CVE-2021-20230

What it is

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.

At a glance

Exploitability

Can be exploited over the network without logging in. High risk of data exposure.

Exploit odds

1.2% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-02-23 17:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 03:33 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 1.2% in the next 30 days (EPSS).
• Severity: 7.5 HIGH (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-02-23 17:15 UTC

Last modified

2026-06-17 03:33 UTC

Exploit probability

1.2% · EPSS, next 30 days

Weakness

CWE-295 · Improper certificate validation

Record status

Modified

Affected

• Stunnel < 5.57

Attack profile CVSS 3.1 · 7.5

Attack vector

Network

Complexity

Low

Privileges needed

None

User interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

None

Availability

None

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2021-20230.md and follow it. Check whether this project is affected by CVE-2021-20230. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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