A flaw was found in the way stunnel, a socket wrapper which can provide SSL support to ordinary applications, performed (re)initialization of PRNG after fork. When accepting a new connection, the server forks and the child process handles the request. The RANDbytes() function of openssl doesn't reset its state after the fork, but seeds the PRNG with the output of time(NULL). The most important consequence is that servers using EC (ECDSA) or DSA certificates may under certain conditions leak their private key.

Stunnel 4.00, and 3.24 and earlier, leaks a privileged file descriptor returned by listen(), which allows local users to hijack the Stunnel server.

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.