Router firmware is the embedded software that runs a network router. It controls core functions such as packet routing, Wi-Fi settings, firewall rules, remote administration, and device boot-up. Unlike apps on a PC, firmware is tightly tied to the hardware model and is usually updated only by the vendor.
It matters because routers sit at the edge of a network and are trusted to move traffic and enforce access controls. If firmware contains a vulnerability, an attacker may gain persistent access, change DNS settings, expose the management interface, or weaken Wi-Fi security. Defenders reduce that risk by checking the exact model and firmware version, installing official updates, disabling unnecessary remote management, and using strong administrator and wireless passwords. In security work, router firmware is treated as part of the trust chain, not just background code.