384,000 sites pull code from sketchy code library recently bought by Chinese firm

More than 384,000 websites are linking to a site that was caught last week performing a supply-chain attack that redirected visitors to malicious sites, researchers said.

For years, the JavaScript code, hosted at polyfill[.]com, was a legitimate open source project that allowed older browsers to handle advanced functions that weren’t natively supported. By linking to cdn.polyfill[.]io, websites could ensure that devices using legacy browsers could render content in newer formats. The free service was popular among websites because all they had to do was embed the link in their sites. The code hosted on the polyfill site did the rest.

The power of supply-chain attacks

In February, China-based company Funnull acquired the domain and the GitHub account that hosted the JavaScript code. On June 25, researchers from security firm Sansec reported that code hosted on the polyfill domain had been changed to redirect users to adult- and gambling-themed websites. The code was deliberately designed to mask the redirections by performing them only at certain times of the day and only against visitors who met specific criteria.