Incident Report – Possible Firmware 5.42 Issue (USG FLEX 200)

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D2IT
D2IT Posts: 20 image  Freshman Member
First Comment Friend Collector Third Anniversary
in Security

Dear Zyxel Support,

We would like to report a possible issue related to Firmware Version 5.42 affecting Zyxel USG FLEX 200 firewalls.

After upgrading several devices to Firmware 5.42, we identified that the Web GUI becomes inaccessible while the firewall continues to operate partially.

Observed Symptoms

  • The Web GUI becomes inaccessible shortly after the firmware upgrade.
  • Console access (via Tera Term) remains fully functional.
  • The firewall continues forwarding traffic, and the network does not experience a complete outage.
  • However, some client devices intermittently lose network connectivity.
  • Router> apply /conf/system-default.confthe Web GUI becomes accessible for approximately 10 seconds, after which it stops responding again.

Troubleshooting Performed

To eliminate any configuration-related issues, we performed several troubleshooting procedures, including:

  • Factory reset via console.
  • Reconfiguring the firewall from factory defaults.
  • Repeated Web GUI access tests.
  • Validation across multiple customer environments.

The behavior remained exactly the same in every test.

Temporary Workaround

The only successful workaround was to rollback the firmware from Version 5.42 to Version 5.41 using the console connection.

Immediately after downgrading to Firmware 5.41, the affected firewalls returned to normal operation, including:

  • Stable Web GUI access.
  • Normal network communication.
  • No further connectivity issues reported.

Current Impact

So far, we have confirmed this issue in three different customer environments.

All three cases were successfully resolved by downgrading to Firmware 5.41.

In addition, we currently have four more USG FLEX 200 firewalls showing similar symptoms. These devices will be investigated in more detail tomorrow, as our engineering team spent most of the day restoring service for the first three customers. Initially, we could not confirm whether the firmware was the root cause of the issue.

Request

We kindly request that Zyxel Engineering investigate whether Firmware Version 5.42 contains any known issue affecting:

  • Web GUI availability.
  • Management services.
  • Overall firewall stability after the firmware upgrade.

If required, we can provide:

  • Configuration files.
  • Firmware details.
  • Additional information from the affected devices.

Thank you for your assistance. We look forward to your analysis and recommendations.

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All Replies

  • Zyxel_Melen
    Zyxel_Melen Posts: 4,992 image  Zyxel Employee
    Zyxel Certified Network Engineer Level 1 - Switch Zyxel Certified Network Administrator - Switch Zyxel Certified Network Administrator - Nebula Zyxel Certified Sales Associate
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    Hi @D2IT

    As our discussion via message, we release a date code firmware to fix this issue. Please help to upgrade all your firewall to this date code firmware version. Thanks.

    Zyxel Melen


  • ABAQUESYSTEM
    ABAQUESYSTEM Posts: 2 image  Freshman Member
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    Hi D2IT,

    We are seeing the same behaviour and ran essentially the same tests as you — factory reset via console, full reconfiguration from defaults, repeated GUI access tests, reproduced across several customer sites. Same result every time: the Web GUI dies a couple of minutes after boot while SSH and console stay fully available.

    A few additional data points from our side that may help narrow this down:

    • Both HTTP and HTTPS still report active: yes in show ip http server status and show ip http server secure status well after the GUI has become unreachable.
    • A packet trace on the LAN interface shows the device answering every SYN on 443 (and 80) with an immediate RST, ~350 microseconds. That is a TCP-stack level reject, i.e. nothing listening on the port — not a firewall drop, which would time out instead.
    • Default security policies are intact (LAN to ZyWALL any/any/any, allow), service control tables are empty, no lockout entries.
    • Reproduced on a unit with no WAN uplink and no internet access at any point, so signature or cloud updates can be excluded as a trigger.

    The important difference from your report: we also see this on firmware older than 5.42. Specifically on a USG FLEX 100 running 5.36 and a USG FLEX 200 running 5.36(ABUI.2), in addition to a FLEX 100 on 5.42(ABUH.1).

    We have opened a support ticket too.

    Has anyone else seen this on pre-5.42 firmware?

  • IQ2004
    IQ2004 Posts: 2 image  Freshman Member
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    could you publish this date code firmware please?
    we need it for at least USG flex 50 and USG Flex 100.
    could we get more information about this issue? we have a few items affected by this issue, and they are in v5.36, but the upgrade doesn't resolve. We also have some of them in 5.36 which are still fine.

  • Zyxel_Melen
    Zyxel_Melen Posts: 4,992 image  Zyxel Employee
    Zyxel Certified Network Engineer Level 1 - Switch Zyxel Certified Network Administrator - Switch Zyxel Certified Network Administrator - Nebula Zyxel Certified Sales Associate
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    Hi @ABAQUESYSTEM @IQ2004

    I will send the date code firmware to you. Please check the message I send.

    Zyxel Melen


  • mMontana
    mMontana Posts: 1,497 image  Guru Member
    Zyxel Certified Network Administrator - Security Zyxel Certified Network Administrator - Switch 50 Answers 1000 Comments
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    I updated 10 devices (USG Flex 50, USG Flex 50W, USG Flex 100 and USG Flex 100W) to 5.42(1) without any issue.

    I will really be glad to receive:
    -info about the next firmware release for these devices
    -any updated weekly firmware with release notes

  • mmmas
    mmmas Posts: 2 image  Freshman Member
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    Can I get the date code? My USG100 and USG50 had the same problem since July 23.

  • Zyxel_Melen
    Zyxel_Melen Posts: 4,992 image  Zyxel Employee
    Zyxel Certified Network Engineer Level 1 - Switch Zyxel Certified Network Administrator - Switch Zyxel Certified Network Administrator - Nebula Zyxel Certified Sales Associate
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    https://community.zyxel.com/en/discussion/comment/86026#Comment_86026

    Hi @mMontana

    What I got from our product team is that the official firmware release will be on 8/3/2026. Please follow this category once the firmware is released.

    Security Gateway New Release - Zyxel Community

    Zyxel Melen


  • Zyxel_Melen
    Zyxel_Melen Posts: 4,992 image  Zyxel Employee
    Zyxel Certified Network Engineer Level 1 - Switch Zyxel Certified Network Administrator - Switch Zyxel Certified Network Administrator - Nebula Zyxel Certified Sales Associate
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    https://community.zyxel.com/en/discussion/comment/86030#Comment_86030

    Hi @mmmas

    The date code firmware is sent by private message.

    Zyxel Melen


  • mMontana
    mMontana Posts: 1,497 image  Guru Member
    Zyxel Certified Network Administrator - Security Zyxel Certified Network Administrator - Switch 50 Answers 1000 Comments
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    https://community.zyxel.com/en/discussion/comment/86096#Comment_86096

    Thanks for sharing.

    I'll assume the release at UTM +8

  • Abudef
    Abudef Posts: 10 image  Freshman Member
    First Comment Friend Collector Eighth Anniversary
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    @Zyxel_Melen

    Hi, may I get the versions for the FLEX 100 and FLEX 50?

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