{"cve":"CVE-2021-21274","nvd":{"published":"2021-02-26T18:15:12.237","modified":"2026-06-17T03:35:10.330","status":"Modified","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.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H","score":6.5},"cwes":["CWE-770","CWE-400"],"references":["https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6","https://github.com/matrix-org/synapse/pull/8950","https://github.com/matrix-org/synapse/releases/tag/v1.25.0","https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8","https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/"],"products":["fedoraproject:fedora","matrix:synapse"],"source":"nvd","synced_at":"2026-10-19T21:15:50.279857260+00:00"},"epss":{"cve":"CVE-2021-21274","score":0.02164,"percentile":0.82237,"date":"2026-11-01"},"kev":null,"osv":[{"id":"GHSA-2hwx-mjrm-v3g8","url":"https://osv.dev/vulnerability/GHSA-2hwx-mjrm-v3g8","summary":"Denial of service attack via .well-known lookups","modified":"2026-09-10T03:49:14.211839612Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":[">=0.99.0 <1.25.0"],"fixed":["1.25.0"]}]},{"id":"PYSEC-2021-132","url":"https://osv.dev/vulnerability/PYSEC-2021-132","summary":"","modified":"2023-11-08T04:04:39.864718Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":[">=0.99.0 <1.25.0"],"fixed":["1.25.0"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}