{"id":"CVE-2021-21333","aliases":["DEBIAN-CVE-2021-21333","EUVD-2021-0126","GHSA-c5f8-35qr-q4fm","PYSEC-2021-134","UBUNTU-CVE-2021-21333"],"status":"published","title":"HTML injection in email and account expiry notifications","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.","severity":{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-10","score":0.01354,"percentile":0.71841,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-74","CWE-79"],"published":"2021-03-26T19:53:04Z","modified":"2026-10-20T10:53:33Z","fixes":[{"ecosystem":"PyPI","package":"matrix-synapse","fixed_in":["1.27.0"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"PyPI","package":"matrix-synapse","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.27.0"}]}]},{"source":"osv","ecosystem":"PyPI","package":"matrix-synapse","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.27.0"}]}],"versions":["0.33.5","0.33.5.1","0.33.6","0.33.6rc1","0.33.7","0.33.7rc1","0.33.7rc2","0.33.8","0.33.8rc2","0.33.9","0.34.0","0.34.0.1","0.34.0rc1","0.34.0rc2","0.34.1.1","0.99.0","0.99.0rc1","0.99.0rc2","0.99.0rc3","0.99.0rc4","0.99.1","0.99.1.1","0.99.1rc1","0.99.1rc2","0.99.2","0.99.2rc1","0.99.3","0.99.3.1","0.99.3.2","0.99.3rc1","0.99.4","0.99.4rc1","0.99.5","0.99.5.1","0.99.5.2","0.99.5rc1","1.0.0","1.0.0rc1","1.0.0rc2","1.0.0rc3","1.1.0","1.1.0rc1","1.1.0rc2","1.10.0","1.10.0rc1","1.10.0rc2","1.10.0rc3","1.10.0rc5","1.10.1","1.11.0","1.11.0rc1","1.11.1","1.12.0","1.12.0rc1","1.12.1","1.12.1rc1","1.12.2","1.12.3","1.12.4","1.12.4rc1","1.13.0","1.13.0rc1","1.13.0rc2","1.13.0rc3","1.14.0","1.14.0rc1","1.14.0rc2","1.15.0","1.15.0rc1","1.15.1","1.15.2","1.16.0","1.16.0rc1","1.16.0rc2","1.16.1","1.17.0","1.17.0rc1","1.18.0","1.18.0rc1","1.18.0rc2","1.19.0","1.19.0rc1","1.19.1","1.19.1rc1","1.19.2","1.19.3","1.2.0","1.2.0rc1","1.2.0rc2","1.2.1","1.20.0","1.20.0rc1","1.20.0rc2","1.20.0rc3","1.20.0rc4","1.20.0rc5","1.20.1","1.21.0","1.21.0rc1","1.21.0rc2","1.21.0rc3","1.21.1","1.21.2","1.22.0","1.22.0rc1","1.22.0rc2","1.22.1","1.23.0","1.23.0rc1","1.23.1","1.24.0","1.24.0rc1","1.24.0rc2","1.25.0","1.25.0rc1","1.26.0","1.26.0rc1","1.26.0rc2","1.27.0rc1","1.27.0rc2","1.3.0","1.3.0rc1","1.3.1","1.4.0","1.4.0rc1","1.4.0rc2","1.4.1","1.4.1rc1","1.5.0","1.5.0rc1","1.5.0rc2","1.5.1","1.6.0","1.6.0rc1","1.6.0rc2","1.6.1","1.7.0","1.7.0rc1","1.7.0rc2","1.7.1","1.7.2","1.7.3","1.8.0","1.8.0rc1","1.9.0","1.9.0.dev1","1.9.0.dev2","1.9.0rc1","1.9.1"]},{"source":"osv","ecosystem":"PyPI","package":"matrix-synapse","ranges":[{"type":"GIT","events":[{"introduced":"0"},{"fixed":"e54746bdf7d5c831eabe4dcea76a7626f1de73df"}]},{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.27.0"}]}],"versions":["0.33.5","0.33.5.1","0.33.6","0.33.6rc1","0.33.7","0.33.7rc1","0.33.7rc2","0.33.8","0.33.8rc2","0.33.9","0.34.0","0.34.0.1","0.34.0rc1","0.34.0rc2","0.34.1.1","0.99.0","0.99.0rc1","0.99.0rc2","0.99.0rc3","0.99.0rc4","0.99.1","0.99.1.1","0.99.1rc1","0.99.1rc2","0.99.2","0.99.2rc1","0.99.3","0.99.3.1","0.99.3.2","0.99.3rc1","0.99.4","0.99.4rc1","0.99.5","0.99.5.1","0.99.5.2","0.99.5rc1","1.0.0","1.0.0rc1","1.0.0rc2","1.0.0rc3","1.1.0","1.1.0rc1","1.1.0rc2","1.10.0","1.10.0rc1","1.10.0rc2","1.10.0rc3","1.10.0rc5","1.10.1","1.11.0","1.11.0rc1","1.11.1","1.12.0","1.12.0rc1","1.12.1","1.12.1rc1","1.12.2","1.12.3","1.12.4","1.12.4rc1","1.13.0","1.13.0rc1","1.13.0rc2","1.13.0rc3","1.14.0","1.14.0rc1","1.14.0rc2","1.15.0","1.15.0rc1","1.15.1","1.15.2","1.16.0","1.16.0rc1","1.16.0rc2","1.16.1","1.17.0","1.17.0rc1","1.18.0","1.18.0rc1","1.18.0rc2","1.19.0","1.19.0rc1","1.19.1","1.19.1rc1","1.19.2","1.19.3","1.2.0","1.2.0rc1","1.2.0rc2","1.2.1","1.20.0","1.20.0rc1","1.20.0rc2","1.20.0rc3","1.20.0rc4","1.20.0rc5","1.20.1","1.21.0","1.21.0rc1","1.21.0rc2","1.21.0rc3","1.21.1","1.21.2","1.22.0","1.22.0rc1","1.22.0rc2","1.22.1","1.23.0","1.23.0rc1","1.23.1","1.24.0","1.24.0rc1","1.24.0rc2","1.25.0","1.25.0rc1","1.26.0","1.26.0rc1","1.26.0rc2","1.27.0rc1","1.27.0rc2","1.3.0","1.3.0rc1","1.3.1","1.4.0","1.4.0rc1","1.4.0rc2","1.4.1","1.4.1rc1","1.5.0","1.5.0rc1","1.5.0rc2","1.5.1","1.6.0","1.6.0rc1","1.6.0rc2","1.6.1","1.7.0","1.7.0rc1","1.7.0rc2","1.7.1","1.7.2","1.7.3","1.8.0","1.8.0rc1","1.9.0","1.9.0.dev1","1.9.0.dev2","1.9.0rc1","1.9.1"]},{"source":"osv","ecosystem":"Ubuntu:Pro:18.04:LTS","package":"matrix-synapse","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"]},{"source":"osv","ecosystem":"Ubuntu:Pro:20.04:LTS","package":"matrix-synapse","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"]}],"products":[{"vendor":"element-hq","product":"synapse"},{"vendor":"fedoraproject","product":"fedora"},{"vendor":"matrix","product":"synapse"},{"vendor":"matrix-org","product":"synapse"}],"references":[{"url":"https://github.com/matrix-org/synapse/releases/tag/v1.27.0","tags":["x_refsource_MISC","x_transferred","Third Party Advisory","advisory","web","report"]},{"url":"https://github.com/matrix-org/synapse/pull/9200","tags":["x_refsource_MISC","x_transferred","Patch","Third Party Advisory","fix","web","report"]},{"url":"https://github.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73df","tags":["x_refsource_MISC","x_transferred","Patch","Third Party Advisory","fix","web","report"]},{"url":"https://github.com/matrix-org/synapse/security/advisories/GHSA-c5f8-35qr-q4fm","tags":["x_refsource_CONFIRM","x_transferred","Patch","Third Party Advisory","fix","web","advisory","report"]},{"url":"https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/","tags":["vendor-advisory","x_refsource_FEDORA","x_transferred","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2021-21333","tags":["advisory"]},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/matrix-synapse/PYSEC-2021-134.yaml","tags":["web"]},{"url":"https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY","tags":["web"]},{"url":"https://github.com/advisories/GHSA-c5f8-35qr-q4fm"},{"url":"https://security-tracker.debian.org/tracker/CVE-2021-21333","tags":["advisory"]},{"url":"https://github.com/matrix-org/synapse","tags":["package"]},{"url":"https://ubuntu.com/security/CVE-2021-21333","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2021-21333","tags":["report"]}],"sources":{"cve":[{"id":"CVE-2021-21333","title":"HTML injection in email and account expiry notifications","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.","status":"published","published":"2021-03-26T20:00:19Z","modified":"2024-08-03T18:09:15.578Z","cvss":[{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium","assessor":"cna"}],"cwes":["CWE-74"],"updated_at":"2026-11-02T14:33:00.137099Z"}],"euvd":[{"id":"EUVD-2021-0126","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.","status":"published","published":"2021-03-26T20:00:19Z","modified":"2024-08-03T18:09:15Z","cvss":[{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium"}],"updated_at":"2026-11-03T03:25:06.913643Z"}],"ghsa":[{"id":"GHSA-c5f8-35qr-q4fm","title":"HTML injection in email and account expiry notifications","description":"### Impact\nThe 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.\n\nThe account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker.\n\n### Patches\nThis issue is fixed in #9200.\n\n### Workarounds\nFor the missed messages notifications:\n\nThe `notif.html`, `notif_mail.html`, and `room.html` templates can be overridden with custom templates that manually escapes the variables using [JInja2's `escape` filter](https://jinja.palletsprojects.com/en/2.11.x/templates/#escape). See the `email.template_dir` setting.\n\nFor the account expiry notifications:\n\n1. Account expiry can be disabled via the `account_validity.enabled` setting.\n\n2. The `notice_expiry.html` template can be overridden with a custom template that manually escapes the variables using [JInja2's `escape` filter](https://jinja.palletsprojects.com/en/2.11.x/templates/#escape). See the `email.template_dir` setting.","status":"published","published":"2021-03-26T19:53:04Z","modified":"2024-09-30T20:35:36Z","cvss":[{"version":"4.0","score":4.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:N/SI:H/SA:N","level":"medium"},{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium"}],"cwes":["CWE-74","CWE-79"],"updated_at":"2026-11-02T14:38:07.852618Z"}],"nvd":[{"id":"CVE-2021-21333","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.","status":"published","published":"2021-03-26T20:15:11.953Z","modified":"2026-06-17T03:35:19.06Z","cvss":[{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium","assessor":"secondary"},{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium","assessor":"primary"},{"version":"2.0","score":2.6,"vector":"AV:N/AC:H/Au:N/C:N/I:P/A:N","level":"low","assessor":"primary"}],"cwes":["CWE-74","CWE-79"],"updated_at":"2026-11-02T14:06:51.197859Z"}],"osv":[{"id":"CVE-2021-21333","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.","status":"published","published":"2021-03-26T20:15:11.953Z","modified":"2026-10-20T10:53:33.353213077Z","cvss":[{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium"}],"updated_at":"2026-11-02T15:44:18.004149Z"},{"id":"DEBIAN-CVE-2021-21333","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.","status":"withdrawn","published":"2021-03-26T20:15:11.953Z","modified":"2026-08-11T20:49:51.602896563Z","cvss":[{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium"}],"updated_at":"2026-11-02T15:45:34.637046Z"},{"id":"GHSA-c5f8-35qr-q4fm","title":"HTML injection in email and account expiry notifications","description":"### Impact\nThe 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.\n\nThe account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker.\n\n### Patches\nThis issue is fixed in #9200.\n\n### Workarounds\nFor the missed messages notifications:\n\nThe `notif.html`, `notif_mail.html`, and `room.html` templates can be overridden with custom templates that manually escapes the variables using [JInja2's `escape` filter](https://jinja.palletsprojects.com/en/2.11.x/templates/#escape). See the `email.template_dir` setting.\n\nFor the account expiry notifications:\n\n1. Account expiry can be disabled via the `account_validity.enabled` setting.\n\n2. The `notice_expiry.html` template can be overridden with a custom template that manually escapes the variables using [JInja2's `escape` filter](https://jinja.palletsprojects.com/en/2.11.x/templates/#escape). See the `email.template_dir` setting.","status":"published","published":"2021-03-26T19:53:04Z","modified":"2026-07-08T06:27:41.174374736Z","cvss":[{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium"},{"version":"4.0","score":4.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:N/SI:H/SA:N","level":"medium"}],"cwes":["CWE-74","CWE-79"],"updated_at":"2026-11-02T15:46:44.571161Z"},{"id":"UBUNTU-CVE-2021-21333","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.","status":"published","published":"2021-03-26T20:15:00Z","modified":"2025-09-08T16:46:57Z","cvss":[{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium"},{"version":"3.1","score":6.1,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N","level":"medium"}],"updated_at":"2026-11-02T15:54:26.29421Z"},{"id":"PYSEC-2021-134","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.","status":"published","published":"2021-03-26T20:15:00Z","modified":"2023-11-08T04:04:41.70106Z","updated_at":"2026-11-02T15:52:52.682031Z"}]}}