High
Security advisory
A flaw was found in stunnel before 5.57, where it improperly validates client certificates when it is configured to use both redirect and verifyChain options. This flaw allows an attacker with a certificate signed by a Certificate Authority, which is not the one accepted by the stunnel server, to access the tunneled service instead of being redirected to the address specified in the redirect option. The highest threat from this vulnerability is to confidentiality.
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2021-20230
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 11:31 UTC · Revision 1
Source evidence
A flaw was found in stunnel before 5.57, where it improperly validates client certificates when it is configured to use both redirect and verifyChain options. This flaw allows an attacker with a certificate signed by a Certificate Authority, which is not the one accepted by the stunnel server, to access the tunneled service instead of being redirected to the address specified in the redirect option. The highest threat from this vulnerability is to confidentiality.
osv:CVE-2021-20230
Complete source record
{
"id": "f984bc7a560755488ad3c2ff48711af524086efc3a5bc14350ecefb2b61d6c26",
"url": "https://osv.dev/vulnerability/CVE-2021-20230",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "CVE-2021-20230",
"summary": "A flaw was found in stunnel before 5.57, where it improperly validates client certificates when it is configured to use both redirect and verifyChain options. This flaw allows an attacker with a certificate signed by a Certificate Authority, which is not the one accepted by the stunnel server, to access the tunneled service instead of being redirected to the address specified in the redirect option. The highest threat from this vulnerability is to confidentiality.",
"evidence": "osv:CVE-2021-20230",
"products": [],
"severity": "high",
"structured": {
"id": "CVE-2021-20230",
"aliases": [
"CVE-2021-20230"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N"
}
],
"affected": [
{
"ranges": [
{
"repo": "https://github.com/mtrojnar/stunnel",
"type": "GIT",
"events": [
{
"introduced": "0"
},
{
"fixed": "ebad9ddc4efb2635f37174c9d800d06206f1edf9"
}
],
"database_specific": {
"extracted_events": [
{
"cpe": "cpe:2.3:a:stunnel:stunnel:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "0"
},
{
"fixed": "5.57"
}
],
"source": "CPE_RANGE"
},
{
"range": [
{
"fixed": "ebad9ddc4efb2635f37174c9d800d06206f1edf9"
}
],
"source": "REFERENCES_COMMIT"
}
]
}
}
],
"versions": [
"stunnel-5.56",
"stunnel-5.55",
"stunnel-5.54",
"stunnel-5.53",
"stunnel-5.52",
"stunnel-5.51",
"stunnel-5.50",
"stunnel-5.49",
"stunnel-5.48",
"stunnel-5.47",
"stunnel-5.46",
"stunnel-5.45",
"stunnel-5.44",
"stunnel-5.43",
"stunnel-5.42",
"stunnel-5.41",
"stunnel-5.40",
"stunnel-5.39",
"stunnel-5.38",
"stunnel-5.37",
"stunnel-5.36",
"stunnel-5.35",
"stunnel-5.34",
"stunnel-5.33",
"stunnel-5.31",
"stunnel-5.30",
"stunnel-5.29",
"stunnel-5.28",
"stunnel-5.27",
"stunnel-5.26",
"stunnel-5.25",
"stunnel-5.24",
"stunnel-5.23",
"stunnel-5.22",
"stunnel-5.21",
"stunnel-5.20",
"stunnel-5.14",
"stunnel-5.13",
"stunnel-5.15",
"stunnel-5.16",
"stunnel-5.17",
"stunnel-5.18",
"stunnel-5.19",
"stunnel-5.08",
"stunnel-5.07",
"stunnel-5.09",
"stunnel-5.10",
"stunnel-5.11",
"stunnel-5.12",
"stunnel-4.57",
"stunnel-4.56",
"stunnel-5.00",
"stunnel-5.01",
"stunnel-5.02",
"stunnel-5.03",
"stunnel-5.04",
"stunnel-5.05",
"stunnel-5.06",
"stunnel-4.48",
"stunnel-4.47",
"stunnel-4.49",
"stunnel-4.50",
"stunnel-4.51",
"stunnel-4.52",
"stunnel-4.53",
"stunnel-4.54",
"stunnel-4.55",
"stunnel-4.40",
"stunnel-4.39",
"stunnel-4.41",
"stunnel-4.42",
"stunnel-4.43",
"stunnel-4.44",
"stunnel-4.45",
"stunnel-4.46",
"stunnel-4.34",
"stunnel-4.33",
"stunnel-4.35",
"stunnel-4.36",
"stunnel-4.37",
"stunnel-4.38",
"stunnel-4.26",
"stunnel-4.25",
"stunnel-4.27",
"stunnel-4.28",
"stunnel-4.29",
"stunnel-4.30",
"stunnel-4.31",
"stunnel-4.32",
"stunnel-4.18",
"stunnel-4.17",
"stunnel-4.19",
"stunnel-4.20",
"stunnel-4.21",
"stunnel-4.22",
"stunnel-4.23",
"stunnel-4.24",
"stunnel-4.09",
"stunnel-4.08",
"stunnel-4.10",
"stunnel-4.11",
"stunnel-4.12",
"stunnel-4.13",
"stunnel-4.14",
"stunnel-4.15",
"stunnel-4.16",
"stunnel-3.22",
"stunnel-3.21",
"stunnel-4.00",
"stunnel-4.01",
"stunnel-4.02",
"stunnel-4.03",
"stunnel-4.04",
"stunnel-4.05",
"stunnel-4.06",
"stunnel-4.07",
"stunnel-1.4",
"stunnel-1.3",
"stunnel-1.5",
"stunnel-1.6",
"stunnel-2.0",
"stunnel-2.1",
"stunnel-3.00",
"stunnel-3.01",
"stunnel-3.02",
"stunnel-3.03",
"stunnel-3.04",
"stunnel-3.05",
"stunnel-3.06",
"stunnel-3.07",
"stunnel-3.08",
"stunnel-3.09",
"stunnel-3.10",
"stunnel-3.11",
"stunnel-3.12",
"stunnel-3.13",
"stunnel-3.14",
"stunnel-3.15",
"stunnel-3.16",
"stunnel-3.17",
"stunnel-3.18",
"stunnel-3.19",
"stunnel-3.20",
"stunnel-1.0",
"stunnel-1.1",
"stunnel-1.2"
],
"database_specific": {
"source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2021-20230.json"
}
}
],
"modified": "2026-10-20T10:54:27.615030692Z",
"provider": "osv",
"references": [
"https://security.gentoo.org/glsa/202105-02",
"https://bugzilla.redhat.com/show_bug.cgi?id=1925226",
"https://github.com/mtrojnar/stunnel/commit/ebad9ddc4efb2635f37174c9d800d06206f1edf9"
],
"provider_metadata": {}
},
"related_cves": [
{
"value": "CVE-2021-20230",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 385377,
"published_at": "2021-02-23T17:15:13.753+00:00"
}