claims-format-masked-phone-numbernpm
claims-format-masked-phone-number is a confirmed malicious npm package (MAL-2026-12617) that executes malicious code on install (malicious version 35.9.8). Do not install it — remove it immediately and rotate any exposed credentials.
Malicious code in claims-format-masked-phone-number (npm)
What this malware does
The package advertises phone-number masking but on every require() loads polyfill.js, which selects an OS/arch-specific asset and fetches an opaque native binary from one of four hardcoded Cloudflare Workers subdomains (oob-worker.cf100-416.workers.dev, oob-worker.cf102-baf.workers.dev, oob-worker.cf103-070.workers.dev, oob-worker.cf99-9b3.workers.dev). The hostnames are assembled at runtime via.join() over fragmented string arrays to defeat static scanners. The fetched bytes are written to /var/tmp/.cache<rand> or %TEMP%\dotnet_diag_<rand>.exe, chmod 0755, and detached via /bin/sh -c "<path> &" or cmd /c start with no hash or signature verification. A fallback channel issues DNS TXT queries against sdk.dl.wel1.ru, ext.dl.wel1.ru, pkg.dl.wel1.ru, and net.dl.wel1.ru, concatenates the chunked TXT responses, and base64-decodes them into an executable buffer that is dropped and run the same way. Cover-story elements (file names like.analytics_state and dotnet_diag_*.exe, comments about telemetry opt-out, DISABLE_TELEMETRY/DO_NOT_TRACK env checks) do not match the package's stated purpose and no binary distribution is part of a phone-number masking library.
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-format-masked-phone-number (version 35.9.8). O3 Security's supply-chain scanner checks every dependency against known-malicious package intelligence at install time and in CI, flagging claims-format-masked-phone-number across your stack and pipelines.
If you installed it — respond
Remove claims-format-masked-phone-number 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-format-masked-phone-number 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-format-masked-phone-number 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-format-masked-phone-number-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.