{"action":"Upgrade matrix-synapse to 1.28.0 or later.","affected":[{"fixed":"1.28.0","introduced":"0"}],"component":"matrix-synapse","cve_ids":["CVE-2021-21393"],"ecosystem":"PyPI","epss":0.01596,"exploited":false,"fixed_versions":["1.28.0"],"ghsa_id":"GHSA-jrh7-mhhx-6h88","id":"tcs_01M5KFPJ6G7K7A9GGKDSC9FCMT","modified_at":"2026-07-08T06:28:42.964990012Z","published_at":"2021-04-13T15:12:40Z","references":["https://osv.dev/vulnerability/GHSA-jrh7-mhhx-6h88","https://github.com/matrix-org/synapse/security/advisories/GHSA-jrh7-mhhx-6h88","https://nvd.nist.gov/vuln/detail/CVE-2021-21393","https://github.com/matrix-org/synapse/pull/9321","https://github.com/matrix-org/synapse/pull/9393","https://github.com/matrix-org/synapse/commit/3f58fc848d0002de4605bed91603a1f9f245d128","https://github.com/matrix-org/synapse/commit/d2f0ec12d5c8f113095408888e87e191ac546499","https://github.com/matrix-org/synapse","https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-26.yaml","https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY","https://pypi.org/project/matrix-synapse"],"severity":{"label":"medium","score":5.3},"sources":[{"source":"osv","source_id":"GHSA-jrh7-mhhx-6h88","url":"https://osv.dev/vulnerability/GHSA-jrh7-mhhx-6h88"}],"summary":"Denial of service (via resource exhaustion) due to improper input validation on groups/communities endpoints"}