Security advisory
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.
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2021-21333
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 11:14 UTC · Revision 1
Source evidence
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.
osv:CVE-2021-21333
Complete source record
{
"id": "f14927c01f256bdfbdc3ce6d87a8dd92e8de71ed6199273e6b1d004f5e3c4f74",
"url": "https://osv.dev/vulnerability/CVE-2021-21333",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "CVE-2021-21333",
"summary": "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.",
"evidence": "osv:CVE-2021-21333",
"products": [],
"severity": "medium",
"structured": {
"id": "CVE-2021-21333",
"aliases": [
"CVE-2021-21333",
"GHSA-c5f8-35qr-q4fm",
"PYSEC-2021-134"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N"
}
],
"affected": [
{
"ranges": [
{
"repo": "https://github.com/matrix-org/synapse",
"type": "GIT",
"events": [
{
"introduced": "0"
},
{
"fixed": "6600f0bd572500f9b85bfeed424eff0300e2fa59"
},
{
"fixed": "e54746bdf7d5c831eabe4dcea76a7626f1de73df"
}
],
"database_specific": {
"extracted_events": [
{
"cpe": "cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "0"
},
{
"fixed": "1.27.0"
}
],
"source": "CPE_RANGE"
},
{
"range": [
{
"fixed": "e54746bdf7d5c831eabe4dcea76a7626f1de73df"
}
],
"source": "REFERENCES_COMMIT"
},
{
"range": [
{
"fixed": "6600f0bd572500f9b85bfeed424eff0300e2fa59"
}
],
"source": "REFERENCES_TAG",
"original_tag": "v1.27.0"
}
]
}
}
],
"versions": [
"v1.24.0rc1",
"v1.23.0rc1",
"v1.17.0rc1",
"v1.18.0rc1",
"v1.16.0rc1",
"v1.15.0rc1",
"v1.14.0rc1",
"v1.12.0rc1",
"v1.6.0rc1",
"v1.11.0rc1",
"v1.3.0rc1",
"v1.1.0rc2",
"v1.1.0rc1",
"v1.10.0rc1",
"v1.9.0rc1",
"v1.9.0.dev2",
"v1.9.0.dev1",
"v1.8.0rc1",
"v1.5.0rc1",
"v1.4.0rc1",
"v0.99.5.1.dev0",
"v0.99.4rc1",
"v0.99.2rc1",
"v0.99.1rc1",
"hhs-8",
"hhs-7",
"hhs-6",
"hhs-5",
"hhs-3",
"hhs-2",
"hhs-1",
"v0.32.2",
"v0.32.0",
"v0.32.0rc1",
"v0.28.0-rc1",
"v0.25.0-rc1",
"v0.23.0-rc2",
"v0.23.0-rc1",
"v0.18.5",
"v0.18.4",
"v0.18.3",
"v0.18.2",
"v0.18.1",
"v0.18.0",
"v0.17.3",
"v0.17.2",
"v0.17.1",
"v0.17.0",
"v0.16.1-r1",
"v0.16.1",
"v0.16.0",
"v0.14.0",
"v0.13.3",
"v0.13.2",
"v0.13.1",
"v0.12.0",
"v0.11.1",
"v0.11.0-r2",
"v0.11.0-r1",
"v0.11.0",
"v0.10.0-r2",
"v0.10.0-r1",
"v0.10.0",
"v0.9.3",
"v0.9.2-r2",
"v0.9.2-r1",
"v0.9.2",
"v0.9.1",
"v0.9.0-r5",
"v0.9.0-r4",
"v0.9.0-r3",
"v0.9.0-r2",
"v0.9.0-r1",
"v0.9.0",
"v0.8.1-r4",
"v0.8.1-r3",
"v0.8.1-r2",
"v0.8.1-r1",
"v0.8.1",
"v0.8.0",
"v0.7.1-r4",
"v0.7.1-r3",
"v0.7.1-r2",
"v0.7.1-r1",
"v0.7.1",
"v0.7.0f",
"v0.7.0e",
"v0.7.0d",
"v0.7.0c",
"v0.7.0b",
"v0.7.0a",
"v0.7.0",
"v0.6.1f",
"v0.6.1e",
"v0.6.1d",
"v0.6.1c",
"v0.6.1b",
"v0.6.1a",
"v0.6.1",
"v0.6.0b",
"v0.6.0a",
"v0.6.0",
"v0.5.4a",
"v0.5.4",
"v0.5.3c",
"v0.5.3b",
"v0.5.3a",
"v0.5.3",
"v0.5.2",
"v0.5.1",
"v0.5.0",
"v0.4.2",
"v0.4.1",
"v0.3.4",
"v0.3.3",
"v0.3.0",
"v0.2.2",
"v0.2.1a",
"v0.2.1",
"v0.2.0"
],
"database_specific": {
"source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2021-21333.json"
}
}
],
"modified": "2026-10-20T10:53:33.353213077Z",
"provider": "osv",
"references": [
"https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/",
"https://github.com/matrix-org/synapse/releases/tag/v1.27.0",
"https://github.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73df",
"https://github.com/matrix-org/synapse/pull/9200",
"https://github.com/matrix-org/synapse/security/advisories/GHSA-c5f8-35qr-q4fm"
],
"provider_metadata": {
"unresolved_ranges": [
{
"cpes": [
"cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*"
],
"vendor_product": "fedoraproject:fedora",
"extracted_events": [
{
"range": [
{
"introduced": "34"
},
{
"last_affected": "34"
}
],
"source": "CPE_STRING"
}
]
}
]
}
},
"related_cves": [
{
"value": "CVE-2021-21333",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 384413,
"published_at": "2021-03-26T20:15:11.953+00:00"
}