For more than a decade, serious flaws had gone undetected in technology underpinning much of the internet.
It took artificial intelligence (AI) to bring them to light and caused a global cyber-security alert.
The vulnerabilities lie within a programming language called PHP, which has been used internationally since the 2000s to build websites.
Governments and cyber-security agencies around the world scrambled to contain the fallout of this discovery on Wednesday, and have urged website operators to install a new emergency fix or consider taking their platforms offline.
Cyber-security firm Wiz - one of the companies whose researchers uncovered the vulnerabilities - described the flaws as the "most severe" ever detected by AI.