Sensitive data stored in reusable resources may be inadvertently leaked to a less privileged user or adversary if not properly cleared. Examples of reusable resources include:
- dynamically allocated memory
- statically allocated memory
- automatically allocated (stack) memory
- memory caches
- disk
- disk caches
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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 either \[[MEM09-A. Do not assume memory allocation routines initialize memory]\]. |
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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In practice, this type of [security flaw|BB. Definitions#security flaw] can expose sensitive information to unintended parties. The Sun tarball vulnerability discussed in _Secure Coding Principles & Practices: Designing and Implementing Secure Applications_ \[[Graf 03|AA. C References#Graf 03]\] and [Sun Security Bulletin #00122 | http://sunsolve.sun.com/search/document.do?assetkey=1-22-00122-1] illustrates a violation of this recommendation, leading to sensitive data being leaked. Attackers may also be able to leverage this defect to retrieve sensitive information using techniques such as _heap inspection_. |
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