Runtime assembly is the practice of building malicious logic while code is executing, rather than delivering one complete, obvious payload file at the start. Attackers may split code into fragments, fetch parts on demand, decode or decrypt content in memory, and only assemble the final behavior when the victim browser or host is already running it.
This matters because it weakens file-based detection. Hashes, signatures, and simple download filters may never see a finished malware sample. In browser abuse, runtime assembly can happen inside workers, scripts, or message-driven components that stitch together requests, redirects, and execution paths. Defenders look for unusual network fetches, suspicious worker registration, dynamic script creation, and unexpected in-memory behavior. The main goal is to catch the assembly process itself, not just the final payload.