Accept optional visible_project_ids filter on project list endpoint (defense-in-depth for cloud private projects)

[jope-bm]

Context

This is a defense-in-depth improvement requested by basic-memory-cloud. It's tracked in the cloud side as part of the discussion on basicmachines-co/basic-memory-cloud#518 and is independent of the is_everyone refactor happening there.

Basic Memory Cloud has a concept of per-project visibility in team workspaces — some projects are visible to all workspace members, others are invite-only. Today, the source of truth for visibility lives in the cloud database (project_permission table), and filtering is enforced at the cloud proxy layer by rewriting the project list response after it comes back from basic-memory.

This is fragile. The proxy must catch every code path that could return project data. We've already had one incident (basicmachines-co/basic-memory-cloud#544) where a trailing-slash variant of /v2/projects/ bypassed the filtering route and exposed other users' private projects. The fix was to add defense-in-depth in the catch-all route, but filtering-at-proxy is still a single layer with a wide attack surface: admin tools, backup flows, future endpoints, or any new direct-DB queries all need to re-implement the filter or risk leaking.

Proposal

Add an optional visible_project_ids filter parameter to basic-memory's project list endpoint (GET /v2/projects/). When provided, basic-memory applies the filter in SQL and only returns projects whose IDs are in the set.

GET /v2/projects/?visible_project_ids=uuid1,uuid2,uuid3

The cloud service would:

1. Compute the set of projects the current user can see (from project_permission + project_share tables)
2. Pass the resulting set as the visible_project_ids parameter on every call into basic-memory
3. basic-memory enforces the filter at query time, not response-rewrite time

Why This Matters

Defense-in-depth: the filter lives in the SQL query, not a response rewriter. URL quirks, routing bugs, or new code paths in the cloud proxy can't bypass it — if the filter isn't passed, basic-memory returns an empty list by convention (or the full list if the parameter is absent, depending on the design choice below).

No migration needed: cloud remains the authoritative source of visibility rules. Nothing moves between databases. basic-memory just gains the ability to honor a filter passed by its caller.

Local-mode compatibility: the parameter is optional. When basic-memory runs in single-user local mode, the parameter is simply not passed and behavior is unchanged — all projects are visible. No new concepts leak into the local product.

Design Questions

1. Parameter semantics when absent:

   • Option A: absent parameter → return all projects (current behavior, preserves local mode)
   • Option B: absent parameter → return empty list (fail-closed, stronger guarantee for cloud but breaks local mode)
   • Recommendation: Option A. The filter is a voluntary restriction a caller can apply. Cloud always passes it; local never does.

2. Parameter format:

   • Comma-separated UUIDs in a query param: simple, works with GET
   • POST body with a JSON array: cleaner for large sets, but changes the endpoint shape
   • Recommendation: comma-separated for simplicity. Workspaces with hundreds of projects are rare, and URL length limits are generous enough.

3. Scope of endpoints:

   • Just GET /v2/projects/ (project list)
   • Also GET /v2/projects/{id}/... endpoints (defense against resolving an ID the user can't see)
   • Recommendation: start with project list. The individual-project endpoints already require a specific ID that the caller provides, so the attack surface is smaller. If cloud decides to add per-project filtering later, the same mechanism extends naturally.

Related

• basicmachines-co/basic-memory-cloud#518 — broader is_everyone refactor discussion
• basicmachines-co/basic-memory-cloud#544 — trailing-slash bypass that motivated this

Not in Scope

• Moving the project_permission table or visibility rules themselves into basic-memory's tenant DB. That's a larger migration with real costs around the local/self-hosted story and is tracked separately.
• Adding per-user identity to basic-memory's API contract beyond what already flows through headers.

[claude]

Claude finished @jope-bm's task in 1m 1s —— View job

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Triage Complete

• Analyze issue
• Explore relevant code
• Check for duplicates
• Add labels and triage summary

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Issue Classification

Attribute	Assessment
Type	Enhancement / Security improvement
Priority	Medium — not a new vulnerability (cloud proxy already filters), but closes a real bypass surface
Component	API (GET /v2/projects/), Cloud
Complexity	Medium

Labels added: enhancement, cloud

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Triage Summary

This is a well-scoped, clearly motivated enhancement. A real bypass incident (trailing-slash on /v2/projects/) already occurred (basicmachines-co/basic-memory-cloud#544), making this a credible defense-in-depth ask rather than speculative hardening.

Implementation scope (based on code review):

The change touches two main areas:

1. GET /v2/projects/ endpoint (src/basic_memory/api/v2/routers/project_router.py:41): Accept an optional visible_project_ids: list[str] | None = Query(None) parameter and pass it through to the service layer.

2. ProjectService.list_projects() (src/basic_memory/services/project_service.py:110): Accept the optional filter and apply it in the query via the repository. No SQL migration needed — this is a WHERE-clause filter on existing external_id column.

There's also a legacy route alias at src/basic_memory/api/app.py:107 (/projects/projects) that imports and reuses the same list_projects handler — it would automatically pick up the new query parameter once the handler is updated.

Design choices confirmed:

• Option A (absent → return all) is correct — preserves local mode, cloud always passes the filter
• Comma-separated UUIDs in query string is reasonable for the expected set sizes
• Scope is just the list endpoint for now (individual-project routes have smaller attack surface)

No duplicates found.