Newlib Unlink Heap Exploitation

In this paper, I introduce the reader to a heap metadata corruption against the latest version of newlib. This allocator is used in embedded systems. The unlink attack on heaps was first introduced by Solar Designer in the year 2000 and was the first generic heap exploitation technique made public. The same attack is possible in modern day uClibc and the attack in newlib is almost identical. In the unlink technique, an attacker corrupts the bk and fd pointers of a free chunk. In a subsequent malloc that recycles this chunk, the chunk is unlinked from its freelist via pointer manipulations. This inadvertently allows an attacker to craft a write-what-where primitive and write what they want where they want in memory. The unlink attack name stemmed from the fact that the unlink macro is the code that performs the pointer manipulation to unlink the free chunk. This macro is unchanged in newlib and is also called unlink.

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