class-synthnpm
class-synth is a confirmed malicious npm package (MAL-2026-5730) that opens a backdoor for remote access (malicious versions 1.0.0, 1.0.1, 1.0.2…). Do not install it — remove it immediately and rotate any exposed credentials.
Malicious code in class-synth (npm)
What this malware does
class-synth is advertised as a small class/style/date utility library, but its main entry (dist/index.js) contains a hidden top-level async IIFE (__init) that fires whenever the package is required or imported. The IIFE dynamically imports node:fs, node:path, node:child_process, node:crypto, and node:https using base64-encoded module names joined at runtime to evade string scanners, and acquires process indirectly via new Function('return typeof process!== "undefined"? process: null;'). It then recursively walks process.cwd() looking for any .css file containing an @sri-hash: marker, base64-decodes that marker, and AES-256-CBC-decrypts it with a hardcoded key (split across an array of hex chunks ['a7b80b01','7e76fb52','fa527621','f76027d2','19014dfc','a59b49ae','3db97ff3','ab4a72fa']) to recover an attacker-controlled URL. The decrypted URL is fetched over HTTPS and the response body is piped directly into child_process.spawn('node', ['-'], {windowsHide: true, stdio: ['pipe','ignore','ignore'], detached: true}), so attacker-supplied JavaScript executes in the developer/CI Node process with no on-disk artifact, suppressed stdio, and a detached/unref'd child. The bundle is padded with ~750 decoy near-duplicate exports (isWithinBoundary1..200, applyPreset1..150, createSequenceStep1..250, mapOperation1..250, checkConstraint1..250) to bury the dropper near the end of the file. The C2 URL is delivered out-of-band via a planted.css file, which defeats URL-based scanning of the package itself. The combination of base64-hidden Node built-ins, split/encrypted C2 location, indirect process access, detached stdin-piped code execution, and large-scale decoy padding leaves no plausible benign reading.
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
Backdoor / remote accessFind it
Scan your lockfiles (package-lock.json, pnpm-lock.yaml, yarn.lock, requirements.txt, poetry.lock, etc.) and build artifacts for class-synth (10 malicious versions). O3 Security's supply-chain scanner checks every dependency against known-malicious package intelligence at install time and in CI, flagging class-synth across your stack and pipelines.
If you installed it — respond
class-synth establishes remote access, so treat any host that installed it as fully compromised. Isolate the machine, remove the package, rotate all credentials it could reach, and rebuild from a trusted image rather than cleaning in place — a backdoor may have planted additional persistence.
Did it already run?
If class-synth 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 class-synth 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 class-synth-class packages before install and in CI — and if it already ran, its runtime egress monitoring catches the C2 callback and severs the channel.