The attackers who stole $81 million from the Bangladesh central bank probably hacked into software from the SWIFT financial platform that is at the heart of the global financial system, said security researchers at British defense contractor BAE Systems.

SWIFT, a cooperative owned by 3,000 financial institutions, confirmed that it was aware of malware targeting its client software. Its spokeswoman Natasha Deteran said SWIFT would release on Monday a software update to thwart the malware, along with a special warning for financial institutions to scrutinize their security procedures.

The new developments now coming to light in the unprecedented heist suggest that an essential lynchpin of the global financial system could be more vulnerable than previously understood to hacking attacks, due to the vulnerabilities that enabled attackers to modify SWIFT's client software.