Software Engineering Institute

Apple OS X uses resource forks to store structured data in files. Data forks are used to store unstructured data.

The AppleDouble standard is specified in RFC 1740:
AppleDouble is the preferred format for a Macintosh file that is to be included in an Internet mail message, because it provides recipients with Macintosh computers the entire document, including icons and other Macintosh specific information, while other users easily can extract the Data fork (the actual data) as it is separated from the AppleDouble encoding.

Apple Mail parses resource forks in mail attachments that use the MIME format AppleDouble.

From the Heise Security report:
Files on a Mac can contain additional information, such as the one that another program should be used to open them. The operating system stores these in the file system in a so-called "resource fork", which is linked to the file. This type of information is usually limited to the local system; however, for emails the MIME format AppleDouble allows resource forks to be attached -- these are automatically analyzed by Apple Mail.

This allows an attacker to create an email with an attachment called picture.jpg that is displayed with a JPEG icon. But when the user tries to open the picture, Apple Mail analyses the resource fork and executes a shell script, for example, without further warning. In this case, even the MIME type displays the attachment as image/jpeg, but careful Mac users may become suspicious when they see that the picture is not immediately displayed as usual.

Note that to successfully exploit this vulnerability an attacker would need to convince a user to open a specially crafted email attachment with Apple Mail. Note that this vulnerability was originally addressed in APPLE-SA-2006-03-01, but was not addressed or reintroduced in Mac OS X Leopard. See US-CERT Technical Cyber Security Alert TA06-062A for more information about APPLE-SA-2006-03-01.

A remote, unauthenticated attacker may be able to execute arbitrary commands with the permissions of the user running Apple Mail.

We are currently unaware of a practical solution to this problem.

Do not open unsolicited email attachments

Use caution when opening email attachments. See US-CERT Cyber Security Tip ST04-010 for more information on how to safely handle email messages.