# polyfill.io CDN turned malicious on 100k+ sites · pwn.watch

> The polyfill.io domain and GitHub account were bought in February 2024 by a Chinese company. The CDN, embedded via cdn.polyfill.io on more than 100,000 sites, then served malicious JavaScript that redirected some mobile visitors to a sports betting site. Sansec exposed it on 2024-06-25. Cloudflare began rewriting polyfill.io links to its own mirror, and Namecheap put the domain on hold.

Page: https://pwn.watch/i/polyfill-io-supply-chain-attack · generated 2026-10-23T03:54:42.456Z

## Status
- Severity: high · status: closed · patch: patched
- Product: polyfill.io CDN
- Started: 2024-06-25 · closed: 2024-06-27
- Affected: Any site loading scripts from cdn.polyfill.io
- Fixed in: Remove polyfill.io references or switch to the Cloudflare/Fastly mirrors

## Follow this incident
- RSS (one item per update): https://pwn.watch/i/polyfill-io-supply-chain-attack/feed.xml
- JSON Feed: https://pwn.watch/i/polyfill-io-supply-chain-attack/feed.json
- Full data (timeline, sources, posts, agentBrief): https://pwn.watch/i/polyfill-io-supply-chain-attack.json
- This file: https://pwn.watch/i/polyfill-io-supply-chain-attack/llms.txt

## Updates (newest first)
- 2024-06-27: Cloudflare rewrites cdn.polyfill.io to its mirror; Namecheap puts the domain on hold (https://sansec.io/research/polyfill-supply-chain-attack)
- 2024-06-25: Sansec reports the polyfill.io supply-chain attack hitting 100k+ sites (https://sansec.io/research/polyfill-supply-chain-attack)

## Fix brief
# Security brief: polyfill.io CDN turned malicious on 100k+ sites

Source: https://pwn.watch/i/polyfill-io-supply-chain-attack · status: closed (closed Jun 27, 2024) · severity: high · updated Jun 27, 2024

## Patch status
**PATCHED: fixed releases exist. Upgrade to a fixed version.**

## What it is
The polyfill.io domain and GitHub account were bought in February 2024 by a Chinese company. The CDN, embedded via cdn.polyfill.io on more than 100,000 sites, then served malicious JavaScript that redirected some mobile visitors to a sports betting site. Sansec exposed it on 2024-06-25. Cloudflare began rewriting polyfill.io links to its own mirror, and Namecheap put the domain on hold.

Severity detail: Supply-chain attack via third-party JavaScript CDN

- Product: polyfill.io CDN

## Affected versions
- Any site loading scripts from cdn.polyfill.io

## Check whether you're affected
No generic command applies (the product is a hosted service, appliance or desktop app). Check the installed version in its admin UI / About screen and compare with the versions above.

## Fix
Upgrade to one of these fixed versions (or later on the same branch):
- Remove polyfill.io references or switch to the Cloudflare/Fastly mirrors

After upgrading, re-run the checks above and redeploy/restart the affected services.

## Latest updates
- Jun 27, 2024: Cloudflare rewrites cdn.polyfill.io to its mirror; Namecheap puts the domain on hold
- Jun 25, 2024: Sansec reports the polyfill.io supply-chain attack hitting 100k+ sites

## Write-ups & news
- [Polyfill supply chain attack hits 100K+ sites](https://sansec.io/research/polyfill-supply-chain-attack)
- [Automatically replacing polyfill.io links with Cloudflare's mirror for a safer Internet](https://blog.cloudflare.com/automatically-replacing-polyfill-io-links-with-cloudflares-mirror-for-a-safer-internet)

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Machine-readable: https://pwn.watch/i/polyfill-io-supply-chain-attack.json · all incidents: https://pwn.watch/incidents.json. Verify against the vendor advisory before changing production systems.