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 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2021-21393
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 11:32 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 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.
osv:CVE-2021-21393
Complete source record
{
"id": "3ba70e379fa2442bfe856b72888b01a0695f63cb3fb6c93df72ae8a151baf4b7",
"url": "https://osv.dev/vulnerability/CVE-2021-21393",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "CVE-2021-21393",
"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 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.",
"evidence": "osv:CVE-2021-21393",
"products": [],
"severity": "medium",
"structured": {
"id": "CVE-2021-21393",
"aliases": [
"CVE-2021-21393",
"GHSA-jrh7-mhhx-6h88",
"PYSEC-2021-26"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"
}
],
"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-2021-21393.json"
}
}
],
"modified": "2026-10-20T10:54:28.523809435Z",
"provider": "osv",
"references": [
"https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/",
"https://github.com/matrix-org/synapse/pull/9321",
"https://github.com/matrix-org/synapse/pull/9393",
"https://github.com/matrix-org/synapse/security/advisories/GHSA-jrh7-mhhx-6h88",
"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-2021-21393",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 385418,
"published_at": "2021-04-12T22:15:13.227+00:00"
}