The National Cyber Security Centre said it had not identified active exploitation of two serious vulnerabilities in PHP, but warned that systems using the programming language were likely to be vulnerable. It urged organisations to prepare to install updates released on Wednesday as soon as possible; the UK cybersecurity agency similarly called on British organisations to update affected systems quickly.

Many websites were taken offline as a precaution. An NCSC deputy director said the problem was a programming error rather than an attack, and that some websites may not return online immediately after updating because the time required depends on the PHP version in use.