feat(connector-mcp): policy-gate declarative stdio transports — default-deny + host allowlist (ADR-0097 security follow-up) #3055

Security follow-up to ADR-0097 / #3017 (#3049), and the precondition for shipping connector-mcp in any default preset (see the companion default-preset issue).

Threat model

A declarative provider: 'mcp' instance may name a stdio transport:

connectors: [{
  name: 'tools',
  provider: 'mcp',
  providerConfig: { transport: { kind: 'stdio', command: 'bash', args: ['-c', '…'] } },
}]

Materialization spawns that command as a local child process (StdioClientTransport). Declarative entries arrive through metadata — including a Studio publish at runtime, which triggers the metadata:reloaded reconcile and materializes new instances on a live server (soft mode skips failures, not successes). Net: anyone who can publish metadata can execute arbitrary commands on the server.

Today the accidental barrier is that the host must consciously install @objectstack/connector-mcp. If the plugin becomes part of the default preset — which the ADR-0097 "expressible and executable as pure metadata" promise wants — that barrier disappears. The gate must become explicit policy instead.

Ask

• createMcpProviderFactory gains a declarativeStdio policy, default deny:
  • undefined / false → a declarative stdio transport is rejected;
  • string[] → allowlist, matched against transport.command by strict equality;
  • true → allow any command (explicit full trust).
• ConnectorMcpPlugin plumbs the option through: new ConnectorMcpPlugin({ declarativeStdio: ['npx'] }).
• A policy violation is a configuration fault — plain throw (fatal at boot, skipped+logged on reload), with an actionable message telling the host exactly how to opt in. It must NOT be classified CONNECTOR_UPSTREAM_UNAVAILABLE (feat(connectors): degrade + retry declarative instances on unreachable upstream (#3017) #3049): a security rejection is not retryable-into-existence.
• http transports stay unaffected (outbound HTTP is the same trust class as the http node / rest connector).
• Hand-wired paths stay unaffected (createMcpConnector / plugin instance options): host code wrote that command — a different trust anchor than metadata.
• Docs: ADR-0097 addendum (+ ADR-0024 §4 cross-ref). Be honest that a command allowlist is a coarse boundary (allowlisting npx ≈ allowing any package); real sandboxing / multi-tenant isolation remains the enterprise tier per ADR-0024 §4.
• Tests: default-deny (plain throw, not unavailable-coded), allowlist hit/miss, true passthrough, http unaffected, hand-wired path unaffected.

Refs

• ADR-0097 (docs/adr/0097-declarative-connector-instances.md), ADR-0024 §4 (trust model)
• feat(connector-mcp): declarative provider — non-fatal / lazy boot for unreachable MCP servers (ADR-0096 follow-up) #3017 / feat(connectors): degrade + retry declarative instances on unreachable upstream (#3017) #3049 (degrade+retry — the fault-classification seam this slots into)
• Implement ADR-0096: declarative connector instances (provider-bound connectors: materialized by generic executors) #2977 (ADR-0097 implementation)