The glibc library provides functions such as free and malloc to help developers manage the heap memory according to their use cases. It is the responsibility of the developer to:
free any memory he/she has obtained using malloc.
Do not free the same memory more than once.
Ensure that memory usage does not go beyond the amount of memory requested, in other terms, prevent heap overflows.
Failing to do makes the software vulnerable to various kinds of attacks. Shellphish, a famous Capture the Flag team from UC Santa Barbara, has done a great job in listing a variety of heap exploitation techniques in how2heap. Attacks described in "The Malloc Maleficarum" by "Phantasmal Phantasmagoria" in an email to the "Bugtraq" mailing list are also described.
A summary of the attacks has been described below:
This is not an attack, it just demonstrates the nature of glibc's allocator
Making malloc return an already allocated fastchunk
Disrupt the fastbin by freeing a chunk twice
Making malloc return a nearly arbitrary pointer
Disrupting fastbin link structure
Getting (nearly)arbitrary write access
Freeing a corrupted chunk and exploiting unlink
Shrinking Free Chunks
Making malloc return a chunk overlapping with an already allocated chunk