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

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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.

The revelation prompted industry-wide calls to take action. Two days after the Sansec report was published, domain registrar Namecheap suspended the domain, a move that effectively prevented the malicious code from running on visitor devices. Even then, content delivery networks such as Cloudflare began automatically replacing pollyfill links with domains leading to safe mirror sites. Google blocked ads for sites embedding the Polyfill[.]io domain. The website blocker uBlock Origin added the domain to its filter list. And Andrew Betts, the original creator of Polyfill.io, urged website owners to remove links to the library immediately.

As of Tuesday, exactly one week after malicious behavior came to light, 384,773 sites continued to link to the site, according to researchers from security firm Censys. Some of the sites were associated with mainstream companies including Hulu, Mercedes-Benz, and Warner Bros. and the federal government. The findings underscore the power of supply-chain attacks, which can spread malware to thousands or millions of people simply by infecting a common source they all rely on.

“Since the domain was suspended, the supply-chain attack has been halted,” Aidan Holland, a member of the Censys Research Team, wrote in an email. “However, if the domain was to be un-suspended or transferred, it could resume its malicious behavior. My hope is that NameCheap properly locked down the domain and would prevent this from occurring.”

What’s more, the Internet scan performed by Censys found more than 1.6 million sites linking to one or more domains that were registered by the same entity that owns polyfill[.]io. At least one of the sites, bootcss[.]com, was observed in June 2023 performing malicious actions similar to those of polyfill. That domain, and three others—bootcdn[.]net, staticfile[.]net, and staticfile[.]org—were also found to have leaked a user’s authentication key for accessing a programming interface provided by Cloudflare.

Censys researchers wrote:

So far, this domain (bootcss.com) is the only one showing any signs of potential malice. The nature of the other associated endpoints remains unknown, and we avoid speculation. However, it wouldn’t be entirely unreasonable to consider the possibility that the same malicious actor responsible for the polyfill.io attack might exploit these other domains for similar activities in the future.

Of the 384,773 sites still linking to polyfill[.]com, 237,700, or almost 62 percent, were located inside Germany-based web host Hetzner.

Censys found that various mainstream sites—both in the public and private sectors—were among those linking to polyfill. They included:

  • Warner Bros. (www.warnerbros.com)
  • Hulu (www.hulu.com)
  • Mercedes-Benz (shop.mercedes-benz.com)
  • Pearson (digital-library-qa.pearson.com, digital-library-stg.pearson.com)
  • ns-static-assets.s3.amazonaws.com

The amazonaws.com address was the most common domain associated with sites still linking to the polyfill site, an indication of widespread usage among users of Amazon’s S3 static website hosting.

Censys also found 182 domains ending in .gov, meaning they are affiliated with a government entity. One such domain—feedthefuture[.]gov—is affiliated with the US federal government. A breakdown of the top 50 affected sites is here.

Attempts to reach Funnull representatives for comment weren’t successful.

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