Stunnel 5.00 through 5.13, when using the redirect option, does not redirect client connections to the expected server after the initial connection, which allows remote attackers to bypass authentication.

Johan Olofsson discovered an authentication bypass vulnerability in Stunnel, a program designed to work as an universal SSL tunnel for network daemons. When Stunnel in server mode is used with the redirect option and certificate-based authentication is enabled with 'verify = 2'or higher, then only the initial connection is redirected to the hosts specified with 'redirect'. This allows a remote attacker to bypass authentication.

This update fixes an authentication bypass when using the 'redirect' option (CVE-2015-3644, bsc#931517, backport from v5.17).

Note that Tenable Network Security has extracted the preceding description block directly from the SUSE security advisory. Tenable has attempted to automatically clean and format it as much as possible without introducing additional issues.

The version of stunnel installed on the remote host is prior to version 5.14. It is, therefore, affected by a vulnerability related to the handling of authentication failures that involve the 'redirect' option. In this case, only the initial connection is forwarded to the hosts specified with 'redirect'; however, subsequent connections established with reused SSL/TLS sessions are forwarded to the hosts specified with 'connect' as if they were already successfully authenticated. A remote attacker can exploit this vulnerability to bypass authentication mechanisms.

Note that Nessus has not tested for this issue but has instead relied only on the application's self-reported version number.

ID	Name	Product	Family	Severity
84509	Debian DSA-3299-1 : stunnel4 - security update	Nessus	Debian Local Security Checks	medium
84206	SUSE SLES12 Security Update : stunnel (SUSE-SU-2015:1062-1)	Nessus	SuSE Local Security Checks	medium
83730	stunnel < 5.14 Authentication Bypass Vulnerability	Nessus	Windows	medium

Detections

Analytics

CVE-2015-3644

critical

Tenable Plugins

medium

medium

medium