ATP100 – critical regression after upgrade to firmware 5.42(ABPS.0), reboots caused by kern

Tibor_SK

Dear Zyxel Support Team,

I would like to report a critical stability issue affecting a Zyxel ATP100, which started immediately after upgrading to the newly released firmware version 5.42(ABPS.0).

Device: Zyxel ATP100

Firmware: 5.42(ABPS.0) (officially released on 2026-02-04)

Status before upgrade: stable operation on previous firmware (5.41)

Problem description

After upgrading to firmware 5.42(ABPS.0), the device starts rebooting periodically. This is not a manual reboot or power-related issue, but a watchdog-triggered reboot caused by a kernel soft lockup.

Crash logs consistently show:

BUG: soft lockup - CPU#1 stuck for 31s! [fqdnobjectd]

The call trace repeatedly points to the fqdn_object kernel module and the fqdnobjectd process, specifically the function:

fqdnObject_syncQ_chrdev_sync()

This strongly indicates a deadlock or infinite wait condition in the FQDN (DNS/domain handling) subsystem, which eventually causes the watchdog to reboot the system.

• The issue started immediately after the upgrade to 5.42
• No configuration changes were made
• The behavior clearly indicates a regression bug in firmware 5.42, as the same configuration was stable on 5.41

I can provide full crash logs if needed.

Thanks.

Zyxel_Melen

Hi @Tibor_SK

I will send you a private message for the full crash logs. Please also help to collect:

1. Device configuration "start-up config".
2. Device diagnostic file.

https://community.zyxel.com/en/discussion/26074/atp-flex-how-to-collect-diagnostic-file-on-firewall-local-web-gui

Zyxel_Melen

Hi @Tibor_SK

I will send you a private message for the full crash logs. Please also help to collect:

1. Device configuration "start-up config".
2. Device diagnostic file.

https://community.zyxel.com/en/discussion/26074/atp-flex-how-to-collect-diagnostic-file-on-firewall-local-web-gui

Zyxel_Cooldia

Hi @Tibor_SK,

To avoid service impact, you can switch the firmware partition by going to “MAINTENANCE > File Manager > Firmware Management”
We are checking this issue and will keep the community updated on the status.

Web GUI:

CLI:

https://community.zyxel.com/en/discussion/25856

Tibor_SK

Dear Support Team,

the requested diagnostic data have been provided. As a temporary mitigation, I have switched the device firmware back to version 5.41, which is currently operating stably.

Please let me know if any further information or testing is required.

Kind regards.

Zyxel_Vic

Hi @Tibor_SK

We sincerely apologize for your inconvenience in certain issue and thanks for your assistance so that we're able to address the root cause in short time. Here we have a firmware which fixed the issue you met.

https://1drv.ms/u/c/a33c4d75c38ddf7d/IQC97Mb7BrkdRq4B1Ww2v5xKAdnuh-bxrB6PpDJWOQgXKGc?e=oPQ3sw

alexey

https://community.zyxel.com/en/discussion/comment/83422#Comment_83422

Great speed. When we can download FW for other series (ATP800 https://community.zyxel.com/en/discussion/32317/atp800-kernel-panic-fqdnobject-syncq-chrdev-sync#latest)?

Zyxel_Vic

Hi @alexey

Thanks for asking. Since we had received several upgrading failure cases recently, we decide to release a patch firmware for all ATP/FLEX Series soon. The schedule will be on next Wed. Apologized for the inconvenience and thanks for your patience.

alexey

https://community.zyxel.com/en/discussion/comment/83425#Comment_83425

Sorry, but its too long. Our company have 15 min downtime every few hours, because ATP rebooting.

You know reason, you already give patched FW. What a problem build new FW for another device?