Saturday, November 24, 2018

$Recycle bin and Undo operations


This week Phil Moore made an excellent finding (link here), one that most of us have seen for years but not investigated. Those $I files that seem orphaned/abandoned without explanation now have one. Phil notes that every time a file/folder is deleted and then restored, the $I file stays behind in the recycle bin. This post is about some follow up testing and results.

I did some quick tests - deleting and restoring files in a few different ways testing $I & $R file creation every time. I'm not even looking at indexes or timestamps, just file creation/deletion. My testing was on Windows 10 (32bit) version 10.0.10586.106

I'm not reproducing all the output here, but only the summary. Here are a few ways to send files to the recycle bin. Remember you cannot do any operations using the command line (as that operates directly at the file system level and does not use the recycle bin abstraction).

First we delete a file (right-click and select Delete). Let's try to restore now using any one of the following ways:

1. Right-click the file, click Restore on file in the recycle bin
2. Cut file from recycle bin and Paste elsewhere
3. Drag the file from recycle bin and into another folder
4. Undo the last operation using Ctrl-Z  OR  right-click & select Undo Delete


Only the 4th method (Undo Delete) results in deletion of the $I file. The other methods leave it behind.

I believe this to be a windows bug. Interestingly, when you Restore a file from the bin (right-click & Restore), then right-click on the desktop (or in any folder), the context menu has an new item called 'Undo Move'. It does NOT say 'Redo Delete'. For every other action, you will see a 'Redo ACTION' in the menu (see figure 2). So I believe when you restore a file, windows just performs a file move (on the $R file) and thus marked it as a move in the last performed action. 

Figure 1 - 'Undo Move' seen after restoring file
Now, clicking on the Undo Move will result in sending the file back to the recycle bin but as a new $I & $R pair, as if it was an entirely new delete operation. 

Figure 2 - 'Redo Rename' seen after using 'Undo Rename' from an unrelated file rename operation
In figure 3 below, you can see multiple abandoned $I files after doing this multiple times (delete, then restore).

Figure 3 - Recycle bin folder after multiple deletes and restores showing abandoned $I files
Let us know (comments or twitter) if you know of more ways to delete/restore.

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