Grackle: Fail-open authorization in the MCP tool layer lets scoped agents perform cross-task and cross-session mutations (IDOR)

Summary

Authorization for scoped (agent) MCP callers is enforced inline, per tool, and is applied inconsistently — several mutating tools silently omit the ancestry/workspace check that their siblings perform. Because the MCP server authenticates all outbound gRPC with the full server API key and the backend gRPC handlers perform no caller-based authorization, the MCP tool layer is the sole authorization boundary. A malicious or prompt-injected scoped agent can therefore perform cross-task and cross-session operations it should not be allowed to (an IDOR / privilege-boundary bypass).

This advisory bundles the audit's Systemic Pattern A findings: F2, F6, F7, F12 (and the duplicate F19).

Affected versions

@grackle-ai/mcp (with @grackle-ai/plugin-core / @grackle-ai/auth) at 0.132.1 and earlier.

Root cause

• mcp-server.ts:111-127 (createGrpcClients) sets Authorization: Bearer ${apiKey} (the full server key) on every outbound gRPC call.
• Backend handlers (updateTask, deleteTask, resumeTask, killAgent, getTask in plugin-core) take only the request message — no AuthContext — and act on whatever ID is passed.
• Therefore scope must be enforced in each MCP tool handler. Some call assertCallerIsAncestor (task_complete, task_start, session_attach, session_send_input); their destructive siblings do not. New tools that forget the check fail open.

F2 — task_update / task_delete / task_resume bypass ancestry (High)

Location: packages/mcp/src/tools/task.ts:226 (task_update), :393 (task_delete), :465 (task_resume).

These accept an arbitrary taskId with only a ROOT_TASK_ID special-case and no assertCallerIsAncestor. The central dispatcher hardcodes its workspace gate to name === "task_show", and these tools' Zod schemas have no workspaceId field (so the blanket injection is stripped). They are not in DEFAULT_SCOPED_MCP_TOOLS but are in the shipped ORCHESTRATOR_MCP_TOOLS / ADMIN_MCP_TOOLS presets.

Exploit: a scoped agent under the orchestrator persona learns a sibling/parent/cross-workspace task ID (via task_list/task_search), then task_delete {taskId: <victim>} (permanently destroys another agent's or the human parent's work) or task_update to flip a sibling to complete/failed or rewrite its dependency DAG and budgets.

F6 — session_kill / session_resume bypass ancestry (Medium)

Location: packages/mcp/src/tools/session.ts:95 (session_resume), :178 (session_kill).

Call client.killAgent / client.resumeAgent directly with no auth context, unlike session_attach/session_send_input. session_kill is in the orchestrator preset. Exploit: cross-session SIGKILL DoS against a sibling agent or the root orchestrator; foreign session resume.

F7 — Workspaceless scoped token fails open to all workspaces (Medium)

Location: packages/mcp/src/mcp-server.ts:565-600.

The per-task cross-workspace check (task_show block) is gated on authContext.workspaceId being truthy. A task created without a workspace (a supported config) mints a scoped token with pid: "" → workspaceId: undefined, so the check is skipped and the injected workspaceId becomes "", which the backend treats as "all workspaces." Exploit: a malicious agent in a workspaceless session calls task_list (no args) → reads every task in every workspace, then task_show {taskId} cross-workspace (title/description/branch/review-notes). schedule_show (in the default scoped allowlist) resolves by ID with no workspace check. Read-only cross-workspace disclosure.

F12 — Scoped-token revocation is dead code (Low)

Location: packages/auth/src/scoped-token.ts:23,142-149.

The consuming check is wired (auth-middleware.ts:90-92), but revokeTask() is never called outside tests — no task-abort/stop flow invokes it — and the backing revokedTasks Map is in-memory only (lost on restart). A compromised agent that exfiltrated its scoped token keeps authenticating for the full 24h TTL regardless of task lifecycle.

Remediation

• Systemic fix: enforce scope centrally in the CallToolRequest dispatcher (mcp-server.ts) via a per-tool targetTaskIdArg / targetSessionIdArg descriptor so any tool that targets a task/session fails closed unless the caller is an ancestor (or self).
• Immediately, add assertCallerIsAncestor (or self-or-ancestor) to task_update, task_delete, task_resume, session_kill, session_resume, mirroring task_complete/task_start.
• F7: do not fail open on empty workspaceId — treat a scoped non-root caller with no workspace as having access to no workspace; apply the task_show membership check whenever the caller is scoped and not ROOT_TASK_ID; add a per-id membership check to schedule_show.
• F12: wire revokeTask() into task-abort/stop flows with SQLite-backed persistence (like channel-grant revocation), or remove the dead API and document the 24h window.
• Add regression tests mirroring the existing task_complete ancestor tests for each mutator.

CWEs

CWE-862 (Missing Authorization), CWE-639 (Authorization Bypass Through User-Controlled Key / IDOR), CWE-613 (Insufficient Session Expiration).