postinstall is an npm lifecycle hook that can run after a package finishes installing. In practice, it lets package authors execute scripts automatically during npm install, which means a dependency can behave like active code instead of a passive download.
This matters in cyber security because install-time execution is a common supply-chain attack path. A typosquatted or compromised package can use postinstall to fetch payloads, modify files, or search for secrets on a developer machine or CI runner. In Web3 environments, that can put wallet credentials, private keys, or seed phrases at risk. Defenders reduce exposure by reviewing package provenance, restricting lifecycle scripts where possible, monitoring for unusual process or network activity during installs, and keeping sensitive wallet material off endpoints that build software.