CVE-2026-91983: Vikunja before 2.6.0 API Token Scope Bypass via expand Parameter

Summary

API 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.

Details

models.CanDoAPIRoute (pkg/models/apiroutes.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.

The task read endpoints accept expand:

- GET /api/v1/tasks/:task, GET /api/v1/tasks (pkg/models/tasks.go ReadOne/ReadAll, pkg/models/taskcollection.go) - GET /api/v2/tasks/:id, GET /api/v2/tasks, GET /api/v2/projects/:project/tasks (pkg/routes/api/v2/tasks.go, taskcollection.go)

Accepted values include comments, commentcount, reactions, timeentriescount. 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.

Result: the taskscomments, reactions, and timeentries permission groups are advisory for any data reachable through a task expansion.

Impact

A holder of an API token scoped to tasks: [readone] or tasks: [readall] (or projectsviewstasks: [readall]) 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.

The 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.

Proof of Concept

Against the test fixtures (pkg/db/fixtures/apitokens.yml, token 1 has {"tasks":["readall","update"]}, plaintext tk2eef46f40ebab3304919ab2e7e39993f75f29d2e):

GET /api/v2/tasks/1/comments Authorization: Bearer tk2eef46f40ebab3304919ab2e7e39993f75f29d2e -> 401 {"code":11,"message":"missing, malformed, expired or otherwise invalid token provided"}

GET /api/v2/tasks?expand=comments&filter=id%3D1 Authorization: Bearer tk2eef46f40ebab3304919ab2e7e39993f75f29d2e -> 200, response items[0].comments contains the full comment objects

Same 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.

Recommended Fix

Enforce 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.

- comments, commentcount -> taskscomments.readall - reactions -> reactions.readall - timeentriescount -> timeentries.readall

(subtasks, buckets, isunread 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/apitokenmethodmatchingtest.go asserting a tasks-only token gets 401 with expand=comments and 200 once taskscomments.readall is added.