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docs(mcp): document Origin validation / CORS secure defaults #530

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13 changes: 13 additions & 0 deletions reference/mcp/configuration.md
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Expand Up @@ -130,6 +130,19 @@ Default: `false`

When `true`, Harper accepts client-issued `DELETE /mcp` requests that explicitly terminate a session. When `false` (the default), `DELETE` returns 405 with an `Allow` header — sessions only end via idle eviction or explicit server-side cleanup.

## Security: Origin validation

The MCP endpoint validates the request `Origin` header to defend against DNS-rebinding attacks (a requirement of the MCP Streamable HTTP transport). Validation reuses each profile's existing CORS configuration rather than introducing a separate MCP setting:

- When CORS is **disabled** (the default), any `Origin` is accepted. This is appropriate for localhost-only or non-browser clients, where no DNS-rebinding vector exists.

@gemini-code-assist gemini-code-assist Bot Jun 16, 2026

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medium

According to the HTTP configuration documentation, http.cors defaults to true (enabled), but http.corsAccessList defaults to null (allowing all origins). Therefore, CORS is not disabled by default; rather, it is enabled without an active allow-list. We should clarify this to avoid confusing users about the default configuration state.

Suggested change
- When CORS is **disabled** (the default), any `Origin` is accepted. This is appropriate for localhost-only or non-browser clients, where no DNS-rebinding vector exists.
- When CORS is **disabled** or no allow-list is configured (the default), any Origin is accepted. This is appropriate for localhost-only or non-browser clients, where no DNS-rebinding vector exists.

- When CORS is **enabled** with an allow-list, a request whose `Origin` is not in the list is rejected with `403 Forbidden`. A `*` entry allows any origin.
- A request with no `Origin` header at all (for example `curl` or server-to-server traffic) is always accepted — DNS rebinding only applies to browser-initiated requests.

**Secure default:** any deployment that exposes the MCP endpoint to browsers beyond loopback should enable CORS with an explicit allow-list — that is what activates Origin-based DNS-rebinding protection.

- Application profile (HTTP port): `http.cors` + `http.corsAccessList`.
- Operations profile (operations port): `operationsApi.network.cors` + `operationsApi.network.corsAccessList`.

## Example

A common deployment pattern that locks down the operations profile to a small explicit set, enables MCP DELETE for graceful client logout, and raises per-tool throughput for the application profile:
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