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Dynamic memory managers are not required to clear freed memory and generally do not because of the additional runtime overhead. Furthermore, dynamic memory managers are free to reallocate this same memory. As a result, it is possible to accidentally leak sensitive information if it is not cleared before calling a function that frees dynamic memory. Programmers also cannot rely on memory being cleared during allocation. (See recommendation MEM09-C. Do not assume memory allocation routines functions initialize memory.)
To prevent information leakage, sensitive information must be cleared from dynamically allocated buffers before they are freed. Calling free() on a block of dynamic memory causes the space to be deallocated; that is, the memory block is made available for future allocation. However, the data stored in the block of memory to be recycled may be preserved. If this memory block contains sensitive information, that information may be unintentionally exposed.
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