From Linuxsecurity.com: "All versions of stunnel from 3.15 to 3.21c are vulnerable to format
string bugs in the functions which implement smtp, pop, and nntp client
negotiations. Using stunnel with the "-n service" option and the "-c"
client mode option, a malicious server could use the format sting
vulnerability to run arbitrary code as the owner of the current stunnel
process. Version 3.22 is not vulnerable to this bug."
January 17, 2002