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Advisories

ENDLESSDOORS: Zbtlink Router rctl/kworker Phone-Home Root Implant

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severity
critical
date
Affecting

  • CPE2801 Firmware 22.10.09

  • WE1026-5G-WD Firmware 21.04.07

  • WE1326 Firmware 22.02.18_1

  • WE2007 Firmware 23.08.12

  • WE2008-DSIM Firmware 23.08.11

  • WE2416 Firmware 21.03.22_1

  • WE3326 Firmware 20.09.30

  • WE5927 Firmware 22.08.10

  • WE5931 Firmware 22.05.31

  • WE5931AC Firmware 22.05.31

  • WE826-T3-DSIM Firmware 21.12.21

  • WG108 Firmware 21.08.06_1

  • WG209 Firmware 21.07.28

  • WG259 Firmware 21.03.23

  • WG1602 Firmware 23.10.11

  • WG1608-DSIM Firmware 23.03.16

  • WG2105 Firmware 22.05.30

  • WG2107 Firmware 22.09.08

  • WG3526 Firmware 22.11.01

  • ZBT-Z8102AX-2SIM Firmware 7.6.7.2-25.0814_114432

CWE
  • CWE-506 Embedded Malicious Code
CVSS
9.3
CVSS V4 Vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
Credit
Jacob Baines of VulnCheck
Description
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.

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