GHSA-w2ch-4xgr-22ww: Vikunja Task Relation Deletion Does Not Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-w2ch-4xgr-22ww",
  "modified" : "2026-10-09T20:54:52Z",
  "published" : "2026-10-09T20:54:52Z",
  "aliases" : [ ],
  "summary" : "Vikunja: Task relation deletion does not check read access to the other task, allowing cross-project relation removal",
  "details" : "## Summary\n\nDeleting a task relation only requires write access on the base task. Unlike relation creation, it does not verify that the caller can read the other task. A user with write access to one project can therefore delete relations whose other end lives in a project they have no access to. Since the delete removes both the forward and inverse rows, the relation also disappears for the other project's members.\n\n## Details\n\n`TaskRelation.CanCreate` (`pkg/models/task_relation_permissions.go:32-52`) requires write access on `TaskID` **and** read access on `OtherTaskID`.\n\n`TaskRelation.CanDelete` (`pkg/models/task_relation_permissions.go:25-29`) only checks `Task{ID: rel.TaskID}.CanUpdate(s, a)`; `OtherTaskID` is never authorized.\n\n`TaskRelation.Delete` (`pkg/models/task_relation.go:314-354`) then deletes both the `(task_id, other_task_id, kind)` row and its inverse, so the relation is removed from the far task as well.\n\nAffects `DELETE /api/v1/tasks/{id}/relations/{kind}/{otherTaskId}` and the equivalent v2 endpoint.\n\n## Impact\n\nLow, integrity only. An authenticated user holding write permission on a shared project can remove task relations that link into projects they cannot read. No data is disclosed and no privilege is gained; the attacker cannot recreate the relation. The far project's owner sees the relation vanish without indication of who removed it.\n\nPreconditions: the attacker has write access to a project containing a task that is already related to a task in a project they cannot access.\n\n## Proof of Concept\n\n1. As `owner`, create project `P_near` with task `near` and project `P_far` with task `far`.\n2. Share `P_near` with `attacker` at write permission (`permission: 1`). Do not share `P_far`.\n3. As `owner`: `PUT /api/v1/tasks/{near}/relations` with `{\"other_task_id\": far, \"relation_kind\": \"related\"}` -> 200.\n4. As `attacker`: `GET /api/v1/tasks/{far}` -> 403 (confirms no access).\n5. As `attacker`: `PUT /api/v1/tasks/{near}/relations` with the same body -> 403 (create path is enforced).\n6. As `attacker`: `DELETE /api/v1/tasks/{near}/relations/related/{far}` -> 200 `\"Successfully deleted.\"`\n7. As `owner`: `GET /api/v1/tasks/{far}` -> `related_tasks` is now empty.\n\nReproduced against `vikunja/vikunja:2.5.0`.\n\n## Recommended Fix\n\nMake `CanDelete` mirror `CanCreate`: after checking `CanUpdate` on the base task, also require `CanRead` on `OtherTaskID`.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/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" : "0.9"
      }, {
        "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-w2ch-4xgr-22ww"
  }, {
    "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-285", "CWE-862" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-09T20:54:52Z",
    "nvd_published_at" : null
  }
}