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multica/server/cmd
Bohan Jiang 607209de7c fix(avatar): serve avatars through a signed endpoint on private buckets (MUL-5393) (#6088)
* fix(avatar): serve avatars through a signed endpoint on private buckets (MUL-5393)

Avatar uploads persisted the raw storage object URL into `avatar_url`. On a
deployment whose bucket is private and has no public CDN domain (S3 with Block
Public Access, R2, MinIO) that URL is a guaranteed 403 in the browser:
ATTACHMENT_DOWNLOAD_MODE only ever applied to the attachment download
endpoint, so every user / agent / squad / workspace avatar rendered broken
even though the upload itself succeeded.

Resolve at read time instead of at upload time. What is persisted stays the
durable object reference, so nothing with a TTL is ever written to the
database and avatars saved by an older build are fixed without a backfill.
What is served is `/api/avatars/<sig>/<key>`, a stable URL the server resolves
per request through the deployment's existing storage download policy
(presigned redirect, CloudFront-signed redirect, or proxied body).

The endpoint is unauthenticated and the HMAC signature is the credential: the
session cookie is SameSite=Strict, so an auth-gated URL cannot be a native
<img src> from Desktop, a mobile webview, or a split-origin self-hosted web
app. The signature covers the storage key and only image extensions resolve,
so an avatar_url pointed at a private document cannot launder it into a
publicly fetchable URL.

Deployments that already work are untouched: a public CDN domain without
per-request signing, and the local-disk backend whose /uploads/* route is
public, both keep returning the raw URL.

Fixes #6024

Co-authored-by: multica-agent <github@multica.ai>

* fix(avatar): only publish avatar-class objects through the signed endpoint (MUL-5393)

Review found that being able to name a storage object was treated as
permission to publish it. `ownedStorageKey` proved only that a URL came from
this deployment's storage, and every image-shaped key was then signed — while
the avatar update endpoints accepted any raw storage URL. A caller who had
seen a private image attachment's URL could submit it as their own avatar, and
the unauthenticated endpoint would keep re-signing it indefinitely. A user
avatar propagates to every workspace that user belongs to, so the leak crossed
workspace boundaries.

Add the missing authorization rule: an object is serveable as an avatar only
when it is avatar-class — a standalone image upload not attached to an issue,
comment, chat session, chat message, or task. The check resolves the backing
attachment row from the id UploadFile embeds in the object filename, so it
needs no lookup by URL and no new index.

It is enforced on both sides. The write side rejects such a value with 403
before anything is stored; the read side re-checks per request, which is what
makes the guarantee hold for rows written before this existed and revokes the
URL if an object is later bound to a comment or chat.

Scope is the `workspaces/` namespace — the only place that can hold content
belonging to someone other than whoever is setting the avatar, covering both
uploads and channel media ingest. Keys elsewhere (the per-user standalone
namespace, or objects an operator placed in the bucket) stay usable, which
keeps the documented "an explicit avatar_url is preserved" contract intact.

Uploader identity is deliberately not part of the rule: duplicating an agent
legitimately reuses the source agent's avatar object, which a different admin
may have uploaded. Publishing someone else's unbound image would require
knowing its URL, and unbound rows appear in no listing endpoint.

Also clamp the 302's cache lifetime to half the signed URL's own TTL (0 ->
no-store). ATTACHMENT_DOWNLOAD_URL_TTL takes any positive duration, so the
fixed 60s could outlive the target it pointed at on a short-TTL deployment.

Co-authored-by: multica-agent <github@multica.ai>

---------

Co-authored-by: Bohan-J <bohan@devv.ai>
Co-authored-by: multica-agent <github@multica.ai>
2026-07-29 15:39:18 +08:00
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