http://www.macnn.com/articles/02/03/12/double-free.zlib/
Double-Free zlib Bug doesn't affect OS X
updated 08:05 pm EST, Tue March 12, 2002
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CERT's Vulnerability Note says that Mac OS X is not affected by a Double Free Bug in zlib Compression Library, noted on the main news page earlier today. The bug affects FreeBSD, most Linux variants, and XFree86.
Junior Member
Joined: Jun 2001
Not Mac OS X, nor FreeBSD
Apple knows Mac OS X is not affected for the same reason that FreeBSD, OpenBSD and NetBSD know that they are not affected.
It is because the *BSD's malloc (memory allocation) system is more robust, and when memory is "free"-ed more than once it recognises it as an error rather that causing a potential security hole.
Just one more reason why Mac OS X and the *BSDs are better than Linux.