The polyfill.js is a popular open source library to support older browsers. 100K+ sites embed it using the cdn.polyfill.io domain. Notable users are JSTOR, Intuit and World Economic Forum. However, in February this year, a Chinese company bought the domain and the Github account. Since then, this domain was caught injecting malware on mobile devices via any site that embeds cdn.polyfill.io. Any complaints were quickly removed (archive here) from the Github repository.
That load-bearing “mostly” is doing a lot of work here.
I invite everybody to find out how everything “mostly” works if you disable “most of” javascript – also have fun deciding which parts to enable because you think they’re trustworthy
I actively do this with uMatrix - granted, I only block non-first-party JavaScript. Most sites I visit only require a few domains to be enabled to function. The ones that don’t are mostly ad-riddled news sites.
There are a few exceptions to this - AWS and Atlassian come to mind - but the majority of what I see on the internet does actually work more or less fine when you block non-first-party JavaScript and some even when you do that. uMatrix also has handy bundles built-in for certain things like sites that embed YouTube, for example, that make this much easier.
Blocking non-first-party like I do does actually solve this issue for the most part, since, according to the article, only bundles that come from the cdn.polyfill.io domain itself that were the problem.
You’re still trusting that the 1st party javascript won’t be vulnerable to supply chain attacks, though