CVE-2021-20230Network attack vector

Authentication bypass via untrusted client certificate

Published Feb 23, 2021 · Updated Aug 3, 2024

Authentication bypass in stunnel 5.14 through 5.56 allows remote attackers to access tunneled services via untrusted client certificates. When redirect and verifyChain are enabled in server mode, certificate validation failure is mishandled and the connection is not redirected. The attack requires a certificate issued by an untrusted CA and the affected option combination, exposing the tunneled service's confidentiality boundary.

CVSS severity5.0
Medium
EPSS probability1.18%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Authentication bypass in stunnel 5.14 through 5.56 allows remote attackers to access tunneled services via untrusted client certificates. When redirect and verifyChain are enabled in server mode, certificate validation failure is mishandled and the connection is not redirected. The attack requires a certificate issued by an untrusted CA and the affected option combination, exposing the tunneled service's confidentiality boundary.

The record

CVE
CVE-2021-20230
Published
Feb 23, 2021
Updated
Aug 3, 2024
Vendor
Michał Trojnara
Product
stunnel
Classifications
T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 23, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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  1. Affected versionversion=stunnel 5.57

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Michał Trojnara · stunnelversion=stunnel 5.57

Published CVSS scores

7.5SUSECVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
5.0stunnelAV:N/AC:L/Au:N/C:P/I:N/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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