Tuesday, November 6, 2018

The ._ (dot-underscore) file format

If you've ever looked at removable media and found several hidden files which start with ._ and there exists one for almost every file (or folder) on the disk, this is the result of having that media being used on macOS.

macOS keeps a copy of file metadata in a separate area, known as Extended Attributes (xattr) on HFS+ or APFS. However, when writing to external media which is not formatted as HFS+ or APFS (thus not having the capability to store extended attributes), it will write this information out as a separate file which will have the same name, just prefixed with dot-underscore  ._  as seen in the screenshot below.

Figure 1 - Screenshot showing exFAT volume on External USB disk
While this is well known for many years, this information is often overlooked in a forensic investigation. On media that has interacted with both macOS and Windows (or even Linux), macOS will create these files and delete them too when the original file is deleted. However, if the file is deleted or renamed on Windows or Linux, then the dot-underscore files will be left behind untouched. A while back, Lee Whitfield touched upon this here specifically pointing out its use for knowing the date & time that a file was copied onto the media. However, there is useful information inside the file too. 

This file can contain useful metadata such as kMDItemWhereFroms (URL of file if downloaded from internet) and kMDItemDownloadedDate (Date & Time when it was downloaded) among other extended attributes.

After a bit of reverse engineering, I wrote an 010 hex editor template to parse this information, it is  available at https://github.com/ydkhatri/MacForensics/blob/master/DotUnderscore_macos.bt

In the screenshot below, you can see it being run on one such file. 

Figure 2 - DotUnderscore_macOS.bt template output

Here is analysis from the data extracted:

Attribute Name Value Meaning
kMDItemWhereFroms https://upload.wikimedia.org/wikipedia/commons/3/3c/Thiruvalluvar_Statue_at_Kanyakumari_beach.jpg URL from where it was downloaded
kMDItemDownloadedDate 0x41BFB51D1CFFA4F8 (11/09/2017 23:32:44) Timestamp when file was downloaded
com.apple.quarantine 0083;5a04e59c;Safari;A451620D-2B49-49BD-ADC1-88DEBEA66582 File was downloaded using the Safari browser*

kMDItemDownloadedDate is stored in a plist as a date value, which is stored as a 64-bit double that is the number of seconds since 01/01/2001
The template does not parse the plist for you, you can export it out and open in any plist viewer to view the human-readable date value.

*The 3rd item in com.apple.quarantine's value (separated by ;) is the Application (agent) name which downloads the file. For more details on this, read Howard Oakley's blog post

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