A single-instance app is software that allows only one active copy to run at a time. When the user starts it again, the new launch is routed to the already running process instead of opening a second instance. Desktop apps use this pattern to restore windows, open files, or process links through a local handoff channel such as IPC.
In cyber security, that handoff is important because it often receives untrusted input from browsers, file associations, or other local programs. If the app does not validate message framing, escape special characters, or separate commands from data, an attacker may inject instructions into the running process. Defenders harden these flows with strict parsing, per-message validation, authentication between components, and removal of legacy helpers that expand the trusted surface.