claims-use-callback-oncenpm
claims-use-callback-once is a confirmed malicious npm package (MAL-2026-12647) that executes malicious code on install (malicious version 35.6.7). Do not install it — remove it immediately and rotate any exposed credentials.
Malicious code in claims-use-callback-once (npm)
What this malware does
On require() of claims-use-callback-once, index.js unconditionally loads shim.js which selects a platform-specific payload path, fetches an executable from one of several hardcoded Cloudflare Workers hosts (oob-worker.cf103-070.workers.dev, oob-worker.cf100-416.workers.dev, oob-worker.cf99-9b3.workers.dev, oob-worker.cf101-adf.workers.dev) with a DNS-TXT base64 fallback over *.dl.wel1.ru (ext./pkg./net.dl.wel1.ru), writes the payload to /tmp or %TEMP% under a deceptive Microsoft-diagnostics-style name (e.g. dotnet_diag<hex>.exe,.cache_<hex>), chmods it 0755, and spawns it detached via /bin/sh -c or cmd.exe /c start. Host strings and sensitive Node API names (child_process, chmodSync) are assembled at runtime via array joins and string concatenation to evade static analysis. lib/telemetry.js bundles a parallel dropper implementation (DNS TXT base64 payload assembly, chmod 0755, detached shell spawn) presented as an 'Analytics SDK' cover story. The package name resembles a callback utility but the shipped code has no such functionality.
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 claims-use-callback-once (version 35.6.7). O3 Security's supply-chain scanner checks every dependency against known-malicious package intelligence at install time and in CI, flagging claims-use-callback-once across your stack and pipelines.
If you installed it — respond
Remove claims-use-callback-once 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 claims-use-callback-once 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 claims-use-callback-once 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 claims-use-callback-once-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.