An issue was discovered on Zyxel GS1900 devices with firmware before 2.50(AAHH.0)C0. The firmware image contains encrypted passwords that are used to authenticate users wishing to access a diagnostics or password-recovery menu. Using the hardcoded cryptographic key found elsewhere in the firmware, these passwords can be decrypted. This is related to fds_sys_passDebugPasswd_ret() and fds_sys_passRecoveryPasswd_ret() in libfds.so.0.0.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-15801
• https://jasper.la/exploring-zyxel-gs1900-firmware-with-ghidra.html
• https://www.zyxel.com/support/gs1900-switch-vulnerabilities.shtml