Diffing Zyxel GS1900 Firmware to Confirm Patch 3 Actually Closes the Root RCE

This short verifies a Zyxel GS1900 follow-up patch by diffing firmware, not by trusting an advisory. We extract both root filesystems, map CGI handlers, trace them to a shell-out sink, and outline a canary regression test for an isolated lab switch. The code and output shown are illustrative shapes, not vendor source. Only analyze firmware you obtained legitimately, and only test hardware you own or are authorized to test. Full walkthrough: https://hackyjs.com/posts/diffing-zyxel-gs1900-firmware-to-confirm-patch-3-actually-closes-the-root-rce

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