{"id":"CVE-2021-21274","state":"PUBLISHED","title":"Denial of service attack via .well-known lookups","description":"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.","publishedAt":"2021-02-26T17:25:16.000Z","updatedAt":"2024-08-03T18:09:15.070Z","severity":{"score":4.3,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L","source":"GitHub_M"},"assigner":"GitHub_M","products":["matrix-org synapse"],"affected":[{"vendor":"matrix-org","product":"synapse","versions":[{"version":">=0.99.0, < 1.25.0","status":"affected"}]}],"references":["https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8","https://github.com/matrix-org/synapse/releases/tag/v1.25.0","https://github.com/matrix-org/synapse/pull/8950","https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6","https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/"],"weaknesses":["CWE-400"],"ingestedAt":"2026-10-29T19:47:59.168Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2021/21xxx/CVE-2021-21274.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.1","cveMetadata":{"cveId":"CVE-2021-21274","state":"PUBLISHED","datePublished":"2021-02-26T17:25:16.000Z","dateUpdated":"2024-08-03T18:09:15.070Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2020-12-22T00:00:00.000Z"},"containers":{"cna":{"title":"Denial of service attack via .well-known lookups","descriptions":[{"lang":"en","value":"Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). 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