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feat(web): enforce localhost-only binding; expand loopback origin allow-list (#1796)

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Closes #1795.

Enforce localhost-only binding in code (the "never 0.0.0.0" rule was only a spec note): refuse an all-interfaces HOST — across every spelling the OS binds as the wildcard (inet_aton decimal/octal/hex, IPv6, IPv4-mapped, zone-scoped, IDNA/Unicode) — unless DANGEROUSLY_BIND_ALL_INTERFACES=true (Docker's sanctioned opt-in). Fixes the reported stdio-connect 403 by expanding the default loopback origin allow-list to all three interchangeable forms.

Hardening surfaced across an extended review: closes an IDNA/fullwidth wildcard bypass, a blank-ALLOWED_ORIGINS fail-open, and the CLI/TUI OAuth-callback wildcard bind (loopback-only now, RFC 8252); canonicalizes the origin allow-list the way a browser does; derives frame-ancestors from the real embedder list (IPv6-literal exclusion verified in headless Chromium); strictPort on all three bind points; and preserves the CLI exit-code map + JSON envelope through the mcp-inspector --cli launcher path. Shared host normalization lives in core/node/hostUrl.ts. Docs + tests + smokes throughout.

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Cliff Hall 2026-07-26 22:15:10 -04:00 • committed by GitHub
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@ -32,6 +32,7 @@ inspector/
│ ├── json/ # JSON + parameter/argument conversion utilities
│ ├── logging/ # Silent pino logger singleton
│ ├── mcp/ # InspectorClient runtime, state stores, transports, config import
│ ├── node/ # Node-only shared helpers: version reader, hostUrl (host normalize/canonicalize + all-interfaces/loopback detection)
│ ├── react/ # React hooks over the state stores
│ └── storage/ # File I/O helpers for the OAuth persist backends
├── test-servers/ # Composable MCP test servers + fixtures used by integration tests
@ -198,7 +199,7 @@ docker build -t mcp-inspector .
docker run --rm -p 6274:6274 mcp-inspector
```
The image defaults to `--web` bound to `0.0.0.0:6274` with browser auto-open disabled; override the args to run another mode (`docker run --rm ghcr.io/modelcontextprotocol/inspector --cli …`). Pass `-e MCP_INSPECTOR_API_TOKEN=…` to set a known token (otherwise one is generated and printed in the logs), or `-e DANGEROUSLY_OMIT_AUTH=true` to disable auth. The image runs as the non-root `node` user and has a `HEALTHCHECK` that probes the web UI — it assumes the default `--web` mode, so add `--no-healthcheck` when running `--cli`/`--tui` (which have no web server).
The image defaults to `--web` bound to `0.0.0.0:6274` with browser auto-open disabled; override the args to run another mode (`docker run --rm ghcr.io/modelcontextprotocol/inspector --cli …`). Pass `-e MCP_INSPECTOR_API_TOKEN=…` to set a known token (otherwise one is generated and printed in the logs), or `-e DANGEROUSLY_OMIT_AUTH=true` to disable auth. Binding `0.0.0.0` (all network interfaces) is refused by default outside a container — it exposes the process-spawning backend to the local network — so the image opts in explicitly with `DANGEROUSLY_BIND_ALL_INTERFACES=true` (already set in the `Dockerfile`); a bare `HOST=0.0.0.0` without that flag exits with an error. If you **remap the published port** (`-p 8080:6274`), the browser's origin (`http://localhost:8080`) no longer matches the in-container port, so set `-e ALLOWED_ORIGINS=http://localhost:8080,http://127.0.0.1:8080` (or run `-e CLIENT_PORT=8080 -p 8080:8080`) or connects will 403. `ALLOWED_ORIGINS` **replaces** the default list rather than merging, so list every loopback form you'll browse from (see the [web README](./clients/web/README.md#host-binding--the-origin-allow-list)). The image runs as the non-root `node` user and has a `HEALTHCHECK` that probes the web UI — it assumes the default `--web` mode, so add `--no-healthcheck` when running `--cli`/`--tui` (which have no web server).
## Contributing — `AGENTS.md` and `CLAUDE.md`