CVE-2021-20230

Name: CVE-2021-20230
Creator: NVD / NIST
Published: 2021-02-23T17:15:13.753+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 7.5/10EPSS 1.18%

Last modified Jun 17, 2026

CVE-2021-20230 is a high-severity vulnerability rated 7.5/10 on the CVSS scale. 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. EPSS estimates a 1.18% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

7.5/10

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

EPSS Probability

1.18%

63.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-295

Affected Software

Vendor	Product	Versions
Stunnel	Stunnel	< 5.57

References

https://bugzilla.redhat.com/show_bug.cgi?id=1925226Issue Tracking, Third Party Advisory
https://github.com/mtrojnar/stunnel/commit/ebad9ddc4efb2635f37174c9d800d06206f1edf9Patch, Third Party Advisory
https://security.gentoo.org/glsa/202105-02Third Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1925226Issue Tracking, Third Party Advisory
https://github.com/mtrojnar/stunnel/commit/ebad9ddc4efb2635f37174c9d800d06206f1edf9Patch, Third Party Advisory
https://security.gentoo.org/glsa/202105-02Third Party Advisory

Timeline

Published: Feb 23, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-20230?

How severe is CVE-2021-20230?

CVE-2021-20230 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 1.18% probability of exploitation in the next 30 days.

How do I fix CVE-2021-20230?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-20230?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST