Swift, the interbank messaging system embroiled in one of the largest cyber heists in history, warned customers that hackers have struck again, attacking a commercial bank client that it didn't name.

The details of a second hack follow a cyber theft in February, when more than $80 million was stolen from Bangladesh's account at the Federal Reserve Bank of New York. Swift last month warned users last month that it was aware of several similar attacks.

This time, the hackers used malware to target a PDF reader used by the customer to check its statement messages, Swift said on Friday. A Swift spokesman declined to reveal the name of the bank, but a U.K.-based security firm, BAE Systems Plc, said in a blog post that it believes the second victim is a commercial bank in Vietnam. BAE isn't directly involved in the investigation, but analyzed malware samples uploaded to public repositories from locations in both Bangladesh and Vietnam and found a match.