{"id":"CVE-2021-21274","aliases":["GHSA-2hwx-mjrm-v3g8","PYSEC-2021-132"],"url":"https://o3.security/vulnerability/CVE-2021-21274","summary":"Denial of service attack via .well-known lookups","details":"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.","published":"2021-02-26T18:15:12.237Z","modified":"2026-07-08T05:59:54.979850663Z","cvss":{"score":6.5,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"PyPI","name":"matrix-synapse","fixedVersion":"1.25.0"}],"fix":{"url":"https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6","label":"matrix-org/synapse@ff5c4da"},"references":[{"type":"WEB","url":"https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/"},{"type":"ADVISORY","url":"https://github.com/matrix-org/synapse/releases/tag/v1.25.0"},{"type":"FIX","url":"https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6"},{"type":"FIX","url":"https://github.com/matrix-org/synapse/pull/8950"},{"type":"FIX","url":"https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-08T05:59:54.979850663Z"}}