Claudio Ortolina
94 lines
4.0 KiB
Markdown
94 lines
4.0 KiB
Markdown
---
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id: ML-5
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title: Document Last.fm OAuth callback trust boundary
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status: Done
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assignee:
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- Claudio Ortolina
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created_date: "2026-04-20 08:48"
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updated_date: "2026-04-24 07:04"
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labels: []
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dependencies: []
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references:
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- "https://github.com/cloud8421/music_library/issues/178"
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priority: low
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---
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## Description
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<!-- SECTION:DESCRIPTION:BEGIN -->
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_GitHub: created 2026-04-16 · updated 2026-04-16_
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## Summary
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`GET /auth/last_fm/callback?token=X` is unauthenticated by design (OAuth flow requires it) but triggers storage of a session key in the encrypted secrets table. The trust model is not captured in comments, so future reviewers may re-flag it.
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## Evidence
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- `lib/music_library_web/controllers/last_fm_controller.ex:7`
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- `lib/music_library_web/router.ex:55` (route outside `:logged_in` pipe)
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Attack surface: cannot forge a valid token (Last.fm validates), but a malicious caller could consume `LastFm.get_session/1` rate-limit quota. Acceptable given the threat model.
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## Fix
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Add a short `@moduledoc` or inline comment in `last_fm_controller.ex` explaining:
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1. Why this endpoint is unauthenticated (OAuth flow cannot carry session cookies across the Last.fm redirect)
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2. What protects it (Last.fm-validated token, rate limiter, single-user scope)
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3. What the failure modes look like (invalid token → store nothing, error logged)
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Similarly in `router.ex:55`, a brief comment noting the deliberate exception from `:logged_in`.
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<!-- SECTION:DESCRIPTION:END -->
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## Acceptance Criteria
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<!-- AC:BEGIN -->
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- [x] #1 last_fm_controller.ex has a comment documenting the trust boundary
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- [x] #2 router.ex:55 notes the deliberate pipeline exemption
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<!-- AC:END -->
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## Implementation Plan
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<!-- SECTION:PLAN:BEGIN -->
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## Implementation Plan
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1. **`lib/music_library_web/controllers/last_fm_controller.ex`** — add `@moduledoc` covering:
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- Why unauthenticated: OAuth redirect from Last.fm cannot carry session cookies across third-party redirect.
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- What protects it: Last.fm validates the `token` via `LastFm.get_session/1`; the call is rate-limited at the `Req` layer (`Req.RateLimiter`, 500ms/req); this is a single-user deployment.
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- Failure mode: invalid/forged tokens cause `get_session/1` to return `{:error, _}`; nothing is written to `secrets`, an error toast is shown, and the Req-layer error is logged.
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2. **`lib/music_library_web/router.ex:55`** — add a brief comment above the route noting deliberate exemption from the `:logged_in` pipeline (with pointer to the controller moduledoc for details).
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3. Run `mise run dev:precommit` to verify formatting, credo (including `ModuleDoc` rule), sobelow, and tests still pass.
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## Notes
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- Controllers are excluded from the `Credo.Check.Readability.ModuleDoc` regex in `.credo.exs`, so `@moduledoc` here is optional for lint purposes — using it is a deliberate choice for this security-sensitive module.
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- No test changes required (documentation-only).
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<!-- SECTION:PLAN:END -->
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## Final Summary
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<!-- SECTION:FINAL_SUMMARY:BEGIN -->
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## Summary
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Documented the Last.fm OAuth callback trust boundary in two places:
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- **`lib/music_library_web/controllers/last_fm_controller.ex`** — added `@moduledoc` explaining why the endpoint is unauthenticated (OAuth redirect from Last.fm cannot carry session cookies), what protects it (Last.fm-validated token, `Req.RateLimiter` bounding quota burn, single-user deployment), and the failure mode (invalid token → nothing stored, error toast shown).
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- **`lib/music_library_web/router.ex`** — added a comment above the `/auth/last_fm/callback` route noting the deliberate exemption from the `:logged_in` pipeline, with a pointer to the controller moduledoc for the full trust boundary.
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## Verification
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`mise run dev:precommit` passes: credo, sobelow, format, translations, unused deps, and tests (827 passed, 43 doctests).
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## Notes
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Controllers are excluded from the `Credo.Check.Readability.ModuleDoc` regex in `.credo.exs`, so the `@moduledoc` here is a deliberate choice for a security-sensitive module, not a lint requirement.
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<!-- SECTION:FINAL_SUMMARY:END -->
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