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Do not use realloc() to resize buffers that store sensitive information.
Heap inspection vulnerabilities occur when sensitive data, such as a password or an encryption key, can be exposed to an attacker because they are not removed from memory.
The realloc() function is commonly used to increase the size of a block of allocated memory. This operation often requires copying the contents of the old memory block into a new and larger block. This operation leaves the contents of the original block intact but inaccessible to the program, preventing the program from being able to scrub sensitive data from memory. If an attacker can later examine the contents of a memory dump, the sensitive data could be exposed.
The following code calls realloc() on a buffer containing sensitive data:
cleartext_buffer = get_secret(); ... cleartext_buffer = realloc(cleartext_buffer, 1024); ... scrub_memory(cleartext_buffer, 1024);
There is an attempt to scrub the sensitive data from memory, but realloc() is used, so a copy of the data can still be exposed in the memory originally allocated for cleartext_buffer.