fix: trust gRPC metadata over payload session vars; mask internal gRPC errors

[patrickleet]

Security summary

The gRPC transport built the request Session by letting the payload session_variables override transport metadata. Behind a trusted gateway that injects authenticated identity as gRPC metadata, a client could spoof identity via the request body.

Exploit (before)

// gRPC metadata (injected by trusted proxy, authenticated): x-hasura-user-id: alice
GrpcRequest {
  command: "order.cancel",
  input: "{...}",
  session_variables: {           // client-controlled body — silently WON
    "x-hasura-user-id": "victim",
    "x-hasura-role": "admin"
  }
}
// Handler saw user=victim, role=admin. Identity spoofed.

Behavior (after)

Payload vars are applied first, then trusted metadata overwrites colliding keys — metadata wins. A client can no longer override a proxy-injected x-hasura-user-id/x-hasura-role from the body. Payload-only keys (no metadata collision) still pass through, preserving the Hasura-action path where Hasura forwards verified claims in the payload and no metadata is injected.

Error masking (consistency fix)

gRPC previously returned raw e.to_string() to clients, which can leak SQL/driver detail; HTTP already masked internal errors. Both transports now route response bodies through a single shared HandlerError::client_facing_message():

• 5xx (Repository/Other) → generic "Internal server error", original logged server-side via eprintln! (the existing sink HTTP already used).
• 4xx (client-fault) → descriptive message preserved.

No duplicated logic: the old http::error_message_for_response was removed and folded into the shared method on HandlerError.

Trust boundary docs

The framework does not authenticate. A trusted proxy must strip client-supplied x-hasura-* headers/metadata and inject only authenticated ones. Documented in:

• Rustdoc on Session (framework-wide trust model + source precedence).
• Rustdoc on gRPC build_session and HTTP session_from_headers.
• README: HTTP/gRPC transport notes + a dedicated Security / Trust Boundary section, and corrected the stale "payload takes precedence" line.

Testing

• cargo fmt --all
• cargo build --features grpc / cargo build --features http — clean
• cargo test --features grpc — 24 microsvc integration + 250 lib unit, all pass
• cargo test --features http — 23 microsvc integration + 261 lib unit, all pass

New tests:

• grpc_payload_session_variables_do_not_override_metadata — metadata trusted-user wins over payload attacker.
• grpc_payload_session_variables_apply_when_metadata_absent — payload-only identity still flows.
• client_supplied_identity_header_is_trusted_verbatim (HTTP) — documents that headers are trusted as-is, which is why the proxy is required.
• Updated HTTP unit tests for the renamed shared masking method.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: b33a3214-a703-43f5-8d7a-cbafb8ebc1de

📥 Commits

Reviewing files that changed from the base of the PR and between da838d7 and 8626ada.

📒 Files selected for processing (7)

• README.md
• src/microsvc/error.rs
• src/microsvc/grpc.rs
• src/microsvc/http.rs
• src/microsvc/session.rs
• tests/microsvc/transport_grpc.rs
• tests/microsvc/transport_http.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch hardening/grpc-session-precedence

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