A flaw was found in stunnel before 5.57, where it improperly validates client certificates when it is configured to use both redirect and verifyChain options. This flaw allows an attacker with a certificate signed by a Certificate Authority, which is not the one accepted by the stunnel server, to access the tunneled service instead of being redirected to the address specified in the redirect option. The highest threat from this vulnerability is to confidentiality.

Stunnel 5.00 through 5.13, when using the redirect option, does not redirect client connections to the expected server after the initial connection, which allows remote attackers to bypass authentication.

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.

stunnel 4.40 and 4.41 might allow remote attackers to execute arbitrary code or cause a denial of service (heap memory corruption) via unspecified vectors.

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.

stunnel 4.0.3 and earlier allows attackers to cause a denial of service (crash) via SIGCHLD signal handler race conditions that cause an inconsistency in the child counter.

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).