Last week, The Wall Street Journal reported that Russian hackers stole National Security Agency (NSA) data after an NSA contractor put highly-classified information on his home computer. According to the WSJ article, “NSA Contractor’s Breach of Cybersecurity Protocol at Heart of Russian Hack,” hackers identified and then targeted NSA files on the contractor’s personal computer by exploiting vulnerabilities in the antivirus software used by the contractor. Notably, given concerns about the particular version of antivirus software used by the contractor, last month the Department of Homeland Security banned all U.S. government departments and agencies from using it.

The stolen data purportedly included details about the NSA’s spying and cyber-defense tactics. Although the NSA has not disclosed or confirmed that the theft occurred, the WSJ reports that the theft “is considered by experts to be one of the most significant security breaches in recent years.” This theft would be the third publicly-known data breach of NSA data stemming from a contractor’s access to classified documents.

TIP: Even companies that have strong data protection measures are vulnerable to—and therefore must carefully protect against—data loss that arises through its contractors, vendors, and other third-parties with access to the company’s networks.