stunnel before 5.00, when using fork threading, does not properly update the state of the OpenSSL pseudo-random number generator (PRNG), which causes subsequent children with the same process ID to use the same entropy pool and allows remote attackers to obtain private keys for EC (ECDSA) or DSA certificates.

stunnel 4.21 through 4.54, when CONNECT protocol negotiation and NTLM authentication are enabled, does not correctly perform integer conversion, which allows remote proxy servers to execute arbitrary code via a crafted request that triggers a buffer overflow.

The OCSP functionality in stunnel before 4.24 does not properly search certificate revocation lists (CRL), which allows remote attackers to bypass intended access restrictions by using revoked certificates.

Unspecified vulnerability in stunnel before 4.23, when running as a service on Windows, allows local users to gain privileges via unknown attack vectors.

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.

OpenSSL does not use RSA blinding by default, which allows local and remote attackers to obtain the server's private key by determining factors using timing differences on (1) the number of extra reductions during Montgomery reduction, and (2) the use of different integer multiplication algorithms ("Karatsuba" and normal).

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.