CVE-2026-66747

Zbtlink router firmware ships an embedded remote-control implant, ENDLESSDOORS, present in every published build across the product line. It is the open-source ycsunjane/rctl tool built in as an OpenWrt package (librctl.so), started at boot and run as root under the process name kworker to blend in with the kernel's [kworker/*] threads. It opens no listening port; it phones home over cleartext TCP to a hardcoded command-and-control server (command channel 7000, interactive-shell callback 7001) with no authentication and no transport encryption, re-attempting contact roughly every 35 seconds. Its command handler passes any received string to popen() as uid=0, and a reserved rctlbash command returns an interactive root shell. Because the channel is unauthenticated and cleartext, control is not limited to whoever planted it: any party that answers at the C2 address, occupies the network path (DNS or route hijack), or acquires the hardcoded fallback domain obtains unauthenticated remote code execution as root.

• CWE-506

• https://github.com/ycsunjane/rctl
• https://www.vulncheck.com/advisories/zbt-endlessdoors
• https://www.vulncheck.com/blog/zbt-endlessdoors
• https://www.zbtlink.com/pages/zbt-router-firmware-download

Published

Aug 5, 2026

Last Modified

Aug 5, 2026

Status

Received

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