The UK's National Cyber Security Centre, a part of GCHQ, is warning businesses to urgently update their Microsoft email servers following a state-sponsored espionage campaign.
Microsoft has warned that multiple groups are taking advantage of a global and indiscriminate hack of its clients' on-premise email servers, attributing the attack to state-sponsored group based in China, with tens of thousands of potential victims worldwide.
The NCSC has stressed the immediate need for organisations to patch their vulnerable Microsoft Exchange servers, amid warnings that the careless techniques used by the attackers could also enable criminals to piggyback into victims' networks.
Sky News understands there were no compromises of public sector organisations in the UK as a result of the state-sponsored attack using vulnerabilities in Microsoft Exchange.
Security officials believe there could be up to 8,000 vulnerable Microsoft servers in the country's private sector, although they estimate roughly half of these may have been patched.
Last week, government security authorities amplified Microsoft's urgent call for customers running on-premise Exchange servers to apply the patch, and the company is now warning that there are multiple groups taking advantage of unpatched systems.
Microsoft initially warned that the state-sponsored group "primarily targets entities in the United States across a number of industry sectors, including infectious disease researchers, law firms, higher education institutions, defence contractors, policy think tanks, and NGOs".
After compromising email servers belonging to these organisations, Microsoft said the attackers created web shells - interfaces which allow them to remotely access the compromised network even after the original vulnerabilities were patched - which is provoking additional concern.
Security officials have addressed 2,300 webshells across businesses in the UK, but more could remain undetected.
The NCSC's director for operations, Paul Chichester, said: "We are working closely with industry and international partners to understand the scale and impact of UK exposure, but it is vital that all organisations take immediate steps to protect their networks.
"Whilst this work is ongoing, the most important action is to install the latest Microsoft updates.
"Organisations should also be alive to the threat of ransomware and familiarise themselves with our guidance. Any incidents affecting UK organisations should be reported to the NCSC," he added.