The tech giants have evidence that Chinese hackers are exploiting the new bug, but warned "multiple actors" are also hacking into affected SharePoint systems.

A security patch released by Microsoft last month failed to fully fix a critical flaw in U.S. tech giant's SharePoint server software that had been identified in May, opening the door to a sweeping global cyber espionage operation.

Two Chinese nation-state actors and one China-based threat actor have tried to exploit the vulnerability for weeks, Microsoft said.