Hardware Hacking a Best-Selling 4G Router: UART Root, Firmware Dump and CVE-2026-12495

A complete hardware-to-firmware walkthrough of the Mercusys MB115-4G, the best-selling router on Amazon Spain: pre-purchase firmware recon, PCB teardown, UART pinout discovery with a voltmeter, a root shell from hardcoded credentials, and a full flash dump over TFTP without an EEPROM programmer. The second half is the root cause analysis of CVE-2026-12495, a pre-authentication stack buffer…