CVE-2026-55064: Medium severity go/code.vikunja.io/api vulnerability

Summary

The fix for CVE-2026-35595 (project re-parenting privilege escalation) only gates reparent operations when parentprojectid > 0. A user with Write (but not Admin) permission on a shared child project can detach it from its parent by sending parentprojectid: 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) go // 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 parentprojectid 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/projectpermissions.go (line 145) go if p.ParentProjectID != 0 && p.ParentProjectID != ol.ParentProjectID { // reparent permission check -- SKIPPED when ParentProjectID == 0 }

File: pkg/models/project.go (line 1065) go colsToUpdate := []string{ "title", "isarchived", "identifier", "hexcolor", "parentprojectid", // <-- 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 "parentprojectid": 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 parentprojectid = 0 because "parentprojectid" 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 parentprojectid (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: bash TOKEN=$(curl -s -X POST http://localhost:3456/api/v1/login \ -H 'Content-Type: application/json' \ -d '{"username":"usera","password":"UserAPassword1!"}' | jq -r '.token')

2. Verify attacker does NOT have Admin (delete should return 403): bash curl -s -o /dev/null -w '%{httpcode}' -X DELETE http://localhost:3456/api/v1/projects/4 \ -H "Authorization: Bearer $TOKEN" Expected: 403

3. Exploit -- detach project from parent: bash 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","parentprojectid":0}'

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

Evidence (3 independent runs)

| Run | parentprojectid BEFORE | DELETE attempt (proves no Admin) | parentprojectid AFTER | Result | |-----|--------------------------|----------------------------------|-------------------------|--------| | 1 | 3 (child of project 3) | HTTP 403 Forbidden | null (detached to root) | CONFIRMED | | 2 | 3 (child of project 3) | HTTP 403 Forbidden | null (detached to root) | CONFIRMED | | 3 | 3 (child of project 3) | HTTP 403 Forbidden | null (detached to root) | CONFIRMED |

Note: "null (detached to root)" means parentprojectid 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 parentprojectid 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.