sh-mcp/packages/shared/src/auth.ts

/**
 * Authorization interface and scope-enforcement guard.
 *
 * This file defines the scope-enforcement contract every tool handler relies on:
 *   1. `ScopeError` — a typed error thrown when a required scope is absent.
 *   2. `requireScope()` — enforces scope presence on an already-decoded
 *      AuthContext. Call this at the top of every tool handler.
 *   3. `AuthProvider` interface — the contract the auth layer implements. Each
 *      server wires an AuthProvider into its request pipeline so that by the
 *      time `handler(input, ctx)` is called the context is already validated.
 *
 * The concrete implementation of `AuthProvider` now lives in `cognito-auth.ts`
 * (`CognitoAuthProvider`) — built and verified after the 0a spike proved the
 * live Cognito access-token shape (verified `sub`/`scope`/`client_id`, no native
 * `aud`). It verifies the JWT signature against the pool JWKS, validates `iss`,
 * enforces the `client_id` allow-list AS the audience boundary (the token has no
 * `aud`), applies the finance TTL ceiling and deny-list, and extracts scopes.
 * This file stays transport- and provider-agnostic; see docs/design.md §2.
 */

import type { AuthContext, Scope } from './types.js';

// ---------------------------------------------------------------------------
// ScopeError
// ---------------------------------------------------------------------------

/**
 * Thrown by `requireScope()` when the caller's AuthContext does not include
 * the required scope.
 *
 * Transport adapters (OpenAPI handler, MCP dispatcher) should catch this and
 * return an appropriate 403 / permission-denied response.
 */
export class ScopeError extends Error {
  /** The scope that was required but absent. */
  readonly requiredScope: Scope;
  /** The `sub` from the AuthContext that triggered the error. */
  readonly sub: string;

  constructor(sub: string, requiredScope: Scope) {
    super(
      `User "${sub}" does not have the required scope "${requiredScope}".`,
    );
    this.name = 'ScopeError';
    this.requiredScope = requiredScope;
    this.sub = sub;
    // Maintain proper prototype chain for `instanceof` checks.
    Object.setPrototypeOf(this, new.target.prototype);
  }
}

// ---------------------------------------------------------------------------
// requireScope
// ---------------------------------------------------------------------------

/**
 * Assert that `ctx` contains `scope`. Throws `ScopeError` if it does not.
 *
 * Call at the top of every tool handler before touching any input or reaching
 * any downstream service:
 *
 * ```ts
 * handler: async (input, ctx) => {
 *   requireScope(ctx, 'finance:read');
 *   // ... safe to proceed
 * },
 * ```
 *
 * NOTE: Server-side enforcement is authoritative. Tool-hiding in the agent UI
 * is a convenience only (design.md §2.5). This guard enforces independently.
 *
 * NOTE: JWT signature/issuer/aud/client_id validation is NOT done here — see
 * the TODO above. By the time `handler` is called, the `AuthProvider` has
 * already validated the token and populated `ctx`.
 */
export function requireScope(ctx: AuthContext, scope: Scope): void {
  if (!ctx.scopes.includes(scope)) {
    throw new ScopeError(ctx.sub, scope);
  }
}

// ---------------------------------------------------------------------------
// AuthProvider interface  (deferred 0a-gated auth layer contract)
// ---------------------------------------------------------------------------

/**
 * Contract that each server's concrete auth layer must implement.
 *
 * The server's request pipeline calls `authenticate(req)` once per inbound
 * request and passes the resolved `AuthContext` into every tool handler.
 *
 * `req` is typed as `unknown` so this interface stays transport-agnostic
 * (works for an Express `Request`, a raw `IncomingMessage`, a Lambda event,
 * or a test-double). The concrete implementation casts to the appropriate type.
 *
 * TODO(auth-layer-0a): Implement this interface in the deferred auth layer.
 * A concrete implementation lives in `servers/sh-mcp-ops/src/auth.ts` and
 * `servers/sh-mcp-finance/src/auth.ts` once that layer is built.
 */
export interface AuthProvider {
  /**
   * Extract and validate the inbound token from `req`, returning a fully
   * populated AuthContext on success.
   *
   * Throws (or rejects) on any validation failure:
   *   - Missing / malformed Authorization header
   *   - Invalid JWT signature
   *   - Wrong issuer, audience, or client_id
   *   - Expired token
   *   - User on the deny-list
   */
  authenticate(req: unknown): Promise<AuthContext>;
}