CVE-2019-15803: Zyxel GS1900-10hp Firmware

Critical severity, CVSS 9.1. EPSS: 1.3% chance of exploitation in the next 30 days.

An issue was discovered on Zyxel GS1900 devices with firmware before 2.50(AAHH.0)C0. Through an undocumented sequence of keypresses, undocumented functionality is triggered. A diagnostics shell is triggered via CTRL-ALT-t, which prompts for the password returned by fds_sys_passDebugPasswd_ret(). The firmware contains access control checks that determine if remote users are allowed to access this functionality. The function that performs this check (fds_sys_remoteDebugEnable_ret in libfds.so) always return TRUE with no actual checks performed. The diagnostics menu allows for reading/writing arbitrary registers and various other configuration parameters which are believed to be related to the network interface chips.

Affected products

• Zyxel GS1900-10hp Firmware: before 2.50\(aazi.0\)c0 (fixed in 2.50\(aazi.0\)c0)
• Zyxel GS1900-16 Firmware: before 2.50\(aahj.0\)c0 (fixed in 2.50\(aahj.0\)c0)
• Zyxel GS1900-24 Firmware: before 2.50\(aahl.0\)c0 (fixed in 2.50\(aahl.0\)c0)
• Zyxel GS1900-24e Firmware: before 2.50\(aahk.0\)c0 (fixed in 2.50\(aahk.0\)c0)
• Zyxel GS1900-24hp Firmware: before 2.50\(aahm.0\)c0 (fixed in 2.50\(aahm.0\)c0)
• Zyxel GS1900-48 Firmware: before 2.50\(aahn.0\)c0 (fixed in 2.50\(aahn.0\)c0)
• Zyxel GS1900-48hp Firmware: before 2.50\(aaho.0\)c0 (fixed in 2.50\(aaho.0\)c0)
• Zyxel GS1900-8 Firmware: before 2.50\(aahh.0\)c0 (fixed in 2.50\(aahh.0\)c0)
• Zyxel GS1900-8hp Firmware: before 2.50\(aahi.0\)c0 (fixed in 2.50\(aahi.0\)c0)

Published 2019-11-14. Last modified 2026-06-17.