Vikunja: Unbounded image decode on avatar and project-background uploads enables decode/resize amplification

{
  "id": "6f0d8ffc90aefeb82492f3773c233abc8264019285681e9dd87a74d772ba7303",
  "url": "https://osv.dev/vulnerability/GHSA-4vh2-39rq-rq8j",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Vikunja: Unbounded image decode on avatar and project-background uploads enables decode/resize amplification",
    "summary": "### Summary The 50-megapixel decode guard exists only on the task-attachment preview path. Avatar and project-background uploads decode uploaded images with no pixel cap. Worse, the avatar resize fixes the output height at 1024 and derives the width from the aspect ratio, so a tiny extreme-aspect-ratio PNG expands to an enormous output image — an input-side pixel cap would not catch it. ### Details The only `maxPixels` check (50MP) is in `TaskAttachment.GetPreview` (`pkg/models/task_attachment.go` ~lines 332-338). No such check guards: - avatar upload: `pkg/modules/avatar/upload/upload.go` (~lines 82, 137, 141) - project background: `pkg/modules/background/handler/background.go` (~lines 174, 269, 289) `imaging.Resize(img, 0, 1024, imaging.Lanczos)` (upload.go ~line 141) fixes height=1024 and derives width from the aspect ratio: a 20000x10 input yields a ~2,048,000 x 1024 output (~2.1 billion pixels), so a few-hundred-byte file drives huge CPU and memory. ### PoC (verified at runtime against v2.5.0) ``` PUT /api/v1/user/settings/avatar/upload avatar=8000x8000 PNG (64MP, 192KB) -> 200 (accepted; exceeds the 50MP attachment cap) PUT /api/v1/user/settings/avatar/upload avatar=20000x10 PNG (681 bytes) -> 200 after ~19.6s of server processing ``` Contrast (guard present): uploading the 64MP PNG as a task attachment and requesting its preview returns in ~1ms without decoding — `GetPreview` rejects it via `maxPixels` and falls back to the raw file. ### Impact A small crafted upload d",
    "evidence": "osv:GHSA-4vh2-39rq-rq8j",
    "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-4vh2-39rq-rq8j",
      "aliases": [
        "CVE-2026-91971",
        "GHSA-4vh2-39rq-rq8j",
        "GO-2026-6717"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
        }
      ],
      "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-4vh2-39rq-rq8j/GHSA-4vh2-39rq-rq8j.json",
            "last_known_affected_version_range": "<= 2.5.0"
          }
        }
      ],
      "modified": "2026-10-19T21:41:15.904955453Z",
      "provider": "osv",
      "references": [
        "https://github.com/go-vikunja/vikunja/security/advisories/GHSA-4vh2-39rq-rq8j",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-91971",
        "https://github.com/go-vikunja/vikunja/pull/3688",
        "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-denial-of-service-via-avatar-upload"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-400"
        ],
        "severity": "HIGH",
        "github_reviewed": true,
        "nvd_published_at": null,
        "github_reviewed_at": "2026-10-09T20:52:15Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-91971",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/database_specific/severity"
  },
  "source_id": "core-osv",
  "document_id": 376790,
  "published_at": "2026-10-09T20:52:15+00:00"
}