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 1.28.0 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.
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2021-21392
- 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 1.28.0 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.
osv:CVE-2021-21392
Complete source record
{
"id": "5e23c6a59fcb0f61f793b86014942949ced5dd22e843783b4840a7fcf72192b2",
"url": "https://osv.dev/vulnerability/CVE-2021-21392",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "CVE-2021-21392",
"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 1.28.0 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.",
"evidence": "osv:CVE-2021-21392",
"products": [],
"severity": "medium",
"structured": {
"id": "CVE-2021-21392",
"aliases": [
"CVE-2021-21392",
"GHSA-5wrh-4jwv-5w78",
"PYSEC-2021-25"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:H/A:N"
}
],
"affected": [
{
"ranges": [
{
"repo": "https://github.com/matrix-org/synapse",
"type": "GIT",
"events": [
{
"introduced": "0"
},
{
"fixed": "2756517f7a6e17d2403de44981569dc18329315b"
}
],
"database_specific": {
"extracted_events": [
{
"cpe": "cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "0"
},
{
"fixed": "1.28.0"
}
],
"source": "CPE_RANGE"
}
]
}
}
],
"versions": [
"v1.28.0rc1",
"v1.27.0rc1",
"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-2021-21392.json"
}
}
],
"modified": "2026-10-20T10:53:33.864211728Z",
"provider": "osv",
"references": [
"https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/",
"https://github.com/matrix-org/synapse/pull/9240",
"https://github.com/matrix-org/synapse/security/advisories/GHSA-5wrh-4jwv-5w78",
"https://pypi.org/project/matrix-synapse/"
],
"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-2021-21392",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 384423,
"published_at": "2021-04-12T22:15:13.147+00:00"
}