CAMediaTimingFunctionBuiltin is a class in QuartzCore. Its initWithCoder: method
reads an Int "index" then passes that to builtin_function
mov ebx, edi <-- controlled unsigned int
mov r14d, ebx
lea r15, __ZL9functions_0 ; functions
mov rax, [r15+r14*8]
if rax is non-null it's returned as an objective-c object pointer and the objective-c retain
selector is sent to it.
Serialized poc in attached file with an index of 12345678.
tested on MacOS 10.12.3 (16D32)
Proof of Concept: