Format string vulnerability in stunnel before 3.22 when used in client mode for (1) smtp, (2) pop, or (3) nntp allows remote malicious servers to execute arbitrary code.
https://exchange.xforce.ibmcloud.com/vulnerabilities/7741
http://www.securityfocus.com/bid/3748
http://www.redhat.com/support/errata/RHSA-2002-002.html
http://www.linux-mandrake.com/en/security/2002/MDKSA-2002-004.php3
http://stunnel.mirt.net/news.html
http://online.securityfocus.com/archive/1/248149