{"id":"CVE-2021-21274","aliases":["GHSA-2hwx-mjrm-v3g8","PYSEC-2021-132"],"title":"Denial of service attack via .well-known lookups","summary":"Denial of service attack via .well-known lookups","severity":"medium","cvss":4.3,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L","vendor":"matrix-synapse","product":"matrix-synapse","ecosystem":"pip","affected":["matrix-synapse >= 0.99.0, < 1.25.0"],"patched":["matrix-synapse 1.25.0"],"published":"2021-03-01","updated":"2026-09-10","sourceUpdated":"2026-09-10T03:49:14.211839612Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-2hwx-mjrm-v3g8","references":[{"url":"https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2021-21274"},{"url":"https://github.com/matrix-org/synapse/pull/8950"},{"url":"https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6"},{"url":"https://github.com/matrix-org/synapse"},{"url":"https://github.com/matrix-org/synapse/releases/tag/v1.25.0"},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-132.yaml"},{"url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY"}],"tags":["osv","pip"],"epss":0.02164,"epssPercentile":0.81434,"ingestedAt":"2026-09-12T03:13:01.637Z","slug":"CVE-2021-21274","body":"## Overview\n\n### Impact\nA 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.\n\nThis affects any server which accepts federation requests from untrusted servers.\n\n### Patches\nIssue is resolved by #8950. A bug not affecting the security aspects of this was fixed in #9108.\n\n### Workarounds\nThe `federation_domain_whitelist` setting can be used to restrict the homeservers communicated with over federation.\n\n## Affected packages\n\n- `matrix-synapse >= 0.99.0, < 1.25.0`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `matrix-synapse 1.25.0`","depth":"sunlit","depthScore":24,"depthScoreParts":{"impact":23.7,"likelihood":0.4,"exploitation":0,"ransomware":0},"changes":[]}