Saturday, 23 May 2020

OBJ_DONT_REPARSE is (mostly) Useless.

Continuing a theme from the last blog post, I think it's great that the two additional OBJECT_ATTRIBUTE flags were documented as a way of mitigating symbolic link attacks. While OBJ_IGNORE_IMPERSONATED_DEVICEMAP is pretty useful, the other flag, OBJ_DONT_REPARSE isn't, at least not for protecting file system access.

To quote the documentation, OBJ_DONT_REPARSE does the following:

"If this flag is set, no reparse points will be followed when parsing the name of the associated object. If any reparses are encountered the attempt will fail and return an STATUS_REPARSE_POINT_ENCOUNTERED result. This can be used to determine if there are any reparse points in the object's path, in security scenarios."

This seems pretty categorical, if any reparse point is encountered then the name parsing stops and STATUS_REPARSE_POINT_ENCOUNTERED is returned. Let's try it out in PS and open the notepad executable file.

PS> Get-NtFile \??\c:\windows\notepad.exe -ObjectAttributes DontReparse
Get-NtFile : (0xC000050B) - The object manager encountered a reparse point while retrieving an object.

Well that's not what you might expect, there should be no reparse points to access notepad, so what went wrong? We'll you're assuming that the documentation meant NTFS reparse points, when it really meant all reparse points. The C: drive symbolic link is still a reparse point, just for the Object Manager. Therefore just accessing a drive path using this Object Attribute flag fails. Still this does means that it will also work to protect you from Registry Symbolic Links as well as that also uses a Reparse Point.

I'm assuming this flag wasn't introduced for file access at all, but instead for named kernel objects where encountering a Symbolic Link is usually less of a problem. Unlike OBJ_IGNORE_IMPERSONATED_DEVICEMAP I can't pinpoint a specific vulnerability this flag was associated with, so I can't say for certain why it was introduced. Still, it's slightly annoying especially considering there is an IO Manager specific flag, IO_STOP_ON_SYMLINK which does what you'd want to avoid file system symbolic links but that can only be accessed in kernel mode with IoCreateFileEx.

Not that this flag completely protects against Object Manager redirection attacks. It doesn't prevent abuse of shadow directories for example which can be used to redirect path lookups.

PS> $d = Get-NtDirectory \Device
PS> $x = New-NtDirectory \BaseNamedObjects\ABC -ShadowDirectory $d
PS> $f = Get-NtFile \BaseNamedObjects\ABC\HarddiskVolume3\windows\notepad.exe -ObjectAttributes DontReparse
PS> $f.FullPath

Oh well...