Microsoft announced that they have added memcpy() to their list of banned functions. The function memcpy() has been responsible for a bunch of security problems in Microsoft and third party products. The reason for this is that memcpy() is not safe and can cause a buffer overflow. So what is a buffer overflow anyway? Here is what wikipedia has on buffer overflow
In computer security and programming, a buffer overflow, or buffer overrun, is an anomaly where a process stores data in a buffer outside the memory the programmer set aside for it. The extra data overwrites adjacent memory, which may contain other data, including program variables and program flow control data. This may result in erratic program behavior, including memory access errors, incorrect results, program termination (a crash), or a breach of system security.
Buffer overflows can be triggered by inputs that are designed to execute code, or alter the way the program operates. They are thus the basis of many software vulnerabilities and can be maliciously exploited. Bounds checking can prevent buffer overflows.
Programming languages commonly associated with buffer overflows include C and C++, which provide no built-in protection against accessing or overwriting data in any part of memory and do not automatically check that data written to an array (the built-in buffer type) is within the boundaries of that array.
Here is a video of a UltraVNC v1.01 client buffer overflow by using Metasploit.
You can also check milw0rm where they list security vulnerabilities exploits, count the buffer overflow vulnerabilities.
So now that memcpy() is banned what should you use? You should use memcpy_s(). The function memcpy_s() takes one extra parameter: this extra parameter is the size of the destination buffer.
You can read more about this here: http://blogs.msdn.com/sdl/archive/2009/05/14/please-join-me-in-welcoming-memcpy-to-the-sdl-rogues-gallery.aspx