bigops-cobrowsing-clientnpm
bigops-cobrowsing-client is a confirmed malicious npm package (MAL-2026-12160) that opens a backdoor for remote access (malicious version 35.1.9). Do not install it — remove it immediately and rotate any exposed credentials.
Malicious code in bigops-cobrowsing-client (npm)
What this malware does
On require() of this package, index.js loads shim.js which triggers an async payload that fetches a platform-specific binary from obfuscated Cloudflare Workers hostnames (oob-worker.cf103-070.workers.dev, oob-worker.cf99-9b3.workers.dev, oob-worker.cf101-adf.workers.dev), writes it to /var/tmp or %TEMP% under a masquerading name (.cache<rand> on Unix, dotnet_diag_<rand>.exe on Windows), chmods it 0o755, and spawns it detached via /bin/sh or cmd.exe. Both the C2 hostnames and a DNS-TXT fallback channel (chunked base64 payload retrieved from TXT records under sdk.dl.wel1.ru, ext.dl.wel1.ru, pkg.dl.wel1.ru, net.dl.wel1.ru) are reconstructed at runtime via.join('') of split string fragments to evade static string detection. The DNS-TXT fallback reads a chunk count from c.<domain> and iterates <n>.<domain> TXT queries, base64-decoding the concatenation into an executable buffer that is written to disk and run. The package advertises itself as a cobrowsing client, but the shipped code contains no cobrowsing functionality — only the dropper.
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 bigops-cobrowsing-client (version 35.1.9). O3 Security's supply-chain scanner checks every dependency against known-malicious package intelligence at install time and in CI, flagging bigops-cobrowsing-client across your stack and pipelines.
If you installed it — respond
bigops-cobrowsing-client 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 bigops-cobrowsing-client 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 bigops-cobrowsing-client 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 bigops-cobrowsing-client-class packages before install and in CI — and if it already ran, its runtime egress monitoring catches the C2 callback and severs the channel.