@jsimplify/errnonpm
@jsimplify/errno is a confirmed malicious npm package (MAL-2026-12006) that executes malicious code on install (malicious versions 2.0.0, 2.0.1, 2.0.5…). Do not install it — remove it immediately and rotate any exposed credentials.
Malicious code in @jsimplify/errno (npm)
What this malware does
The package declares a scripts.install hook that spawns bun index.js detached and unref'd on npm install, and pulls in bun as a declared dependency to guarantee the alternate runtime is present. index.js decrypts a hardcoded hex ciphertext with AES-256-CBC using an embedded key and IV to reconstruct a remote URL, fetches that URL, and passes the response body directly to eval(). This is a lifecycle-time dropper that executes attacker-controlled code on the installer's machine with the privileges of the user running npm install, with the destination URL obfuscated behind AES so it cannot be read from the source. The package presents itself as an errno-code lookup library, copying the code, README, description, and author string (Rod Vagg @rvagg) of the legitimate errno package while pointing repository.url at a different GitHub account (maxpsmith/node-errno); the errno-lookup functionality is a cover for the install-time execution path and there is no legitimate reason for an errno lookup library to AES-decrypt and eval remote content at install.
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 @jsimplify/errno (6 malicious versions). O3 Security's supply-chain scanner checks every dependency against known-malicious package intelligence at install time and in CI, flagging @jsimplify/errno across your stack and pipelines.
If you installed it — respond
Remove @jsimplify/errno 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 @jsimplify/errno 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 @jsimplify/errno 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 @jsimplify/errno-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.