macOS – How Unix File Permissions are Stored in ZIP Files

archivefinderjavapermissionzip

I am trying to extract a ZIP file in Java which was created on a Mac using the built-in archive functionality of the Finder. As the ZIP file contains an application bundle, it is important to retain the Unix file permissions, especially the executable flag.

When extracting the ZIP file using the Finder, the permissions are retained. So I know they're stored somewhere in there.

Now my question is: how does macOS store Unix file permissions in ZIP files? Unfortunately I could not find any information about this on the net.

Best Answer

The permissions are stored in the same manner as they are on other Unixoid systems, such as for example Linux.

If you look at the central directory header in the zip-file, you have for each file stored in the Zip-file a field called "external file attributes". The field is at byte 36 in the header and is 4 bytes long.

The two last bytes in that field contains the file permissions on macOS - to be precise it is the last 9 bits (the least significant 9 bits). Their meaning is just like traditional Unix file permissions.

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MacOS – How to retrieve image files from a damaged zip archive file

are you sure that the file is an actual Zip file as you will get this message on a file that has been renamed as .zip when it isnt in fact a Zip file.

The only real way to tell is to use a HEX editor such as 0xED to check the first few bytes are the characters PK.

MacOS – What does “@” signify in unix file permissions

If the file or directory has extended attributes, you'll see an @ in the permissions field. For extended security information (ACLs), you'll see a +.

From man ls on OS X:

-@      Display extended attribute keys and sizes in long (-l) output.

The xattr program can be used to display and manipulate extended attributes. Take a look at what's there before deciding to go while deleting those attributes, though. Definitely take a quick look at the man page for xattr too.

A quick example from some example PHP compromise code I keep around for reference:

$ ls -@l php-compromise.php 
-rw-r--r--@ 1 user  group  502620 Jul  5  2011 php-compromise.php
    com.apple.FinderInfo        32 
    com.apple.TextEncoding      15

Best Answer

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