Vikunja: Read-only project members can obtain any link share's access hash via the single-share read endpoint (v1 and v2) and escalate to the share's permission level

{
  "id": "f69f8306c4d697879b6dfc6333e8fd840dc10664d79226b4b23ff0eec74dbd04",
  "url": "https://osv.dev/vulnerability/GHSA-qfwc-vx6f-3g6g",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Vikunja: Read-only project members can obtain any link share's access hash via the single-share read endpoint (v1 and v2) and escalate to the share's permission level",
    "summary": "## Summary A user who has only read permission on a project can call the single link-share read endpoint and receive the share's `hash` field — the secret credential that the anonymous `POST /shares/{share}/auth` endpoint exchanges for a link-share JWT carrying the share's permission (read / read-write / admin). A read-only member can therefore mint a write- or admin-level token for the project and perform writes they are not entitled to, while their own user token is correctly refused. This is the remaining variant of the link-share hash disclosure class: GHSA-8hp8-9fhr-pfm9 fixed the list endpoint (ReadAll now requires project admin) but the single-read endpoint's gate was never aligned. Verified on Vikunja 2.5.0; the weak gate has existed since the endpoint, so earlier versions are likely affected too. ## Details Affected endpoints (both verified with a complete chain): - v1: `GET /api/v1/projects/{project}/shares/{share}` - v2: `GET /api/v2/projects/{project}/shares/{share}` Permission gate: `LinkSharing.CanRead` (`pkg/models/link_sharing_permissions.go`) delegates to `project.CanRead(s, a)` — i.e. any user with read access to the project passes. The response serializes the `hash` field (`pkg/models/link_sharing.go`, field tag `json:\"hash\"`), which is the share's bearer credential. The share `password` field is correctly cleared before returning, but the hash is not restricted. Inconsistent with the sibling list endpoint `GET /projects/{project}/shares` (`LinkSharing.Read",
    "evidence": "osv:GHSA-qfwc-vx6f-3g6g",
    "products": [
      {
        "name": {
          "value": "code.vikunja.io/api",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "2.6.0",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "GHSA-qfwc-vx6f-3g6g",
      "aliases": [
        "CVE-2026-91985",
        "GHSA-qfwc-vx6f-3g6g",
        "GO-2026-6747"
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "2.6.0"
                }
              ]
            }
          ],
          "package": {
            "name": "code.vikunja.io/api",
            "purl": "pkg:golang/code.vikunja.io/api",
            "ecosystem": "Go"
          },
          "database_specific": {
            "source": "https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-qfwc-vx6f-3g6g/GHSA-qfwc-vx6f-3g6g.json",
            "last_known_affected_version_range": "<= 2.5.0"
          }
        }
      ],
      "modified": "2026-10-19T21:40:54.885980828Z",
      "provider": "osv",
      "references": [
        "https://github.com/go-vikunja/vikunja/security/advisories/GHSA-qfwc-vx6f-3g6g",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-91985",
        "https://github.com/go-vikunja/vikunja/pull/3688",
        "https://github.com/go-vikunja/vikunja/commit/077dc4de79ce6f1ab59215a2c7bf9b30423685f2",
        "https://github.com/go-vikunja/vikunja",
        "https://github.com/go-vikunja/vikunja/releases/tag/v2.6.0",
        "https://www.vulncheck.com/advisories/vikunja-before-2.6.0-privilege-escalation-via-link-share-hash"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-200",
          "CWE-639",
          "CWE-862"
        ],
        "severity": "HIGH",
        "github_reviewed": true,
        "nvd_published_at": null,
        "github_reviewed_at": "2026-10-09T20:51:29Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-91985",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/database_specific/severity"
  },
  "source_id": "core-osv",
  "document_id": 376755,
  "published_at": "2026-10-09T20:51:29+00:00"
}