If i understand it right you are mixing a release version of OpenSSL with a debug version of your program that causes different CRT versions to be used, since you haven't posted actual settings it may be even worse that OpenSSL is using DLL CRT while your code static. Please post what kind of CRT is your program using (can be found by clicking Properties on the project and then Configuration Properties -> C/C++ -> Code Generation -> Runtime Libarary). Either use proper OpenSSL version (e.g. build it with debug info and linked to debug CRT) or, ...