Developer Builds HMAC Hook Scanner After AI Agent Was Fed Fake Security Warnings

A developer discovered that malicious code had injected a fake security warning into their Claude Code tool output, mimicking a legitimate hook response without any actual package install occurring. The incident highlighted a critical gap: there is no native way to distinguish intentionally installed AI agent hooks from those secretly planted by malicious npm packages buried in transitive dependencies. In response, the developer built an open-source tool called claude-hookscanner, which uses HMAC-SHA256 signatures to verify that hook scripts were authored by the legitimate user. The tool also applies heuristics to flag unsigned hooks found inside node_modules, high-risk command patterns like curl piped to bash, and scripts resolving outside expected directories. A content-hash acknowledgement list prevents repeated review of already-vetted benign files, reducing alert fatigue during routine scans.