The developers of widely used software are releasing patches for two critical vulnerabilities, as organisations unable to apply the fixes are warned to take affected websites offline.
The PHP programming language is used across large parts of the internet, and speculation has circulated over the past day that websites running the software could be exposed to serious cyber intrusions.
"PHP is everywhere," cybersecurity company Huntress said in a blog post.
"It quietly powers WordPress, Drupal, Joomla, and Magento sites, shared hosting platforms, control panels, and an awful lot of software that doesn't even advertise that it runs PHP under the hood."
In a security update, PHP said both vulnerabilities were rated 9.2 on the CVSS v4.0 severity scale - with 10 being the highest score - and urged users to "upgrade as soon as the release is available".
It said a patch for both vulnerabilities was scheduled for 2pm GMT on Wednesday (1am AEDT on Thursday).
The European Union Computer Emergency Response Team warned the 9.2 rating indicated a risk of unauthenticated remote code execution, a type of flaw that can allow attackers to run malicious code on vulnerable systems without logging in.
It urged organisations to prepare for the immediate patching of affected assets and said those unable to do so should consider taking affected sites offline.
The Australian Cyber Security Centre issued similar guidance, urging organisations to identify PHP systems in their environments, prepare to apply patches and monitor applications for signs of malicious activity.
"ACSC is currently not aware of active exploitation related to these vulnerabilities," the agency said.
Both said all PHP versions from PHP 5 onward could be affected, although PHP has not disclosed which versions are vulnerable.
Web hosting provider GoDaddy said the flaws could allow attackers to execute code on affected servers over the internet, potentially leading to data theft, website defacement or full system compromise.
Dane Knecht, the chief technology officer of web and cloud security firm Cloudflare, said attacks were emerging and Cloudflare was updating its firewall rules as a stopgap measure.
"We are seeing early attacks emerging and are updating mitigations as new vectors show up. This is a STOPGAP only. Patch as soon as PHP releases the update," he said on X.
PHP's security team said details of the vulnerabilities, affected versions and the fixed releases would be published when patches were issued.
With Reuters