Medium
Security advisory
AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth/*/fallback/web or /_matrix/client/unstable/auth/*/fallback/web Synapse endpoints.
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2020-26891
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 11:09 UTC · Revision 1
Source evidence
AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth/*/fallback/web or /_matrix/client/unstable/auth/*/fallback/web Synapse endpoints.
osv:CVE-2020-26891
Complete source record
{
"id": "e41f7980c745e822a1887b1df15512958e7afc46789be00162cc920f778bc02a",
"url": "https://osv.dev/vulnerability/CVE-2020-26891",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "CVE-2020-26891",
"summary": "AuthRestServlet in Matrix Synapse before 1.21.0 is vulnerable to XSS due to unsafe interpolation of the session GET parameter. This allows a remote attacker to execute an XSS attack on the domain Synapse is hosted on, by supplying the victim user with a malicious URL to the /_matrix/client/r0/auth/*/fallback/web or /_matrix/client/unstable/auth/*/fallback/web Synapse endpoints.",
"evidence": "osv:CVE-2020-26891",
"products": [],
"severity": "medium",
"structured": {
"id": "CVE-2020-26891",
"aliases": [
"CVE-2020-26891",
"GHSA-3x8c-fmpc-5rmq",
"PYSEC-2020-238"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
}
],
"affected": [
{
"ranges": [
{
"repo": "https://github.com/matrix-org/synapse",
"type": "GIT",
"events": [
{
"introduced": "0"
},
{
"fixed": "cd0f65d2c71ce8f6cadfa84a6eb6b882d97e36c0"
},
{
"fixed": "9b8a53c7b9e1a3ca5f46e417b9fa705f8bacb494"
}
],
"database_specific": {
"extracted_events": [
{
"cpe": "cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "0"
},
{
"fixed": "1.21.0"
}
],
"source": "CPE_RANGE"
},
{
"range": [
{
"fixed": "9b8a53c7b9e1a3ca5f46e417b9fa705f8bacb494"
}
],
"source": "REFERENCES_TAG",
"original_tag": "v1.21.2"
}
]
}
}
],
"versions": [
"v1.21.0rc3",
"v1.21.0rc2",
"v1.21.0rc1",
"v1.17.0rc1",
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],
"database_specific": {
"source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2020-26891.json"
}
}
],
"modified": "2026-10-20T10:52:58.205647467Z",
"provider": "osv",
"references": [
"https://github.com/matrix-org/synapse/releases/tag/v1.21.2",
"https://matrix.org/blog/2020/10/15/synapse-1-21-2-released-and-security-advisory",
"https://github.com/matrix-org/synapse/pull/8444",
"https://github.com/matrix-org/synapse/security/advisories/GHSA-3x8c-fmpc-5rmq"
],
"provider_metadata": {}
},
"related_cves": [
{
"value": "CVE-2020-26891",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 384105,
"published_at": "2020-10-19T17:15:13.22+00:00"
}