CVE-2021-20230: Stunnel

High severity, CVSS 7.5. EPSS: 1.2% chance of exploitation in the next 30 days.

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.

Affected products

• Stunnel Stunnel: before 5.57 (fixed in 5.57)

Published 2021-02-23. Last modified 2026-06-17.