Vikunja: Link-share token reads any tenant's kanban buckets and enumerates usernames/IDs instance-wide (BOLA)

Summary

The task-collection endpoint GET /api/v1/projects/{project}/views/{view}/tasks loads the requested project view straight from the URL path without verifying the caller is authorized for it. For a link-share token, the task query is correctly pinned to the share's own project, but the view is taken from the attacker-controlled path and never re-validated against the share. A holder of a share link to any project can therefore point the endpoint at any other tenant's kanban view and receive that view's bucket records — bucket titles plus the full created_by user object (username, name, id) — for every view in the instance. The same missing pre-authorization view load also yields a project/view-ID existence oracle (404 vs. non-404) usable by link shares and ordinary authenticated users alike.

Details

Root cause is in TaskCollection.ReadAll (pkg/models/task_collection.go).

1. The view is resolved from the URL params before any permission check:

// pkg/models/task_collection.go  (ReadAll)
if tf.ProjectViewID != 0 {
    view, err = GetProjectViewByIDAndProject(s, tf.ProjectViewID, tf.ProjectID) // (URL params)
    ...
}

GetProjectViewByIDAndProject (pkg/models/project_view.go) filters only on WHERE id = ? AND project_id = ? — it performs no authorization. A valid (project, view) pair returns the view; an invalid pair returns ErrProjectViewDoesNotExist (HTTP 404).

2. For a link-share caller, the code pins the task scope to the token's own project but reuses the attacker-supplied view:

shareAuth, is := a.(*LinkSharing)
if is {
    project, err := GetProjectSimpleByID(s, shareAuth.ProjectID)   // token's project
    ...
    return getTaskOrTasksInBuckets(s, a, []*Project{project}, view, opts, filteringForBucket, tf.forceFlatTasks)
    // `view` is the foreign, attacker-controlled view — never validated against shareAuth.ProjectID
}

3. getTaskOrTasksInBuckets → GetTasksInBucketsForView (pkg/models/kanban.go) reads the buckets of that foreign view directly:

err = s.Where("project_view_id = ?", view.ID).OrderBy("position").Find(&buckets)
...
users, err := getUsersOrLinkSharesFromIDs(s, userIDs) // resolves each bucket's created_by

Each returned Bucket serializes CreatedBy *user.User as json:"created_by" (pkg/models/kanban.go), exposing the creating user's username, name, and id.

The normal (non-share) path is protected because it runs getRelevantProjectsFromCollection, which calls project.CanRead on the URL project and returns 403 before any bucket data is produced. Only the LinkSharing branch skips that gate, which is why the bucket disclosure is link-share-specific. ReadAllWeb (pkg/web/handler/read_all.go) performs no permission check of its own, so all authorization for this endpoint lives inside ReadAll.

Scope of the leak (verified against fixtures): the tasks returned inside the buckets are still constrained to the share's own project (getRawTasksForProjects filters tasks.project_id IN opts.projectIDs, derived from the share's project). So victim task contents do not leak; what leaks is the foreign view's bucket structure and, critically, the created_by user identities of arbitrary buckets instance-wide.

Introduced in v0.24.0 by the per-view kanban feature (feat(views)!: return tasks in buckets by view), which added the views/{view}/tasks endpoint and the foreign-view bucket load. The link-share branch predates it, but the two only combine into this bug from v0.24.0 onward.

Impact

A holder of a link-share token to any single project (link shares are designed to be handed out, often semi-publicly) can:

• Enumerate all project and view IDs instance-wide. IDs are sequential auto-increment integers; an invalid (project, view) pair returns 404 while a valid one returns 200, giving a reliable existence oracle across all tenants.
• Disclose the created_by user of any kanban view's buckets — username, display name, and user id. Because Vikunja auto-creates a default project with a Kanban view for every user, iterating view IDs enumerates usernames and user IDs for essentially every account on the instance.
• Disclose bucket titles of every kanban view in the instance.

This is a cross-tenant broken-object-level-authorization bypass. It does not disclose victim task contents through this path, but instance-wide username/user-id enumeration plus kanban structure disclosure is a meaningful PII and reconnaissance exposure that defeats the tenant isolation the permission model is meant to enforce. The ID-enumeration oracle additionally works for any ordinary authenticated user (invalid combo → 404 vs. inaccessible combo → 403), because the view is loaded before the CanRead check.

Proof of Concept

Prerequisites: link sharing enabled (default), a share link to any project (call it project A), and any second project B (e.g. another tenant's) with a kanban view V_B.

1. Authenticate the share link to obtain a share JWT: POST /api/v1/shares/{shareHash}/auth
2. Using that token, request a foreign kanban view's tasks: GET /api/v1/projects/{B}/views/{V_B}/tasks
3. The 200 response is a list of Bucket objects belonging to view V_B — each with title and a populated created_by (username, name, id) — even though the token has no relationship to project B.
4. Iterate {B}/{V_B} over sequential integers: valid pairs return bucket lists (harvest usernames/ids), invalid pairs return 404.

Verified locally against the model fixtures: a LinkSharing{ProjectID: 1} (project 1 owned by user 1) calling TaskCollection{ProjectID: 2, ProjectViewID: 8}.ReadAll (project 2 owned by user 3) returns project 2's buckets ("testbucket4 - other project", "testbucket40") with their created_by user objects populated, and no error — while a nonexistent view id returns ErrProjectViewDoesNotExist.

Recommended Fix

In TaskCollection.ReadAll, before the view is loaded, pin the requested project to the link share's own project so any foreign view resolves to a 404 instead of leaking:

if shareAuth, is := a.(*LinkSharing); is {
    tf.ProjectID = shareAuth.ProjectID
}

This is consistent with the existing behavior that already forces the task query to shareAuth.ProjectID, and it closes both the bucket disclosure and the share-token half of the enumeration oracle. As defence-in-depth, resolve the view's authorization against the authenticated caller for every auth type (not just link shares), so the endpoint never loads a view the caller cannot read — closing the 404-vs-403 existence oracle for ordinary users as well.