What does the bigops-cobrowsing-client malware do?

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_ on Unix, dotnet_diag_ .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

Which versions of bigops-cobrowsing-client are malicious?

The following npm versions of bigops-cobrowsing-client are flagged malicious: 35.1.9. Treat any of these as compromised.

How do I remediate bigops-cobrowsing-client if I installed it?

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.

I already installed bigops-cobrowsing-client — how do I know if it already compromised me?

If bigops-cobrowsing-client was installed, its post-install payload may already have run. Removing the package does not undo that — check outbound connections and credential use from the install window onward.

Is bigops-cobrowsing-client part of a known attack campaign?

Yes — bigops-cobrowsing-client is associated with the IN-MAL-2026-014410 campaign. Packages in the same campaign often share infrastructure and payloads, so check whether any related packages are also in your dependency tree.

How do I find bigops-cobrowsing-client across my projects, lockfiles, and CI pipelines?

Search your lockfiles and build artifacts for bigops-cobrowsing-client (version 35.1.9).

Malicious package

bigops-cobrowsing-clientnpm

Advisory published Aug 5, 2026Updated Aug 5, 2026

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)

MAL-2026-12160

Immediate action

Remove the package, then rotate any secrets the build/runtime could reach.

npm uninstall bigops-cobrowsing-client

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

1 flagged

35.1.9

Indicators of compromise (SHA-256)

25ce2f0f7a7411a7bfedb5d07449d26620388d3936e5951883e52463de01ef50

Detection & response playbook

Backdoor / remote access

Find it
Search your lockfiles and build artifacts for bigops-cobrowsing-client (version 35.1.9).
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 installed, its post-install payload may already have run. Removing the package does not undo that — check outbound connections and credential use from the install window onward.

Frequently asked questions

No. bigops-cobrowsing-client on npm has been identified as a malicious package (version 35.1.9 flagged). It should be removed immediately — do not install or keep it in your dependency tree.

Campaign

IN-MAL-2026-014410

References

npmjs.com

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.

Malware detection Runtime egress monitoring

Explore

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