CVE-2026-91983: Vikunja API Token Scopes Bypassed Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-9rg3-v78m-26q8",
  "modified" : "2026-10-09T20:51:52Z",
  "published" : "2026-10-09T20:51:52Z",
  "aliases" : [ "CVE-2026-91983" ],
  "summary" : "Vikunja: API token scopes bypassed via task expand parameter (comments, reactions, time entry counts)",
  "details" : "## Summary\n\nAPI token permission checks only match the HTTP method and route path. The `expand` query parameter on task read endpoints embeds data from other permission groups (task comments, reactions, time entry counts) without checking whether the token holds those scopes. A token scoped only to `tasks` read permissions can therefore read task comments and reactions it was explicitly not granted.\n\n## Details\n\n`models.CanDoAPIRoute` (`pkg/models/api_routes.go`, ~line 440) authorises API tokens purely by comparing `method` and `c.Path()` against the routes stored for each granted permission group. The query string is never consulted.\n\nThe task read endpoints accept `expand`:\n\n- `GET /api/v1/tasks/:task`, `GET /api/v1/tasks` (`pkg/models/tasks.go` ReadOne/ReadAll, `pkg/models/task_collection.go`)\n- `GET /api/v2/tasks/:id`, `GET /api/v2/tasks`, `GET /api/v2/projects/:project/tasks` (`pkg/routes/api/v2/tasks.go`, `task_collection.go`)\n\nAccepted values include `comments`, `comment_count`, `reactions`, `time_entries_count`. `addMoreInfoToTasks` (`pkg/models/tasks.go`, ~line 683) loads and embeds that data. At that layer only a `web.Auth` (the plain owner user, resolved by `auth.GetAuthFromClaims`) is available, so the token's scopes cannot be enforced there either.\n\nResult: the `tasks_comments`, `reactions`, and `time_entries` permission groups are advisory for any data reachable through a task expansion.\n\n## Impact\n\nA holder of an API token scoped to `tasks: [read_one]` or `tasks: [read_all]` (or `projects_views_tasks: [read_all]`) can read the full bodies of task comments, all reactions, and time entry counts on every task the token owner can access, despite `GET /api/v1|v2/tasks/:task/comments` and the reactions endpoints correctly returning 401 for the same token.\n\nThe leak is limited to data the token owner can already see, and is read-only. The realistic victim is a user who grants a narrowly scoped token to a third-party integration expecting comments to stay private.\n\n## Proof of Concept\n\nAgainst the test fixtures (`pkg/db/fixtures/api_tokens.yml`, token 1 has `{\"tasks\":[\"read_all\",\"update\"]}`, plaintext `tk_2eef46f40ebab3304919ab2e7e39993f75f29d2e`):\n\n```\nGET /api/v2/tasks/1/comments\nAuthorization: Bearer tk_2eef46f40ebab3304919ab2e7e39993f75f29d2e\n-> 401 {\"code\":11,\"message\":\"missing, malformed, expired or otherwise invalid token provided\"}\n\nGET /api/v2/tasks?expand=comments&filter=id%3D1\nAuthorization: Bearer tk_2eef46f40ebab3304919ab2e7e39993f75f29d2e\n-> 200, response items[0].comments contains the full comment objects\n```\n\nSame behaviour with `expand=reactions`, and on the v1 endpoints `GET /api/v1/tasks?expand=comments` / `GET /api/v1/tasks/1?expand=comments`. Any user-created token with only `tasks` read permissions reproduces this.\n\n## Recommended Fix\n\nEnforce expansions in the single existing choke point, `models.CanDoAPIRoute`: after the method/path match succeeds, read `c.QueryParams()[\"expand\"]` and require the token to hold the owning group's read permission for each value, e.g.\n\n- `comments`, `comment_count` -> `tasks_comments.read_all`\n- `reactions` -> `reactions.read_all`\n- `time_entries_count` -> `time_entries.read_all`\n\n(`subtasks`, `buckets`, `is_unread` are task-level data and need nothing extra.) Doing this in the middleware covers both v1 and v2 without handler or model changes. Add a table-driven test alongside `pkg/webtests/api_token_method_matching_test.go` asserting a `tasks`-only token gets 401 with `expand=comments` and 200 once `tasks_comments.read_all` is added.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "code.vikunja.io/api"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.0.0"
      }, {
        "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-9rg3-v78m-26q8"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-91983"
  }, {
    "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-api-token-scope-bypass-via-expand-parameter"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-863" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-09T20:51:52Z",
    "nvd_published_at" : null
  }
}