NVD / CVE Program· CNA redhat

CVE-2021-20230: A flaw was found in stunnel before 5.57, where it improperly validates client certificates when it is configured to use both redirect and…

HighCVSS 7.5 · v3.1—No exploit Fix available

Noroxi assessment

Priority: Monitor (action score 30/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 1.1% (FIRST EPSS); higher than 68% of all records.
Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
Fixed version: stunnel (AlmaLinux:8) 5.56-5.el8_3; stunnel4 (Debian:12) 3:5.56+dfsg-8; stunnel (openSUSE:Leap 15.2) 5.57-lp152.2.6.1.

Low priority for now. Upgrade to stunnel (AlmaLinux:8) 5.56-5.el8_3; stunnel4 (Debian:12) 3:5.56+dfsg-8; stunnel (openSUSE:Leap 15.2) 5.57-lp152.2.6.1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

Published: Feb 23, 2021
Updated: Jun 16, 2026
EPSS: 1.1% · 68th percentile
CWE: CWE-295

Report tools

JSON

Action score

Monitor

Low priority for now.

CVSS: 30 / 40 · 7.5 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 1.1%

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Noroxi analysis

What does it mean in your environment?

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Affected systems

Is your version affected?

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Vendor	Product	CPE
stunnel	stunnel	cpe:2.3:a:stunnel:stunnel

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Versions reported by the vendor

Affected version ranges reported by the assigning authority (redhat). Independent of NVD's CPE analysis and usually ahead of it.

n/a stunnel
- stunnel 5.57affected

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

stunnel stunnelbefore 5.57

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
AlmaLinux:8	stunnel	before 5.56-5.el8_3	5.56-5.el8_3
Debian:12	stunnel4	before 3:5.56+dfsg-8	3:5.56+dfsg-8
openSUSE:Leap 15.2	stunnel	before 5.57-lp152.2.6.1	5.57-lp152.2.6.1
Red Hat:enterprise_linux:8::baseos	stunnel	before 0:5.56-5.el8_3	0:5.56-5.el8_3
Red Hat:enterprise_linux:8::baseos	stunnel-debuginfo	before 0:5.56-5.el8_3	0:5.56-5.el8_3
Red Hat:enterprise_linux:8::baseos	stunnel-debugsource	before 0:5.56-5.el8_3	0:5.56-5.el8_3
Red Hat:rhel_eus:8.1::baseos	stunnel	before 0:5.48-6.el8_1	0:5.48-6.el8_1
Red Hat:rhel_eus:8.1::baseos	stunnel-debuginfo	before 0:5.48-6.el8_1	0:5.48-6.el8_1
Red Hat:rhel_eus:8.1::baseos	stunnel-debugsource	before 0:5.48-6.el8_1	0:5.48-6.el8_1
Red Hat:rhel_eus:8.2::baseos	stunnel	before 0:5.48-6.el8_2	0:5.48-6.el8_2
Red Hat:rhel_eus:8.2::baseos	stunnel-debuginfo	before 0:5.48-6.el8_2	0:5.48-6.el8_2
Red Hat:rhel_eus:8.2::baseos	stunnel-debugsource	before 0:5.48-6.el8_2	0:5.48-6.el8_2
Rocky Linux:8	stunnel	before 0:5.56-5.el8_3	0:5.56-5.el8_3
SUSE:Linux Enterprise High Performance Computing 15-ESPOS	stunnel	before 5.44-3.8.1	5.44-3.8.1
SUSE:Linux Enterprise Module for Server Applications 15 SP2	stunnel	before 5.57-3.11.1	5.57-3.11.1

Same product

stunnel: all records

Other highest-scoring records for the same primary product.

CVE-2001-0060Format string vulnerability in stunnel 3.8 and earlier allows attackers to execute arbitrary commands via a malformed ident username.
41Plan
CVE-2011-2940stunnel 4.40 and 4.41 might allow remote attackers to execute arbitrary code or cause a denial of service (heap memory corruption) via…
39Monitor
CVE-2002-0002Format string vulnerability in stunnel before 3.22 when used in client mode for (1) smtp, (2) pop, or (3) nntp allows remote malicious…
32Monitor

Published before and after this one in the same product

← EarlierCVE-2013-1762Mar 8, 2013 · stunnel 4.21 through 4.54, when CONNECT protocol negotiation and NTLM authentication are enabled, does not correctly perform integer…
27Monitor
← EarlierCVE-2014-0016Mar 24, 2014 · stunnel before 5.00, when using fork threading, does not properly update the state of the OpenSSL pseudo-random number generator (PRNG)…
18Monitor
← EarlierCVE-2015-3644May 13, 2015 · Stunnel 5.00 through 5.13, when using the redirect option, does not redirect client connections to the expected server after the initial…
24Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
almalinux:stunnel	5.56-5.el8_3 · AlmaLinux:8	Package registry (OSV)
debian:stunnel4	3:5.56+dfsg-8 · Debian:12	Package registry (OSV)
opensuse:stunnel	5.57-lp152.2.6.1 · openSUSE:Leap 15.2	Package registry (OSV)
red hat:stunnel	0:5.56-5.el8_3 · Red Hat:enterprise_linux:8::baseos	Package registry (OSV)
red hat:stunnel-debuginfo	0:5.56-5.el8_3 · Red Hat:enterprise_linux:8::baseos	Package registry (OSV)
red hat:stunnel-debugsource	0:5.56-5.el8_3 · Red Hat:enterprise_linux:8::baseos	Package registry (OSV)
rocky linux:stunnel	0:5.56-5.el8_3 · Rocky Linux:8	Package registry (OSV)
suse:stunnel	5.57-3.11.1 · SUSE:Linux Enterprise Module for Server Applications 15 SP2	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

mtrojnar/stunnelcommit ebad9dd

Not on record for this CVE: EPSS history · researcher credit · variant or chain candidate · public bounty program.

Technical details

Attack conditions

Anyone who can reach it over the internet can trigger it.
No account or password is required.
No user action is required.
No special conditions are required; it is repeatable.

If successful

Confidentiality: high · data can be read
Integrity: none
Availability: none

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality impact: High
Integrity impact: None
Availability impact: None

Weakness class (CWE)

CWE-295 · Improper Certificate Validation

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

nvd-primary

Root cause

A certificate validation error is silently swallowed instead of terminating the connection. Often, code added for a test environment makes its way into production.

The bug in code

The whole CWE-295 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

kotlin

1override fun checkServerTrusted(chain: Array<X509Certificate>, type: String) {
2  // Left empty for the test environment
3}

Fixed

kotlin

1val client = OkHttpClient.Builder()
2  .certificatePinner(
3    CertificatePinner.Builder()
4      .add("api.sirius.example", "sha256/…")
5      .build()
6  ).build()

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Attack patterns (CAPEC)

ATT&CK techniques

—

Change log (1)

Fix✗ → ✓Sep 28, 2026, 10:45 PM UTC

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References

bugzilla.redhat.com/show_bug.cgi?id=1925226

Show all references (2 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

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