In my recent post I described how NTFS Sparse files could be used to make life of reversers, forensic investigators, and sandbox developers a bit more difficult. There are  more way to to make their life more miserable and here I describe another potential one.

Every once in a while I re-read the Naming Files, Paths, and Namespaces article on Microsoft page. I do it because I don’t remember all the gory details and I know that this page is updated every once in a while so sometimes you can find some new content there.

New content == new ideas.

While the change I refer to is not very very new, it is very interesting especially from the reversers’ and sandbox developers’ perspective:

Maximum Path Length Limitation

In the Windows API (with some exceptions discussed in the following paragraphs), the maximum length for a path is MAX_PATH, which is defined as 260 characters.

[…]

Tip Starting in Windows 10, version 1607, MAX_PATH limitations have been removed from common Win32 file and directory functions. However, you must opt-in to the new behavior.

A registry key allows you to enable or disable the new long path behavior. To enable long path behavior set the registry key at HKLM\SYSTEM\CurrentControlSet\Control\FileSystem LongPathsEnabled (Type: REG_DWORD). The key’s value will be cached by the system (per process) after the first call to an affected Win32 file or directory function (list follows). The registry key will not be reloaded during the lifetime of the process. In order for all apps on the system to recognize the value of the key, a reboot might be required because some processes may have started before the key was set.

The registry key can also be controlled via Group Policy at Computer Configuration > Administrative Templates > System > Filesystem > Enable NTFS long paths.

This is very interesting.

If there is no limitation, malware could abuse it relying on the fact limitations apply to all software that is not aware of this change.

For instance, if sandbox is copying files from the sandbox to external world, using a script/program that runs inside the sandbox, and does not use the extended-length path’s prefix \\?\ when it accesses files it will fail to copy files out as it won’t ‘see’ them (they are too deep). Notably, the very same limitation applies to the old trick where the known device name is used as a file name and such file (e.g. c:\con) can only be accessed via that extended-length path’s prefix.

But are all sandbox developers aware of this? (unless they use external access to the sandbox disk using e.g. their own NTFS parser)

And as for reversers… if the tool they use to browse the system, open the file don’t use the extended-length path’s prefix they will find themselves unable to browse/open files inside these deeply nested folders.

And if you are a forensic investigator, there is yet another Registry key to learn about:

HKLM\SYSTEM\CurrentControlSet\Control\FileSystem
LongPathsEnabled

So, not entirely new (the c:\con trick is at least 10 years old), but perhaps new to you.