Apparently a big vendor of proprietary network, system and infrastructure software, SolarWinds, has been broken into, and Microsoft systems were - among others - compromised, giving the attackers access to Microsoft source code.
Not sure why it makes the news, as Microsoft source code seems to have been stolen and made public a number of times previously.
Interesting is how the "Microsoft Security Response Center" explains how this breach was not a problem:
"At Microsoft, we have an inner source approach - the use of open source software development best practices and an open source-like culture - to making source code viewable within Microsoft. This means we do not rely on the secrecy of source code for the security of products, and our threat models assume that attackers have knowledge of source code. So viewing source code isn't tied to elevation of risk," the MSRC said.
From Deciper's article "SolarWinds Attackers Accessed, But Did Not Modify, Microsoft Source Code" by Dennis Fisher.
Microsoft using "open source software development best practices" and "an open source-like culture". Whaat.