ASUS RT-AX53U - Unable to flash new firmware since 25.12.0 upgrade

[bitestring]

Since upgrade from 24.10 to 25.12.0, two of my identical ASUS RT-AX53U routers lost the ability to flash new firmware. For example, LuCI shows a new version 25.12.1 is available for upgrade, but when I upgrade via either LuCI's Attended Sysupgrade or owut, system reboots back to 25.12.0. It's the same behavior if I flash a build generated by the firmware-selector.

Model: ASUS RT-AX53U
Architecture: MediaTek MT7621 ver:1 eco:3
Target Platform: ramips/mt7621
Firmware Version: OpenWrt 25.12.0 r32713-f919e7899d / LuCI openwrt-25.12 branch 26.063.83382~971a52d

Any help would be appreciated. Thanks in advance.

[tojestzart]

The OpenWrt Firmware Selector

If I had to guess, I would say you are using adblock and have a corrupted /dev/null. Can you check ls -l /dev/null like they did here:

The OpenWrt Firmware Selector

Yes, this file is broken. There was a bug in a recent adblock version which deleted this special file and recreated it as a regular file. The consequence was that all log and everything would be stored in that file rather than being discarded. Your router was/is out of disk space during upgrade, so it failed. The images you have downloaded are all functional correct. If you have never used adblock, than uninstall it. Then log in to the router and do this: rm /dev/null; mknod -m 666 /dev/null c…

hope it helps for you

EDIT 2:
first finding

Adblock support thread

I think there's a problem with the new adb_errorlog pointing to /dev/null and wiping out /dev/null's special attributes. Restarting adblock causes the file to become "normal". If I enable adb_debug which changes adb_errorlog to something besides /dev/null, it doesn't clobber the file. This is before and after a restart: root@router:~# ls -l /dev/null; service adblock restart; sleep 5; ls -l /dev/null crw-rw-rw- 1 root root 1, 3 Mar 4 21:27 /dev/null -rw-r--r-- 1 root ro…

[bitestring]

Thanks. Let me try it later and report the results. But I can see /dev/null is corrupted in my routers as reported in adblock thread.

[bitestring]

Suggested solution worked. It was adblock indeed. Probably this is why programs should not run as root.