Reverse Polish notation (RPN) is a way to enter calculations by typing the operands first and the operator last. Instead of writing 3 + 4, an RPN user enters 3 4 +. The calculator or program keeps values on a stack, then applies each operation to the most recent entries. This makes nested expressions easier to process because parentheses are often unnecessary.
In cyber security, RPN matters because many tools are parser-driven and stack-based, from calculators and scripting environments to packet processing and command interpreters. Attackers may abuse expression parsers, malformed stack handling, or input validation weaknesses to trigger crashes or unintended behavior. Defenders need to understand the notation when reviewing legacy software, embedded devices, or custom automation, since a small input mistake can change results or reveal bugs in how values are stored and combined.