Apple's Preview app passes on unnoticed data fragments
The Preview app, a standard component of Mac OS X present on every Mac, contains a bug with potential data protection ramifications. When copying an area marked using the select tool from a PDF file, it does not merely copy the selected area; it copies the entire contents of the page – hidden and thus unnoticed by the user – into a new document created from the clipboard. The hidden part is, however, retained when the new PDF is saved and subsequently edited. The problem has been described by developer Christian Kienle, who has posted a video demonstrating the problem.
In Preview 5.5 in Mac OS X 10.7 Lion, the hidden part of the PDF can only be viewed by rotating the text, but older versions (such as version 5.0.3 in Mac OS X 10.6.8) display the entire contents of the page if the "document", rather than the default "selection", view is selected from the menu.
While Preview explicitly notes, when cropping selected areas, that "cropping a PDF document doesn't delete the content outside the selection", but merely hides it, this warning is not displayed when copying the selected area. As a result, users may find themselves inadvertently passing on sensitive content. Until Apple fixes this, users who create or edit PDF files using the Preview app should remember that the entire page is retained and can be viewed with certain applications.
Kienle has submitted a bug report to Apple and is planning to offer a tool to check for and display any hidden content in PDF files shortly.