CVE-2021-20230 [High] | Security Vulnerability in Stunnel

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.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-06-15	0.20%	1.18%	+0.98%
2	2025-03-30	0.69%	0.20%	-0.49%
3	2025-03-29	—	0.69%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-06-15

0.20%

1.18%

+0.98%

2025-03-30

0.69%

0.20%

-0.49%

2025-03-29

—

0.69%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
7.5	3.1	HIGH	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network—not just someone sitting at the machine. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:N) No account or special rights needed—anonymous or random user is enough. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:U) Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems. Confidentiality (C:H) Serious risk that confidential data gets exposed in a big way. Integrity (I:N) Data isn’t meaningfully altered or forged. Availability (A:N) Service keeps running; no real outage angle.	3.9	3.6	[email protected]
5.0	2.0	MEDIUM	`AV:N/AC:L/Au:N/C:P/I:N/A:N` Click to expand Access vector (AV:N) Can be exploited remotely over network reachability. Access complexity (AC:L) Exploitation conditions are straightforward and predictable. Authentication (AU:N) No authentication is required. Confidentiality impact (C:P) Partial confidentiality impact. Integrity impact (I:N) No integrity impact. Availability impact (A:N) No availability impact.	10.0	2.9	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

7.5

3.1

HIGH

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

Attack vector (AV:N): Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L): Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N): No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U): Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H): Serious risk that confidential data gets exposed in a big way.
Integrity (I:N): Data isn’t meaningfully altered or forged.
Availability (A:N): Service keeps running; no real outage angle.

3.9

3.6

[email protected]

5.0

2.0

MEDIUM

AV:N/AC:L/Au:N/C:P/I:N/A:N Click to expand

Access vector (AV:N): Can be exploited remotely over network reachability.
Access complexity (AC:L): Exploitation conditions are straightforward and predictable.
Authentication (AU:N): No authentication is required.
Confidentiality impact (C:P): Partial confidentiality impact.
Integrity impact (I:N): No integrity impact.
Availability impact (A:N): No availability impact.

10.0

2.9

[email protected]

OS Trackers for CVE-2021-20230

vendor	priority	summary	link
`debian`	not yet assigned	CVE-2021-20230 not yet assigned priority: Debian including 1 source packages (stunnel4), 4 status rows across 4 suites (bookworm, bullseye, sid, trixie): resolved 4.	https://security-tracker.debian.org/tracker/CVE-2021-20230
`gentoo`	low	CVE-2021-20230: 1 GLSA(s) (202105-02), 1 atom(s) (net-misc/stunnel); latest impact low.	https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2021-20230
`redhat`	high	—	https://access.redhat.com/security/cve/CVE-2021-20230
`suse`	high	CVE-2021-20230 severity important: SUSE including 8 source package names (stunnel, stunnel-5.44-3.8.1, …), 45 product×package rows across 44 product lines (HPE Helion OpenStack 8, SUSE CaaS Platform 4.0, … (44 product lines)): Fixed 25, Known Not Affected 20.	https://www.suse.com/security/cve/CVE-2021-20230/
`ubuntu`	medium	CVE-2021-20230 medium priority: Ubuntu including 1 source packages (stunnel4), 13 status rows across 13 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, trusty, upstream, xenial): not-affected 7, ignored 3, released 3.	https://ubuntu.com/security/CVE-2021-20230

Affected software / configurations for CVE-2021-20230

Vendor	Product	Version	Raw CPE
stunnel	stunnel	< 5.57	cpe:2.3:a:stunnel:stunnel::::::::

References for CVE-2021-20230

URL	Tags
https://bugzilla.redhat.com/show_bug.cgi?id=1925226	Issue Tracking Third Party Advisory
https://github.com/mtrojnar/stunnel/commit/ebad9ddc4efb2635f37174c9d800d06206f1edf9	Patch Third Party Advisory
https://security.gentoo.org/glsa/202105-02	Third Party Advisory

URL

CVE-2021-20230

Exploit prediction scoring system (EPSS) score for CVE-2021-20230

Common vulnerability scoring system (CVSS) metrics for CVE-2021-20230

Weakness enumeration for CVE-2021-20230

OS Trackers for CVE-2021-20230

Affected software / configurations for CVE-2021-20230

References for CVE-2021-20230