HOMEVULNERABILITIESCVE-2021-20230
HIGH

CVE-2021-20230

CWE-295Published: February 23, 2021· Updated: Jun 17, 2026

7.5
CVSS v3.1

Official 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.

NVD Source

Technical Analysis

CVE-2021-20230 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.

The vulnerability requires no privileges and no user interaction, making it a prime target for automated exploitation campaigns and worm-like propagation.

A successful exploit results in complete confidentiality breach (data exposure), with a CVSS base score of 7.5.

CVSS v3.1 Vector Breakdown

Exploitability
Attack VectorNetwork
Attack ComplexityLow
Privileges Req.None
User InteractionNone
ScopeUnchanged
Impact
ConfidentialityHigh
IntegrityNone
AvailabilityNone
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Affected Vendors & Products

stunnel1 product(s)
stunnel
Source: NVD CPE · 1 total CPE entries

Exploit & PoC Resources

NO KNOWN EXPLOITNo public exploit confirmed at this time
Exploit-DB
Offensive Security exploit database
GitHub PoC Search
Community PoC repositories
Rapid7 / Metasploit
Metasploit module search
PacketStorm
Security advisories & exploits
NVD Detail
Official NIST entry
MITRE CVE
CVE Program entry
External links open in a new tab. Always verify in a controlled environment before use.

Official Patches & Advisories

PATCHhttps://github.com/mtrojnar/stunnel/commit/ebad9ddc4efb2635f37174c9d800d06206f1edf9
PATCHhttps://github.com/mtrojnar/stunnel/commit/ebad9ddc4efb2635f37174c9d800d06206f1edf9
ADVISORYhttps://bugzilla.redhat.com/show_bug.cgi?id=1925226
ADVISORYhttps://security.gentoo.org/glsa/202105-02
ADVISORYhttps://bugzilla.redhat.com/show_bug.cgi?id=1925226
ADVISORYhttps://security.gentoo.org/glsa/202105-02

All References (6)

→
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

Quick Facts

CVE IDCVE-2021-20230
CVSS Score7.5 / 10
SeverityHIGH
WeaknessCWE-295
CISA KEVNo
Affected1 vendor(s)
PublishedFeb 23, 2021

Related CVEs (CWE-295)

CVE-2026-4370
10.0
CVE-2020-7956
9.8
CVE-2019-18632
9.8
CVE-2018-21029
9.8
CVE-2019-18633
9.8
CVE-2019-18847
9.8

Recommended Actions

  • →Apply vendor patches immediately
  • →Monitor CVE-2021-20230 in threat intel feeds
  • →Review IDS/IPS signatures for exploitation attempts
Data sourced from NVD (NIST), CISA KEV, and EPSS (FIRST). Analysis generated by CTIWATCH.COM. CVE data is provided under the NVD usage policy.
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