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.
You’d be surprised how much code people blindly reuse without even looking at it, especially in JavaScript. A bunch of it is from projects owned by random individuals. The JS standard library is ridiculously small, so nearly all JS apps import third-party code of some sort. One JS framework can pull in hundreds of third-party modules.
It’s much less of an issue with languages like C# and even PHP, where the first-party libraries are often sufficient for building a small or mid-sized app.
