CVE-2021-20230: High severity stunnel vulnerability
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.
Other sources
stunnel before version 5.57 does not correctly verify the client certificate when options redirect and verifyChain are used. redirect redirects TLS client connections to another address when there is a certificate-based authentication failure and verifyChain is used to verify the client certificate starting from the root CA (specified in CAfile or CApath). When these options are used together, the stunnel server does not correctly validate the client certificate, allowing a client with a certificate not signed by the right CA to access the service without being redirected to the specified redirect address.
Upstream 5.57 release patch: https://github.com/mtrojnar/stunnel/commit/ebad9ddc4efb2635f37174c9d800d06206f1edf9
— Red Hat
Affected Software
Remediation
Event History
Frequently Asked Questions
What is the severity of CVE-2021-20230?
CVE-2021-20230 has a medium severity rating due to improper validation of client certificates.
How do I fix CVE-2021-20230?
To fix CVE-2021-20230, upgrade stunnel to version 5.57 or later.
Which versions of stunnel are affected by CVE-2021-20230?
Versions of stunnel prior to 5.57 are affected by CVE-2021-20230.
What areas of stunnel are impacted by CVE-2021-20230?
CVE-2021-20230 impacts stunnel's client certificate validation when using both redirect and verifyChain options.
Is there a specific platform where CVE-2021-20230 occurs?
CVE-2021-20230 occurs in stunnel implementations that haven't been updated to version 5.57 or later across various Linux distributions.