The UK government says it is working to actively identify and patch any potential exposure in its online services, after vulnerabilities were identified in the PHP programming language that underpins a significant number of the world's websites.
Governments and some of the world's biggest companies, including Amazon, were scrambling to protect their systems on Thursday before hackers could exploit them.
The flaws were first discovered by AI, after which governments and agencies urged websites to address them.
Amazon was targeted in multiple attempts, which it said occurred before an embargo on the warning was lifted. It added that it was unclear how actors knew about the vulnerabilities.
A UK government spokesperson urged "all organisations, private and public, large and small" to follow guidance to protect their systems.
The UK's National Cyber Security Centre (NCSC), as well as its counterparts in the EU and Australia, earlier urged those who use PHP to apply the fixes.
"Where immediate remediation is not feasible, organisations should consider taking the system offline until it can be fully mitigated," the NCSC said in a note, external.
Amazon announced on Wednesday that it had identified a "PHP2Shell exploitation attempt" that occurred on 17 October.
In the attack, an anonymous internet address attempted to target weaknesses in one of Amazon's decoy or "honeypot" systems, the company's security chief CJ Moses said.
The use of an anonymous IP address suggests that the activity was conducted by a "threat actor", he said.
While the PHP Foundation intended to release its discovery later this month along with a complete fix for the problem, usually referred to by software developers as a "patch", details of the flaw were leaked on social media on Tuesday.
In subsequent attacks on Amazon that occurred on 21 October, "the actual payloads and choices in anonymisation infrastructure varied, suggesting these were multiple actors," Moses said.
The attacks occurred after the leak, but before the embargo was lifted on the PHP warning.
The use of AI means that "more actors can act within hours or even minutes once technical details of a vulnerability are disclosed", Moses added.
The PHP incident has "demonstrated how quickly AI is changing our cybersecurity landscape", Australian Council on AI Strategy CEO Anna-Maria Arabia told the BBC, as "what used to take months, to expose a vulnerability, is now taking days".
She warned that threats would only become more commonplace as AI technology rapidly improves. It is "not going backwards, in other words", she said.
The PHP flaws went undetected for at least a decade.
Cyber-security firm Wiz has estimated that "roughly 70%" of the internet is affected, though the BBC is unable to independently verify that figure.
The Dutch cybersecurity agency went offline as a precaution, and Finland's government shut down its public web services. Wikipedia also took down parts of its website.
This was in line with advice issued by governments, including the US, to take websites down if the flaws could not immediately be fixed.
The US National Security Agency said that the vulnerabilities identified "often provide a direct path to the server's file system or database".
It could allow attackers to "steal sensitive data, disrupt services, or move deeper into a network. These vulnerabilities are flaws in PHP itself, not in the hosted application or its configuration," the NSA said in a later update.
OpenAI called the discovery, "one of the most impactful security issues in recent memory".