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.
For the stable distribution (jessie), this problem has been fixed in version 3:5.06-2+deb8u1.
For the testing distribution (stretch), this problem has been fixed in version 3:5.18-1.
For the unstable distribution (sid), this problem has been fixed in version 3:5.18-1.
We recommend that you upgrade your stunnel4 packages.