cobrowsing-utilsnpm
Advisory published Updated
cobrowsing-utils is a confirmed malicious npm package (MAL-2026-12659) that executes malicious code on install (malicious version 35.7.3). Do not install it — remove it immediately and rotate any exposed credentials.
Malicious code in cobrowsing-utils (npm)
What this malware does
[email protected] executes a binary dropper as a side effect of requiring the package. index.js loads helpers.js which auto-runs main() at import time. main() assembles Cloudflare Workers hostnames from split string fragments (e.g. oob-worker.cf101-adf.workers.dev, oob-worker.cf99-9b3.workers.dev, oob-worker.cf102-baf.workers.dev), fetches a platform-specific binary via https.get, writes it to /var/tmp or %TEMP% under masquerading names (.cache<hex>, dotnet_diag_<hex>.exe, analytics_state), chmods 0755, and spawns it detached via /bin/sh -c "<path> &" or cmd.exe /c start /b. When HTTPS fetch fails, the code falls back to a DNS-TXT covert channel: it queries c.<domain> under *.dl.wel1.ru subdomains (sdk.dl.wel1.ru, ext.dl.wel1.ru, pkg.dl.wel1.ru, net.dl.wel1.ru) for a chunk count, reassembles indexed TXT records, base64-decodes the payload, writes it to disk, and executes it. Hostnames, discovery domains, file names, and function identifiers are chosen to blend with legitimate telemetry (_l, _p, activate, analytics_state, dotnet_diag). The package advertises itself as shared utilities; the fetched binary has no relation to that stated purpose.
Malicious versions
Indicators of compromise (SHA-256)
Detection & response playbook
Malicious packageFind it
Search your lockfiles and build artifacts for cobrowsing-utils (version 35.7.3).
If you installed it — respond
Remove cobrowsing-utils 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 cobrowsing-utils 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
Campaign
References
Credits
- Amazon Inspector · finder
Detect & block this
O3 blocks cobrowsing-utils-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.