Arbitrary file write via failed symlink validation

Symlink validation in xar on Apple macOS allows local users to write arbitrary files when extracting a crafted XAR archive. xar processes its archive table of contents in reverse, creates a directory symlink, continues after the corresponding real-directory creation fails, and writes the archived file through the symlink. Exploitation requires a local user to extract an attacker-crafted XAR archive; the resulting write targets a location selected by the attacker.