There is an info leak when decoding the SGBigUTF8String class using [SGBigUTF8String initWithCoder:]. This class initializes the string using [SGBigUTF8String initWithUTF8DataNullTerminated:] even though there is no guarantee the bytes provided to the decoder are null terminated. It should use [SGBigUTF8String initWithUTF8Data:] instead.
While this class is included in iMessage, it is more likely that this bug could be useful in local attacks.
To reproduce this issue:
1) Compile decodeleak.m
clang -o decodeleak -g decodeleak.m -fobjc-arc -framework CoreSuggestionsInternals -F/System/Library/PrivateFrameworks
leaked memory will be printed to the screen.
Proof of Concept: