GHSA-fh55-fq5h-h352: Malicious code in postcss-semantic-themes (npm)

Published Oct 26, 2026
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Updated

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Source: amazon-inspector (21c91b39929f03c30e501726fc2d1c1fa43f17bf2ed77bda52380e563c1171e0) The package advertises itself as a zero-runtime CSS theming kit but on require/import executes a chain of obfuscated JavaScript modules. lib.mjs imports tokens.mjs, which top-level imports cache-warm.mjs — a ~279KB single-line file consisting of a hex-named string array of XOR/base64-encoded blobs decoded at runtime. Three additional sibling modules of the same shape (build-cache.mjs ~203KB, cache-sweep.mjs ~295KB, workspaces.js ~290KB) are shipped and listed in the published files manifest. Alongside the JavaScript loader, the tarball bundles prebuilt N-API native binaries at native/{linux,darwin,win32}/{arm64,x64}/cache-probe.node whose exported symbols implement extensive hypervisor, container, and sandbox detection (VMDetect/VMDetector classes, hypervisorbit, qemufwcfg, systemdvirt, dockerenv, bluestacks, vboxmodule, podmanfile, wslprocsubdir, amdsevmsr, azure, chassisvendor, macaddresscheck, isx86processonarm). None of this has any role in CSS custom-property emission. The README instructs installation as npm install -D tailwindcss@npm:postcss-semantic-themes, aliasing the legitimate tailwindcss name onto this package so existing import 'tailwindcss' call sites resolve to the obfuscated loader. The combination of an import-time obfuscated loader, multiple additional staged obfuscated payloads, environment-fingerprinting native code to gate execution to non-analyst hosts, and a brandjacking install instruction is the structural shape of a staged supply-chain payload rather than a theming plugin.

Source: ghsa-malware (d188bb5d3c330ed5c438865b3787cca124861eefe2c8e094e58511bbca437e09) Any computer that has this package installed or running should be considered fully compromised. All secrets and keys stored on that computer should be rotated immediately from a different computer. The package should be removed, but as full control of the computer may have been given to an outside entity, there is no guarantee that removing the package will remove all malicious software resulting from installing it.

Affected Software

2 affected components
npm/postcss-semantic-themes*
npm/postcss-semantic-themes>=0

Remediation

Recommended actions to resolve this vulnerability, in priority order.

  1. Remove

    Remove npm/postcss-semantic-themes from your environment.

    Remove or uninstall the postcss-semantic-themes package; removing it may not remove all malicious software resulting from its installation.

  2. Operational

    Rotate all secrets and keys stored on any computer that has this package installed or running, immediately and from a different computer.

Event History

Oct 26, 2026
Advisory Published
via GitHub·04:17 PM
Data Sourced
via GitHub·04:17 PM
DescriptionWeaknessAffected Software
Data Sourced
via OSV·04:17 PM
DescriptionAffected Software

Frequently Asked Questions

1

When does the suspicious code run?

The package executes its obfuscated module chain when it is required or imported. Its lib.mjs file imports tokens.mjs, which top-level imports cache-warm.mjs.

2

Which environments may be exposed to the bundled native code?

The published tarball includes N-API binaries for Linux, macOS, and Windows on both arm64 and x64. These binaries contain hypervisor, container, and sandbox detection functionality.

3

How can I check whether the installed package contains the suspicious components?

Inspect the package contents for large single-line modules named cache-warm.mjs, build-cache.mjs, cache-sweep.mjs, and workspaces.js, along with native/*/*/cache-probe.node binaries. The modules are listed in the published files manifest.

4

Is the behavior limited to CSS processing functionality?

No. The package includes obfuscated runtime-decoded blobs and native environment-detection code, neither of which has a stated role in emitting CSS custom properties.

Contact

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