CVE-2026-33312: Read-only Vikunja users can delete project background images via broken object-level authorization

Summary

The DELETE /api/v1/projects/:project/background endpoint checks CanRead permission instead of CanUpdate, allowing any user with read-only access to a project to permanently delete its background image.

Details

The RemoveProjectBackground handler (pkg/modules/background/handler/background.go) reuses checkProjectBackgroundRights, a helper originally written for the read-only GetProjectBackground endpoint. This helper only verifies CanRead permission. In contrast, the handler for setting a background (setBackgroundPreparations) correctly checks CanUpdate.

As a result, destructive write operations (deleting the background file from storage and clearing the project's backgroundfileid and backgroundblurhash fields) are gated behind a read-only permission check.

Impact

A user with read-only access to a project — via direct sharing, team membership, link share tokens with read permission, or read-scoped API tokens — can permanently delete the project's background image. The background file is removed from storage and cannot be recovered. This constitutes unauthorized data destruction.

Reproduction

1. User A creates a project and sets a background image. 2. User A shares the project with User B with read-only permission. 3. User B sends: DELETE /api/v1/projects/{projectid}/background with a valid auth token. 4. The request succeeds. The background image is permanently deleted.

References

- pkg/modules/background/handler/background.go — RemoveProjectBackground (line 416), checkProjectBackgroundRights (line 304), setBackgroundPreparations (line 106) - pkg/routes/routes.go line 665 — route registration

Credits

This vulnerability was found using GitHub Security Lab Taskflows.