Your RSA-2048 keys break in 2030. Find every one of them before attackers do.
Malicious package

css-field-sizing-polyfillnpm

Advisory published Updated

css-field-sizing-polyfill is a confirmed malicious npm package (MAL-2026-17478) that opens a backdoor for remote access (malicious version 1.0.0). Do not install it — remove it immediately and rotate any exposed credentials.

Malicious code in css-field-sizing-polyfill (npm)

MAL-2026-17478
Immediate action
Remove the package, then rotate any secrets the build/runtime could reach.
npm uninstall css-field-sizing-polyfill

What this malware does

The package advertises itself as a CSS field-sizing polyfill but ships thunderboltRegistry.js, which runs an IIFE at module load time. The IIFE shells out via child_process.execSync to run id, whoami, uname -a, ifconfig/ip addr, and cat /etc/hosts, then transmits the collected output together with hostname, Node version, platform, and pid as query-string parameters to the hardcoded plaintext endpoint http://dxpoc.gt.tc/callback.php/bb8968d0f67000433bf7005dd5ad2d1f. The module also exports proxied stubs for Wix-internal names (thunderboltRegistry, siteAssetsRegistry, documentManagementRegistry, editorRegistry, corvidRegistry) and ships a registry-manifest.min.json pointing at static.parastorage.com, indicating a dependency-confusion impersonation of Wix internal packages so that an internal resolver pulling this public name will trigger the beacon. The reconnaissance behavior is unrelated to the declared CSS polyfill purpose and fires on any require() of the package.

Malicious versions

1 flagged
1.0.0

Indicators of compromise (SHA-256)

549bb8820cb6a8a35853826d17b64df14c0561b83790192a33c5dc76dbad53b9

Detection & response playbook

Backdoor / remote access

  1. Find it

    Search your lockfiles and build artifacts for css-field-sizing-polyfill (version 1.0.0).

  2. If you installed it — respond

    css-field-sizing-polyfill establishes remote access, so treat any host that installed it as fully compromised. Isolate the machine, remove the package, rotate all credentials it could reach, and rebuild from a trusted image rather than cleaning in place — a backdoor may have planted additional persistence.

  3. Did it already run?

    If css-field-sizing-polyfill was installed, its post-install payload may already have run. Removing the package does not undo that — check outbound connections and credential use from the install window onward.

Frequently asked questions

No. css-field-sizing-polyfill on npm has been identified as a malicious package (version 1.0.0 flagged). It should be removed immediately — do not install or keep it in your dependency tree.

Campaign

IN-MAL-2026-020885

References

Credits

  • Amazon Inspector · finder

Detect & block this

O3 blocks css-field-sizing-polyfill-class packages before install and in CI — and if it already ran, its runtime egress monitoring catches the C2 callback and severs the channel.

Malware detection Runtime egress monitoring

Explore

All malware advisories How O3 detects malware Vulnerability database
css-field-sizing-polyfill (npm) malicious package — MAL-2026-17478 | O3 Security