A security flaw in the WordPress blogging software has let hackers attack and deface tens of thousands of sites.
One estimate suggests more than 1.5 million pages on blogs have been defaced.
The security firm that found the vulnerability said some hackers were now trying to use it to take over sites rather than just spoil pages.
WordPress urged site owners to update software to avoid falling victim.
The vulnerability is found in an add-on for the WordPress blogging software that was introduced in versions released at the end of 2016.
Security firm Sucuri found the "severe" bug and informed WordPress about it on 20 January.
In a blogpost, WordPress said it delayed going public about the flaw so it could prompt hosting firms to update their software to a fixed version.
The patched version of WordPress was formally released on 26 January and led to many sites and blogs automatically applying the update.
However, many blogs have not followed suit leaving them open to defacement attacks.
Security firm WordFence said it had seen evidence that 20 hacker groups were trying to meddle with vulnerable sites. About 40,000 blogs are believed to have been hit.
The vulnerability had set off a "feeding frenzy" among hacker groups, WordFence founder Mark Maunder told the Bleeping Computer tech news site.
"During the past 48 hours we have seen over 800,000 attacks exploiting this specific vulnerability across the WordPress sites we monitor," he added.
Sucuri said some hacker groups had moved on from defacement to attempts to use the bug to hijack sites for their own ends.
"Attackers are starting to think of ways to monetise this vulnerability," wrote Sucuri founder Daniel Cid. "Defacements don't offer economic returns, so that will likely die soon."
Hackers were keen to use the vulnerable sites as proxies for spam or malware campaigns, he said.