Use GitHub's private vulnerability reporting: on the repository, open the Security tab and click Report a vulnerability. Include your macOS version, the Chiaro version, and steps to reproduce.

This is a personal project rather than a staffed one, so treat response times accordingly: the maintainer will acknowledge a report within a week.

A report about Chiaro can reasonably be about:

• The MCP server (ADR 0008). It binds to 127.0.0.1 only, listening on port 24242 with a fallback to an ephemeral port if that's taken. It checks the Origin header by exact host match against 127.0.0.1, ::1, and localhost, and caps request bodies at 16 MB. It has no authentication, by design: the threat model is that any process already running on the machine is trusted, the same as a local Unix socket. A report that the server accepts requests from another machine, or from a spoofed Origin, is in scope; a report that it has no login is not, since that's the design.
• File handling. Sidecar JSON decoding, and RAW decoding through Apple's own frameworks (CIRAWFilter and friends).
• Distribution. The DMG is ad-hoc signed, not notarized. Chiaro asks GitHub for the latest release tag and points you to the release page; it never downloads or replaces itself (ADR 0014), so there is no update channel to spoof. Get new versions from the GitHub release page rather than a mirror.

Reports that require a compromised local machine to work. If an attacker already has code execution on the machine Chiaro runs on, Chiaro's local trust model doesn't hold, and that's true of most local-first software.