In July, Senator Patrick Leahy (D-VT), introduced a data breach bill called the Personal Data Privacy and Security Act. Two previous data breach bills of his have failed. The Act is supposed to make it more difficult for retailers to conceal information about breaches. But critics of the bill argue that the proposed law is full of holes.
In a blog post, Mozilla said that it shut down the Mozilla Store as a preventative step, and that it also closed the International Mozilla Store, run by a different third-party vendor, as a precaution.
Mozilla said that GatewayCDI is in the process of investigating the cause and the extent of the breach. Affected customers are supposed to be contacted by GatewayCDI.
"Mozilla is committed to user privacy and the store will only be reinstated once we have a satisfactory assurance of ongoing login security and data privacy," the company said.
Mozilla has been mulling how to make the Mozilla Store more compelling to members of its community. In May, Mozilla developer David Boswell wrote a blog post that asked whether the social interaction at online T-shirt vendor Threadless offers a model that could be emulated to improve the Mozilla Store experience.
Update: Added GatewayCDI comment.
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