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.
Affected products
| Vendor | Product | Affected | Fixed |
|---|---|---|---|
| — | stunnel | stunnel 5.57 | — |
Fixed in Linux distributions
Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.
Debian
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian 13 | stunnel4 | 3:5.56+dfsg-8 | — |
| Debian 12 | stunnel4 | 3:5.56+dfsg-8 | — |
Ubuntu
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Ubuntu 20.04 LTS | stunnel4 | 3:5.56-1ubuntu0.2 | USN-6901-1 |
Show 1 more (older releases, extended support, other products)Fewer releases
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Ubuntu Pro 18.04 LTS | stunnel4 | 3:5.44-1ubuntu3+esm1 | USN-6901-1 |
Red Hat
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| RHEL 8 | stunnel | 5.56-5.el8_3 | RHSA-2021:0618 |
Show 2 more (older releases, extended support, other products)Fewer releases
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| RHEL 8.2 EUS | stunnel | 5.48-6.el8_2 | RHSA-2021:0619 |
| RHEL 8.1 EUS | stunnel | 5.48-6.el8_1 | RHSA-2021:0620 |
AlmaLinux
Errata ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| AlmaLinux 8 | stunnel | 5.56-5.el8_3 | ALSA-2021:0618 |
Rocky Linux
Errata ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Rocky Linux 8 | stunnel | 5.56-5.el8_3 | RLSA-2021:0618 |
SUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| SLES 15 SP2 | stunnel | 5.57-3.11.1 | SUSE-SU-2021:0772-1 |
| SLES 15 SP1 | stunnel | 5.44-3.8.1 | SUSE-SU-2021:1465-1 |
| SLES 15 | stunnel | 5.44-3.8.1 | SUSE-SU-2021:1465-1 |
Show 8 more (older releases, extended support, other products)Fewer releases
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| SLES for SAP 15 SP1 | stunnel | 5.44-3.8.1 | SUSE-SU-2021:1465-1 |
| SLES for SAP 15 | stunnel | 5.44-3.8.1 | SUSE-SU-2021:1465-1 |
| SLE HPC 15 SP1 | stunnel | 5.44-3.8.1 | SUSE-SU-2021:1465-1 |
| SLE HPC 15 | stunnel | 5.44-3.8.1 | SUSE-SU-2021:1465-1 |
| SUSE Manager Server 4.0 | stunnel | 5.44-3.8.1 | SUSE-SU-2021:1465-1 |
| SUSE Manager Retail Branch Server 4.0 | stunnel | 5.44-3.8.1 | SUSE-SU-2021:1465-1 |
| SUSE Manager Proxy 4.0 | stunnel | 5.44-3.8.1 | SUSE-SU-2021:1465-1 |
| SUSE Enterprise Storage 6 | stunnel | 5.44-3.8.1 | SUSE-SU-2021:1465-1 |
openSUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| openSUSE Leap 15.2 | stunnel | 5.57-lp152.2.6.1 | openSUSE-SU-2021:0409-1 |
Azure Linux
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Azure Linux 2 (CBL-Mariner) | stunnel | 5.70-1 | — |
Oracle Linux
| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Oracle Linux 8 | stunnel | 5.56-5.el8_3 | ELSA-2021-0618 |
Advisories 11
- ALSA-2021:0618 Important: stunnel security update AlmaLinux Security Advisories · Oct 29, 2026
- USN-6901-1 stunnel4 vulnerability Ubuntu Security Notices (USN) · Feb 10, 2026
- RLSA-2021:0618 Important: stunnel security update Rocky Linux Security Advisories · Feb 4, 2026
- SUSE-SU-2021:1465-1 Security update for stunnel SUSE / openSUSE Security Updates · Feb 4, 2026
- openSUSE-SU-2021:0409-1 Security update for stunnel SUSE / openSUSE Security Updates · Feb 4, 2026
- SUSE-SU-2021:0772-1 Security update for stunnel SUSE / openSUSE Security Updates · Feb 4, 2026
- RHSA-2021:0619 Red Hat Security Advisory: stunnel security update Red Hat Security Advisories (RHSA) · Nov 22, 2025
- RHSA-2021:0620 Red Hat Security Advisory: stunnel security update Red Hat Security Advisories (RHSA) · Nov 22, 2025
- RHSA-2021:0618 Red Hat Security Advisory: stunnel security update Red Hat Security Advisories (RHSA) · Nov 22, 2025
- WID-SEC-2024-1674 Red Hat Enterprise Linux: Schwachstelle ermöglicht Umgehen von Sicherheitsvorkehrungen BSI CERT-Bund (WID) · Jul 17, 2024
- ELSA-2021-0618 Important: stunnel security update (Oracle Linux 8) Oracle Linux Security Advisories (ELSA) · Feb 23, 2021