Flask-Reuploaded: Extension-denylist bypass via case-folding asymmetry in name-override path (incomplete-fix variant of CVE-2026-27641)

{
  "id": "7e2f8294e7eb0839d421bc248cda7ee03ae54cc581ea47e4409f82f5e233d3db",
  "url": "https://osv.dev/vulnerability/CVE-2026-54567",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Flask-Reuploaded: Extension-denylist bypass via case-folding asymmetry in name-override path (incomplete-fix variant of CVE-2026-27641)",
    "summary": "Flask-Reuploaded provides file uploads for Flask. From 1.5.0 until 1.6.0, UploadSet.save(storage, name=...) in src/flask_uploads/flask_uploads.py applies lowercase_ext to the default upload path but uses the case-preserving extension helper for a caller-supplied name before extension_allowed evaluates an AllExcept denylist. An attacker who controls the name override can use a mixed-case dangerous extension to bypass a lowercase denylist and store the file in the served upload directory. Exploitation requires a denylist configuration, a user-influenced name override, and a deployment that resolves or executes extensions case-insensitively; pure allowlists remain protected and path containment is not bypassed. On an execution-capable upload directory, the stored file can execute with the web server's privileges and affect confidentiality, integrity, and availability. This issue is fixed in version 1.6.0.",
    "evidence": "osv:CVE-2026-54567",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-54567",
      "aliases": [
        "CVE-2026-54567",
        "GHSA-937x-gpqr-72gg",
        "PYSEC-2026-3466"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/jugmac00/flask-reuploaded",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "5ded76092429c6eb8a4af941b14fbde40a38fff4"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "fixed": "5ded76092429c6eb8a4af941b14fbde40a38fff4"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  }
                ]
              }
            }
          ],
          "versions": [
            "0.2.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-54567.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:27.091531050Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/54xxx/CVE-2026-54567.json",
        "https://github.com/jugmac00/flask-reuploaded/security/advisories/GHSA-937x-gpqr-72gg",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-54567",
        "https://github.com/jugmac00/flask-reuploaded/commit/5ded76092429c6eb8a4af941b14fbde40a38fff4",
        "https://github.com/jugmac00/flask-reuploaded/pull/186"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-178",
          "CWE-434"
        ],
        "cna_assigner": "GitHub_M",
        "unresolved_ranges": [
          {
            "extracted_events": [
              {
                "range": [
                  {
                    "fixed": "1.6.0"
                  }
                ],
                "source": "AFFECTED_FIELD",
                "strategy": "StringRangeExpression"
              }
            ]
          },
          {
            "extracted_events": [
              {
                "range": [
                  {
                    "fixed": "1.6.0"
                  }
                ],
                "source": "DESCRIPTION"
              }
            ]
          }
        ],
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/54xxx/CVE-2026-54567.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-54567",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 493867,
  "published_at": "2026-09-14T17:15:49.272+00:00"
}