ASan does not make your code memory safe! It is quite good at catching unintentional bugs/oob memory writes in your code, and it is quite reliable (authors claim no false positives), but it has false negatives i.e. won't detect everything. Especially if you're against someone who tries to corrupt your memory intentionally.

ASan works by (simplifying a lot) padding allocations and surrounding them with untouchable "red zone". So with some luck even this can work:

  char *a = new char[100];
  char *b = new char[1000];
  a[500] = 0; // may end up in b