It was found that float-parsing code used in Mono before 4.2 is derived from code vulnerable to CVE-2009-0689. The issue concerns the `freelist` array, which is a global array of 16 pointers to `Bigint`. This array is part of a memory allocation and reuse system which attempts to reduce the number of `malloc` and `free` calls. The system allocates blocks in power-of-two sizes, from 2^0 through 2^15, and stores freed blocks of each size in a linked list rooted at the corresponding cell of `freelist`. The `Balloc` and `Bfree` functions which operate this system fail to check if the size parameter `k` is within the allocated 0..15 range. As a result, a sufficiently large allocation will have k=16 and treat the word immediately after `freelist` as a pointer to a previously-allocated chunk. The specific results may vary significantly based on the version, platform, and compiler, since they depend on the layout of variables in memory. An attacker who can cause a carefully-chosen string to be converted to a floating-point number can cause a crash and potentially induce arbitrary code execution.

Patch:

https://gist.github.com/directhex/01e853567fd2cc74ed39

CVE request (contains reproducer):

http://www.openwall.com/lists/oss-security/2015/12/19/3