{"cve":"CVE-2021-21392","nvd":{"published":"2021-04-12T22:15:13.147","modified":"2026-06-17T03:35:28.643","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 requests to user provided domains were not restricted to external IP addresses when transitional IPv6 addresses were used. Outbound requests to federation, identity servers, when calculating the key validity for third-party invite events, sending push notifications, and generating URL previews are affected. This could cause Synapse to make requests to internal infrastructure on dual-stack networks. See referenced GitHub security advisory for details and workarounds.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N","score":6.3},"cwes":["CWE-601"],"references":["https://github.com/matrix-org/synapse/pull/9240","https://github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78","https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/","https://pypi.org/project/matrix-synapse/","https://github.com/matrix-org/synapse/pull/9240"],"products":["fedoraproject:fedora","matrix:synapse"],"source":"nvd","synced_at":"2026-10-19T21:17:01.373894544+00:00"},"epss":{"cve":"CVE-2021-21392","score":0.00944,"percentile":0.61076,"date":"2026-11-01"},"kev":null,"osv":[{"id":"GHSA-5wrh-4jwv-5w78","url":"https://osv.dev/vulnerability/GHSA-5wrh-4jwv-5w78","summary":"Open redirect via transitional IPv6 addresses on dual-stack networks","modified":"2026-07-08T06:28:51.474360522Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":["<1.28.0rc1"],"fixed":["1.28.0rc1"]}]},{"id":"PYSEC-2021-25","url":"https://osv.dev/vulnerability/PYSEC-2021-25","summary":"","modified":"2023-11-08T04:04:44.343565Z","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."]}