Researchers Detail EEPROM Hijacking Method Targeting 8051 Microcontrollers

A technical methodology has been outlined for intercepting and redirecting execution flow in 8051-architecture microcontrollers, which remain widely used in constrained embedded systems. The technique involves inserting a binary hook — a long jump instruction — at the start of an SDCC-compiled validation function to redirect execution to a custom assembly routine stored in unused memory, known as slack space. A key challenge addressed is byte-alignment: replacing a 4-byte prologue with a 3-byte LJMP instruction requires a padding NOP to prevent program counter corruption and system crashes. The injected assembly routine conditionally bypasses a critical logic or security check only when a specific trigger value is detected in memory, otherwise resuming normal program execution. The approach is noted for its minimal memory footprint, making it particularly relevant in environments where available storage is measured in just a few kilobytes.