{"cve":"CVE-2021-21393","nvd":{"published":"2021-04-12T22:15:13.227","modified":"2026-06-17T03:35:28.770","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 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","score":6.5},"cwes":["CWE-20"],"references":["https://github.com/matrix-org/synapse/pull/9321","https://github.com/matrix-org/synapse/pull/9393","https://github.com/matrix-org/synapse/security/advisories/GHSA-jrh7-mhhx-6h88","https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/","https://pypi.org/project/matrix-synapse/"],"products":["fedoraproject:fedora","matrix:synapse"],"source":"nvd","synced_at":"2026-10-19T21:17:01.373894544+00:00"},"epss":{"cve":"CVE-2021-21393","score":0.01596,"percentile":0.75745,"date":"2026-11-01"},"kev":null,"osv":[{"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","modified":"2026-07-08T06:28:42.964990012Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":["<1.28.0"],"fixed":["1.28.0"]}]},{"id":"PYSEC-2021-26","url":"https://osv.dev/vulnerability/PYSEC-2021-26","summary":"","modified":"2023-11-08T04:04:44.409892Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":["<1.28.0"],"fixed":["1.28.0"]}]}],"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."]}