{"cve":"CVE-2021-21333","nvd":{"published":"2021-03-26T20:15:11.953","modified":"2026-06-17T03:35:19.060","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 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","score":6.1},"cwes":["CWE-79","CWE-74"],"references":["https://github.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73df","https://github.com/matrix-org/synapse/pull/9200","https://github.com/matrix-org/synapse/releases/tag/v1.27.0","https://github.com/matrix-org/synapse/security/advisories/GHSA-c5f8-35qr-q4fm","https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/"],"products":["fedoraproject:fedora","matrix:synapse"],"source":"nvd","synced_at":"2026-10-19T21:17:01.373894544+00:00"},"epss":{"cve":"CVE-2021-21333","score":0.01354,"percentile":0.71615,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-c5f8-35qr-q4fm","url":"https://osv.dev/vulnerability/GHSA-c5f8-35qr-q4fm","summary":"HTML injection in email and account expiry notifications","modified":"2026-07-08T06:27:41.174374736Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":["<1.27.0"],"fixed":["1.27.0"]}]},{"id":"PYSEC-2021-134","url":"https://osv.dev/vulnerability/PYSEC-2021-134","summary":"","modified":"2023-11-08T04:04:41.701060Z","packages":[{"ecosystem":"PyPI","name":"matrix-synapse","purl":"pkg:pypi/matrix-synapse","ranges":["<1.27.0"],"fixed":["1.27.0"]}]}],"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."]}