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Vulnerability record · CVE-2021-20230 · published 23 February 2021

CVE-2021-20230: Stunnel improper certificate validation vulnerability

Stunnel · Stunnel

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.

7.5 CVSS 3.1 High EPSS 1.2% · top 33.6% CWE-295 · Improper certificate validation
7.5CVSS 3.1 base score, v2 5.0
1.2%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
6References
17 Jun 2026Last modified by NVD

Description

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://bugzilla.redhat.com/show_bug.cgi?id=1925226 Issue TrackingThird Party Advisory
https://github.com/mtrojnar/stunnel/commit/ebad9ddc4efb2635f37174c9d800d06206f1edf9 PatchThird Party Advisory
https://security.gentoo.org/glsa/202105-02 Third Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1925226 Issue TrackingThird Party Advisory
https://github.com/mtrojnar/stunnel/commit/ebad9ddc4efb2635f37174c9d800d06206f1edf9 PatchThird Party Advisory
https://security.gentoo.org/glsa/202105-02 Third Party Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

10.0CVE-2001-0060Stunnel vulnerabilityFormat string vulnerability in stunnel 3.8 and earlier allows attackers to execute arbitrary commands via a malformed ident username.EPSS 2.8%9.3CVE-2011-2940Stunnel memory buffer overflow vulnerabilitystunnel 4.40 and 4.41 might allow remote attackers to execute arbitrary code or cause a denial of service (heap memory corruption) via unspecified ve…EPSS 5.7%7.5CVE-2002-0002Stunnel vulnerabilityFormat string vulnerability in stunnel before 3.22 when used in client mode for (1) smtp, (2) pop, or (3) nntp allows remote malicious servers to exe…EPSS 5.3%7.2CVE-2008-2400Stunnel permissions and access controls vulnerabilityUnspecified vulnerability in stunnel before 4.23, when running as a service on Windows, allows local users to gain privileges via unknown attack vect…EPSS 0.35%6.8CVE-2008-2420Stunnel permissions and access controls vulnerabilityThe OCSP functionality in stunnel before 4.24 does not properly search certificate revocation lists (CRL), which allows remote attackers to bypass in…EPSS 1.4%6.6CVE-2013-1762Stunnel code injection vulnerabilitystunnel 4.21 through 4.54, when CONNECT protocol negotiation and NTLM authentication are enabled, does not correctly perform integer conversion, whic…EPSS 2.9%5.8CVE-2015-3644Stunnel improper access control vulnerabilityStunnel 5.00 through 5.13, when using the redirect option, does not redirect client connections to the expected server after the initial connection, …EPSS 2.1%5.0CVE-2003-0147Openpkg vulnerabilityOpenSSL does not use RSA blinding by default, which allows local and remote attackers to obtain the server's private key by determining factors using…EPSS 6.4%

Source: NIST National Vulnerability Database (record CVE-2021-20230), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.