{"id":"UBUNTU-CVE-2021-21274","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.","modified":"2025-09-08T16:46:57Z","published":"2021-02-26T18:15:00Z","upstream":["CVE-2021-21274"],"references":[{"type":"REPORT","url":"https://ubuntu.com/security/CVE-2021-21274"},{"type":"REPORT","url":"https://github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8"},{"type":"REPORT","url":"https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6"},{"type":"REPORT","url":"https://github.com/matrix-org/synapse/pull/8950"},{"type":"REPORT","url":"https://github.com/matrix-org/synapse/releases/tag/v1.25.0"},{"type":"REPORT","url":"https://www.cve.org/CVERecord?id=CVE-2021-21274"}],"affected":[{"package":{"name":"matrix-synapse","ecosystem":"Ubuntu:Pro:18.04:LTS","purl":"pkg:deb/ubuntu/matrix-synapse@0.24.0+dfsg-1ubuntu0.1~esm4?arch=source&distro=esm-apps/bionic"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["0.19.2+dfsg-6","0.24.0+dfsg-1","0.24.0+dfsg-1ubuntu0.1~esm1","0.24.0+dfsg-1ubuntu0.1~esm4"],"ecosystem_specific":{"binaries":[{"binary_version":"0.24.0+dfsg-1ubuntu0.1~esm4","binary_name":"matrix-synapse"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2021/UBUNTU-CVE-2021-21274.json"}},{"package":{"name":"matrix-synapse","ecosystem":"Ubuntu:Pro:20.04:LTS","purl":"pkg:deb/ubuntu/matrix-synapse@1.11.0-1ubuntu0.1~esm2?arch=source&distro=esm-apps/focal"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.3.0-1","1.4.0-1","1.5.0-1","1.5.1-1","1.6.0-1","1.6.1-1","1.7.0-2","1.7.1-1","1.7.2-1","1.7.3-1","1.8.0-1","1.9.0-1","1.9.1-1","1.10.0-1","1.10.0-2","1.11.0-1","1.11.0-1ubuntu0.1~esm2"],"ecosystem_specific":{"binaries":[{"binary_version":"1.11.0-1ubuntu0.1~esm2","binary_name":"matrix-synapse"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2021/UBUNTU-CVE-2021-21274.json"}}],"schema_version":"1.7.3","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L"},{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H"},{"type":"Ubuntu","score":"medium"}]}