# unlink {% hint style="success" %} Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\ Learn & practice GCP Hacking: [**HackTricks Training GCP Red Team Expert (GRTE)**](https://training.hacktricks.xyz/courses/grte)
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{% endhint %} ### Code ```c // From https://github.com/bminor/glibc/blob/master/malloc/malloc.c /* Take a chunk off a bin list. */ static void unlink_chunk (mstate av, mchunkptr p) { if (chunksize (p) != prev_size (next_chunk (p))) malloc_printerr ("corrupted size vs. prev_size"); mchunkptr fd = p->fd; mchunkptr bk = p->bk; if (__builtin_expect (fd->bk != p || bk->fd != p, 0)) malloc_printerr ("corrupted double-linked list"); fd->bk = bk; bk->fd = fd; if (!in_smallbin_range (chunksize_nomask (p)) && p->fd_nextsize != NULL) { if (p->fd_nextsize->bk_nextsize != p || p->bk_nextsize->fd_nextsize != p) malloc_printerr ("corrupted double-linked list (not small)"); // Added: If the FD is not in the nextsize list if (fd->fd_nextsize == NULL) { if (p->fd_nextsize == p) fd->fd_nextsize = fd->bk_nextsize = fd; else // Link the nexsize list in when removing the new chunk { fd->fd_nextsize = p->fd_nextsize; fd->bk_nextsize = p->bk_nextsize; p->fd_nextsize->bk_nextsize = fd; p->bk_nextsize->fd_nextsize = fd; } } else { p->fd_nextsize->bk_nextsize = p->bk_nextsize; p->bk_nextsize->fd_nextsize = p->fd_nextsize; } } } ``` ### Graphical Explanation Check this great graphical explanation of the unlink process:

https://ctf-wiki.mahaloz.re/pwn/linux/glibc-heap/implementation/figure/unlink_smallbin_intro.png

### Security Checks * Check if the indicated size of the chunk is the same as the prev\_size indicated in the next chunk * Check also that `P->fd->bk == P` and `P->bk->fw == P` * If the chunk is not small, check that `P->fd_nextsize->bk_nextsize == P` and `P->bk_nextsize->fd_nextsize == P` ### Leaks An unlinked chunk is not cleaning the allocated addreses, so having access to rad it, it's possible to leak some interesting addresses: Libc Leaks: * If P is located in the head of the doubly linked list, `bk` will be pointing to `malloc_state` in libc * If P is located at the end of the doubly linked list, `fd` will be pointing to `malloc_state` in libc * When the doubly linked list contains only one free chunk, P is in the doubly linked list, and both `fd` and `bk` can leak the address inside `malloc_state`. Heap leaks: * If P is located in the head of the doubly linked list, `fd` will be pointing to an available chunk in the heap * If P is located at the end of the doubly linked list, `bk` will be pointing to an available chunk in the heap * If P is in the doubly linked list, both `fd` and `bk` will be pointing to an available chunk in the heap {% hint style="success" %} Learn & practice AWS Hacking:[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)\ Learn & practice GCP Hacking: [**HackTricks Training GCP Red Team Expert (GRTE)**](https://training.hacktricks.xyz/courses/grte)
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{% endhint %}