Security advisory

{
  "id": "d318cd95350461db87c5d5f9a20f0a6b20bc5f5101976338b120cd06a6289772",
  "url": "https://osv.dev/vulnerability/CVE-2021-21274",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "CVE-2021-21274",
    "summary": "Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.25.0, a malicious homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver. This affects any server which accepts federation requests from untrusted servers. Issue is resolved in version 1.25.0. As a workaround the `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation.",
    "evidence": "osv:CVE-2021-21274",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2021-21274",
      "aliases": [
        "CVE-2021-21274",
        "GHSA-2hwx-mjrm-v3g8",
        "PYSEC-2021-132"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/matrix-org/synapse",
              "type": "GIT",
              "events": [
                {
                  "introduced": "3bd9daf4b86ed811c9ced95a4adb9aa58b681399"
                },
                {
                  "fixed": "ebd534b58d9d53dc5086a9dd9cb176868c7b93ef"
                },
                {
                  "fixed": "ff5c4da1289cb5e097902b3e55b771be342c29d6"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "0.99.0"
                      },
                      {
                        "fixed": "1.25.0"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "range": [
                      {
                        "fixed": "ff5c4da1289cb5e097902b3e55b771be342c29d6"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "ebd534b58d9d53dc5086a9dd9cb176868c7b93ef"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "v1.25.0"
                  }
                ]
              }
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2021-21274.json"
          }
        }
      ],
      "modified": "2026-10-20T10:54:27.913971945Z",
      "provider": "osv",
      "references": [
        "https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/",
        "https://github.com/matrix-org/synapse/releases/tag/v1.25.0",
        "https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6",
        "https://github.com/matrix-org/synapse/pull/8950",
        "https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8"
      ],
      "provider_metadata": {
        "unresolved_ranges": [
          {
            "cpes": [
              "cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*"
            ],
            "vendor_product": "fedoraproject:fedora",
            "extracted_events": [
              {
                "range": [
                  {
                    "introduced": "34"
                  },
                  {
                    "last_affected": "34"
                  }
                ],
                "source": "CPE_STRING"
              }
            ]
          }
        ]
      }
    },
    "related_cves": [
      {
        "value": "CVE-2021-21274",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 385392,
  "published_at": "2021-02-26T18:15:12.237+00:00"
}