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As noted in bullet 169 of Appendix J, the behavior a program is undefined when

the pointer argument to the free or realloc function does not match a pointer earlier returned by calloc, malloc, or realloc, or the space has been deallocated by a call to free or realloc.

Freeing memory multiple times has similar consequences to accessing memory after it is freed. The underlying data structures that manage the heap can become corrupted in a way that can introduce security vulnerabilities into a program. These types of issues are referred to as double-free vulnerabilities. In practice, double-free vulnerabilities can be exploited to execute arbitrary code. VU#623332, which describes a double-free vulnerability in the MIT Kerberos 5 function krb5_recvauth(), is one example.

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