Governments and businesses across Europe took websites offline after cybersecurity agencies issued urgent warnings about critical vulnerabilities in PHP, a programming language that underpins much of the internet.
The flaws, identified Wednesday with the help of artificial intelligence, prompted PHP developers to release emergency fixes.
Authorities in Europe, Britain, Australia and Japan urged website operators to install the patches immediately or take vulnerable sites offline until they could.
Technology firms warned that hackers may already be exploiting the flaws, Reuters reported. Security experts cautioned that a wave of intrusion attempts could follow as attackers race to hit systems before they are patched.
"This issue impacts the majority of the Internet," said Tiffany Farriss, interim chief executive of the Drupal Association, which oversees one of the most popular PHP-based website platforms. Mrs. Farriss warned that servers running PHP versions released over the past two decades could be affected.
PHP runs a large share of the world's websites, including popular content management systems such as WordPress and Drupal.
In the Netherlands, the national cybersecurity agency, the Port of Rotterdam, the Royal Dutch Football Association, hospitals, IT service providers and the websites of the country's five largest political parties were among those that took precautionary steps, according to reports. France's Interior Ministry also took protective action.
The White House said it was aware of the situation.
The discovery highlights the growing role of AI in cybersecurity.
AI tools can uncover long-hidden flaws far faster than human researchers, but experts warn that the same capabilities could help attackers find and exploit weaknesses before defenders can respond.