A Russian hacker group known on underground forums under the name Fxmsp started advertising in April that they had persistent remote access to the networks of multiple antivirus companies with offices in the United States. They offered to sell access to source code related to various products from the victims: antivirus software base code, artificial intelligence model, web security model, customer panels, and utilities. One company changed the file extensions in an attempt to make it more difficult to identify the source code files. Another company changed all extensions on purpose, while another removed all extensions in a bid to confuse anyone who wants to look for source code.
Source: https://www.bleepingcomputer.com/news/security/new-details-emerge-of-fxmsps-hacking-of-antivirus-companies/