The U.S. agency responsible for maintaining and designing the nation’s cache of nuclear weapons was among those breached by a hack of Microsoft Corp.’s SharePoint document management software, according to a person with knowledge of the matter.

At least 54 organizations have been breached in the attacks. Some of the attacks that targeted organizations using an exploit in Microsoft’s SharePoint server platform over the last few days have been linked to hacking groups affiliated with the Chinese government, according to a new Microsoft security blog.

Microsoft says Chinese hackers exploited security vulnerabilities in the company’s SharePoint platform, which it first reported over the weekend.