Modern applications combine multiple components into single processes, leading to complex tradeoffs between isolation, performance, and programmability. We present the Endoprocess, a unique, microkernel-based approach for protection within process spaces. An endoprocess safely multiplexes process resources by exporting a low-level abstraction, the subprocess, that is transparently overlaid on existing process interfaces (like mmap, mprotect, etc), and provides extensibility and programmability through custom application-layer modules. We report experimental results of an initial prototype and highlight several application domains. Overall, the endoprocess presents a path for protection within processes while remaining compatible with existing OS abstractions and multiplexing them in a secure and extensible way.