A runtime environment is the live execution context in which software runs. It includes the process state, available data, loaded libraries, system calls, files, network access, and the permissions granted to the program or agent. In plain terms, it is the space where code stops being static and starts acting.
In cyber security, the runtime environment matters because controls and risks become real there. An input that looks harmless on disk may become dangerous after it is decrypted, parsed, or executed inside a trusted runtime. That is why prompt injection and similar attacks target AI agents, browsers, and terminal tools after they have access to files, commands, or secrets. Defenders reduce risk by tightening permissions, isolating execution, limiting tool access, and treating external content as untrusted until the final moment.