A null pointer dereference was addressed with improved validation. This issue is fixed in macOS High Sierra 10.13, iCloud for Windows 7.0, watchOS 4, iOS 11, iTunes 12.7 for Windows. Processing maliciously crafted XML may lead to an unexpected application termination or arbitrary code execution.
https://support.apple.com/en-us/HT208144
https://support.apple.com/en-us/HT208142
https://support.apple.com/en-us/HT208141