Unfortunately, that worry came true on June 25, 2024 as the polyfill.io service was being used to inject nefarious code that, under certain circumstances, redirected users to other websites.

We have taken the exceptional step of using our ability to modify HTML on the fly to replace references to the polyfill.io CDN in our customers’ websites with links to our own, safe, mirror created back in February.

Cloudflare proxies millions of websites, and a large portion of these sites are on our free plan. Free plan customers tend to have simpler applications while not having the resources to update and react quickly to security concerns. We therefore decided to turn on the feature by default for sites on our free plan, as the likelihood of causing issues is reduced while also helping keep safe a very large portion of applications using polyfill.io.

Paid plan customers, on the other hand, have more complex applications and react quicker to security notices. We are confident that most paid customers using polyfill.io and Cloudflare will appreciate the ability to virtually patch the issue with a single click, while controlling when to do so.

This is a pretty good response IMO. An acknowledgment that intercepting and changing sites like that on the fly is an exceptional measure and not a desirable path, the recognition that advanced users want the extra control of not having the decision made for them, and the probably correct recognition that less advanced users are going to be a massive liability to the internet at large if they don’t intervene.