Summary

When the Vikunja API returns tasks, it populates the related_tasks field with full task objects for all related tasks without checking whether the requesting user has read permission on those tasks' projects. An authenticated user who can read a task that has cross-project relations will receive full details (title, description, due dates, priority, percent completion, project ID, etc.) of tasks in projects they have no access to.

Details

The vulnerability is in addRelatedTasksToTasks() at pkg/models/tasks.go:496-548. This function is called by addMoreInfoToTasks() (line 773) during every task read operation — both project task listings (GET /api/v1/projects/{id}/views/{id}/tasks) and single task reads (GET /api/v1/tasks/{id}).

The function fetches all related tasks directly from the database without any permission filtering:

// pkg/models/tasks.go:496-548
func addRelatedTasksToTasks(s *xorm.Session, taskIDs []int64, taskMap map[int64]*Task, a web.Auth) (err error) {
    relatedTasks := []*TaskRelation{}
    err = s.In("task_id", taskIDs).Find(&relatedTasks)
    // ...
    fullRelatedTasks := make(map[int64]*Task)
    err = s.In("id", relatedTaskIDs).Find(&fullRelatedTasks)  // Line 514: NO permission check
    // ...
    for _, rt := range relatedTasks {
        // Directly adds to response without checking if user can read the related task
        taskMap[rt.TaskID].RelatedTasks[rt.RelationKind] = append(
            taskMap[rt.TaskID].RelatedTasks[rt.RelationKind], otherTask)
    }
}

The a web.Auth parameter is received but only used for determining favorites (line 519), never for access control on the related tasks themselves.

In contrast, addBucketsToTasks() (line 550+) in the same file correctly filters enrichment data by calling getAllRawProjects(s, a, ...) to scope results to projects the requesting user can access.

While task relation creation properly enforces authorization (task_relation_permissions.go:32-52 checks write access on the base task and read access on the other task), the relation display path does not re-check permissions for the current reader. This means a privileged user can create a relation that then leaks data to all other users who can read the base task.

PoC

Setup: Two users (User A, User B), two projects (Project-Shared, Project-Private).

• User A has access to both projects.
• User B has access only to Project-Shared.
• Task 1 exists in Project-Shared, Task 2 exists in Project-Private.

Step 1: User A creates a relation between the two tasks

# As User A (who has access to both projects)
curl -X PUT "http://localhost:3456/api/v1/tasks/TASK1_ID/relations" \
  -H "Authorization: Bearer USER_A_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"other_task_id": TASK2_ID, "relation_kind": "related"}'

Expected: 201 Created (User A has write on Task 1, read on Task 2).

Step 2: User B reads tasks from the shared project

# As User B (who has NO access to Project-Private)
curl "http://localhost:3456/api/v1/projects/PROJECT_SHARED_ID/views/VIEW_ID/tasks" \
  -H "Authorization: Bearer USER_B_TOKEN"

Expected: Task 1 should be returned, but related_tasks should NOT include Task 2.

Actual result: The response includes Task 1 with the related_tasks field containing the full Task 2 object, including its title, description, due_date, priority, percent_done, project_id, and other metadata — despite User B having no access to Project-Private.

Impact

• Information disclosure: Any authenticated user can read the full metadata of tasks in projects they do not have access to, as long as a relation exists from a task they can read.
• Leaked fields include: title, description, due dates, start dates, priority, percent completion, project ID, hex color, task index, done status, repeat configuration, cover image attachment ID, and creation/update timestamps.
• Project structure disclosure: The project_id field reveals the existence and IDs of private projects.
• No user interaction required: Once a privileged user creates a cross-project relation (which is intentionally allowed), the data leak is automatic for all readers of the base task.
• Blast radius: Affects all Vikunja instances with cross-project task relations. In multi-tenant or team environments where projects have different access scopes, this undermines project-level access control.

Recommended Fix

Filter related tasks by the requesting user's read permissions before adding them to the response. In addRelatedTasksToTasks(), after fetching full task objects, check that the user can read each related task's project:

func addRelatedTasksToTasks(s *xorm.Session, taskIDs []int64, taskMap map[int64]*Task, a web.Auth) (err error) {
    relatedTasks := []*TaskRelation{}
    err = s.In("task_id", taskIDs).Find(&relatedTasks)
    if err != nil {
        return
    }

    var relatedTaskIDs []int64
    for _, rt := range relatedTasks {
        relatedTaskIDs = append(relatedTaskIDs, rt.OtherTaskID)
    }

    if len(relatedTaskIDs) == 0 {
        return
    }

    fullRelatedTasks := make(map[int64]*Task)
    err = s.In("id", relatedTaskIDs).Find(&fullRelatedTasks)
    if err != nil {
        return
    }

    // Filter related tasks by user's read permission
    allowedProjectIDs := make(map[int64]bool)
    checkedProjectIDs := make(map[int64]bool)
    for _, t := range fullRelatedTasks {
        if checkedProjectIDs[t.ProjectID] {
            continue
        }
        checkedProjectIDs[t.ProjectID] = true
        p := &Project{ID: t.ProjectID}
        canRead, _, err := p.CanRead(s, a)
        if err != nil {
            log.Errorf("Could not check project read permission: %v", err)
            continue
        }
        if canRead {
            allowedProjectIDs[t.ProjectID] = true
        }
    }

    taskFavorites, err := getFavorites(s, relatedTaskIDs, a, FavoriteKindTask)
    if err != nil {
        return err
    }

    for _, rt := range relatedTasks {
        task, has := fullRelatedTasks[rt.OtherTaskID]
        if !has {
            continue
        }
        // Skip related tasks the user cannot access
        if !allowedProjectIDs[task.ProjectID] {
            continue
        }
        fullRelatedTasks[rt.OtherTaskID].IsFavorite = taskFavorites[rt.OtherTaskID]
        otherTask := &Task{}
        err = copier.Copy(otherTask, fullRelatedTasks[rt.OtherTaskID])
        if err != nil {
            log.Errorf("Could not duplicate task object: %v", err)
            continue
        }
        otherTask.RelatedTasks = nil
        taskMap[rt.TaskID].RelatedTasks[rt.RelationKind] = append(
            taskMap[rt.TaskID].RelatedTasks[rt.RelationKind], otherTask)
    }

    return
}

This checks project-level read permission once per unique project ID (cached in allowedProjectIDs) and skips related tasks from projects the user cannot access.