SolarWinds hackers accessed Microsoft source code, the company says

FILE PHOTO: A Microsoft logo is seen on an office building in New York City on July 28, 2015. REUTERS/Mike Segar

WASHINGTON (Reuters) - The hacking group behind the SolarWinds compromise was able to break into Microsoft Corp and access some of its source code, Microsoft said on Thursday, something experts said sent a worrying signal about the spies' ambition.

Source code - the underlying set of instructions that run a piece of software or operating system - is typically among a technology company's most closely guarded secrets and Microsoft has historically been particularly careful about protecting it.