Question

I want to programmatically determine if the current user (or process) has access to create symbolic links. In Windows (Vista and greater), one cannot create a symbolic link without the SeCreateSymbolicLinkPrivilege and by default, this is only assigned to administrators. If one attempts to create a symbolic link without this privilege, a Windows error 1314 (A required privilege is not held by the client) occurs.

To demonstrate this restriction, I created a clean install of Windows, logged in as the initial Administrator account (restricted through UAC), and was unable to create a symlink in the home directory.

After running the Command Prompt as Administrator or disabling UAC, that command executes without error.

According to this article, “every process executed on behalf of the user has a copy of the [access] token”.

So I’ve created a Python script to query for the permissions. For convenience and posterity, I include an excerpt below.

The idea behind the script is to enumerate all privileges and determine if the process has the required privilege. Unfortunately, I find that the current process does not in fact have the desired privilege, even when it can create symlinks.

I suspect the problem is that even though the current user’s privileges does not explicitly include the privilege, his group membership does afford that privilege.

In short, how can I determine if a given process will have privilege to create symbolic links (without attempting to create one)? An example in C or C++ or Python is preferred, though anything utilizing the Windows API will be suitable.

def get_process_token():
    token = wintypes.HANDLE()
    TOKEN_ALL_ACCESS = 0xf01ff
    res = OpenProcessToken(GetCurrentProcess(), TOKEN_ALL_ACCESS, token)
    if not res > 0: raise RuntimeError("Couldn't get process token")
    return token

def get_privilege_information():
    # first call with zero length to determine what size buffer we need

    return_length = wintypes.DWORD()
    params = [
        get_process_token(),
        TOKEN_INFORMATION_CLASS.TokenPrivileges,
        None,
        0,
        return_length,
        ]

    res = GetTokenInformation(*params)

    # assume we now have the necessary length in return_length

    buffer = ctypes.create_string_buffer(return_length.value)
    params[2] = buffer
    params[3] = return_length.value

    res = GetTokenInformation(*params)
    assert res > 0, "Error in second GetTokenInformation (%d)" % res

    privileges = ctypes.cast(buffer, ctypes.POINTER(TOKEN_PRIVILEGES)).contents
    return privileges

privileges = get_privilege_information()
print("found {0} privileges".format(privileges.count))
map(print, privileges)

Answer 1

I found a solution. The following Python code is a fully-functional script under Python 2.6 or 3.1 that demonstrates how one might determine privilege to create symlinks. Running this under the Administrator account responds with success, and running it under the Guest account responds with failure.

Note, the first 3/4 of the script is mostly API definitions. The novel work begins with get_process_token().

from __future__ import print_function
import ctypes
from ctypes import wintypes

GetCurrentProcess = ctypes.windll.kernel32.GetCurrentProcess
GetCurrentProcess.restype = wintypes.HANDLE
OpenProcessToken = ctypes.windll.advapi32.OpenProcessToken
OpenProcessToken.argtypes = (wintypes.HANDLE, wintypes.DWORD, ctypes.POINTER(wintypes.HANDLE))
OpenProcessToken.restype = wintypes.BOOL

class LUID(ctypes.Structure):
    _fields_ = [
        ('low_part', wintypes.DWORD),
        ('high_part', wintypes.LONG),
        ]

    def __eq__(self, other):
        return (
            self.high_part == other.high_part and
            self.low_part == other.low_part
            )

    def __ne__(self, other):
        return not (self==other)

LookupPrivilegeValue = ctypes.windll.advapi32.LookupPrivilegeValueW
LookupPrivilegeValue.argtypes = (
    wintypes.LPWSTR, # system name
    wintypes.LPWSTR, # name
    ctypes.POINTER(LUID),
    )
LookupPrivilegeValue.restype = wintypes.BOOL

class TOKEN_INFORMATION_CLASS:
    TokenUser = 1
    TokenGroups = 2
    TokenPrivileges = 3
    # ... see http://msdn.microsoft.com/en-us/library/aa379626%28VS.85%29.aspx

SE_PRIVILEGE_ENABLED_BY_DEFAULT = (0x00000001)
SE_PRIVILEGE_ENABLED            = (0x00000002)
SE_PRIVILEGE_REMOVED            = (0x00000004)
SE_PRIVILEGE_USED_FOR_ACCESS    = (0x80000000)

class LUID_AND_ATTRIBUTES(ctypes.Structure):
    _fields_ = [
        ('LUID', LUID),
        ('attributes', wintypes.DWORD),
        ]

    def is_enabled(self):
        return bool(self.attributes & SE_PRIVILEGE_ENABLED)

    def enable(self):
        self.attributes |= SE_PRIVILEGE_ENABLED

    def get_name(self):
        size = wintypes.DWORD(10240)
        buf = ctypes.create_unicode_buffer(size.value)
        res = LookupPrivilegeName(None, self.LUID, buf, size)
        if res == 0: raise RuntimeError
        return buf[:size.value]

    def __str__(self):
        res = self.get_name()
        if self.is_enabled(): res += ' (enabled)'
        return res

LookupPrivilegeName = ctypes.windll.advapi32.LookupPrivilegeNameW
LookupPrivilegeName.argtypes = (
    wintypes.LPWSTR, # lpSystemName
    ctypes.POINTER(LUID), # lpLuid
    wintypes.LPWSTR, # lpName
    ctypes.POINTER(wintypes.DWORD), #cchName
    )
LookupPrivilegeName.restype = wintypes.BOOL

class TOKEN_PRIVILEGES(ctypes.Structure):
    _fields_ = [
        ('count', wintypes.DWORD),
        ('privileges', LUID_AND_ATTRIBUTES*0),
        ]

    def get_array(self):
        array_type = LUID_AND_ATTRIBUTES*self.count
        privileges = ctypes.cast(self.privileges, ctypes.POINTER(array_type)).contents
        return privileges

    def __iter__(self):
        return iter(self.get_array())

PTOKEN_PRIVILEGES = ctypes.POINTER(TOKEN_PRIVILEGES)

GetTokenInformation = ctypes.windll.advapi32.GetTokenInformation
GetTokenInformation.argtypes = [
    wintypes.HANDLE, # TokenHandle
    ctypes.c_uint, # TOKEN_INFORMATION_CLASS value
    ctypes.c_void_p, # TokenInformation
    wintypes.DWORD, # TokenInformationLength
    ctypes.POINTER(wintypes.DWORD), # ReturnLength
    ]
GetTokenInformation.restype = wintypes.BOOL

# http://msdn.microsoft.com/en-us/library/aa375202%28VS.85%29.aspx
AdjustTokenPrivileges = ctypes.windll.advapi32.AdjustTokenPrivileges
AdjustTokenPrivileges.restype = wintypes.BOOL
AdjustTokenPrivileges.argtypes = [
    wintypes.HANDLE,                # TokenHandle
    wintypes.BOOL,                  # DisableAllPrivileges
    PTOKEN_PRIVILEGES,              # NewState (optional)
    wintypes.DWORD,                 # BufferLength of PreviousState
    PTOKEN_PRIVILEGES,              # PreviousState (out, optional)
    ctypes.POINTER(wintypes.DWORD), # ReturnLength
    ]

def get_process_token():
    """
    Get the current process token
    """
    token = wintypes.HANDLE()
    TOKEN_ALL_ACCESS = 0xf01ff
    res = OpenProcessToken(GetCurrentProcess(), TOKEN_ALL_ACCESS, token)
    if not res > 0: raise RuntimeError("Couldn't get process token")
    return token

def get_symlink_luid():
    """
    Get the LUID for the SeCreateSymbolicLinkPrivilege
    """
    symlink_luid = LUID()
    res = LookupPrivilegeValue(None, "SeCreateSymbolicLinkPrivilege", symlink_luid)
    if not res > 0: raise RuntimeError("Couldn't lookup privilege value")
    return symlink_luid

def get_privilege_information():
    """
    Get all privileges associated with the current process.
    """
    # first call with zero length to determine what size buffer we need

    return_length = wintypes.DWORD()
    params = [
        get_process_token(),
        TOKEN_INFORMATION_CLASS.TokenPrivileges,
        None,
        0,
        return_length,
        ]

    res = GetTokenInformation(*params)

    # assume we now have the necessary length in return_length

    buffer = ctypes.create_string_buffer(return_length.value)
    params[2] = buffer
    params[3] = return_length.value

    res = GetTokenInformation(*params)
    assert res > 0, "Error in second GetTokenInformation (%d)" % res

    privileges = ctypes.cast(buffer, ctypes.POINTER(TOKEN_PRIVILEGES)).contents
    return privileges

def report_privilege_information():
    """
    Report all privilege information assigned to the current process.
    """
    privileges = get_privilege_information()
    print("found {0} privileges".format(privileges.count))
    tuple(map(print, privileges))

def enable_symlink_privilege():
    """
    Try to assign the symlink privilege to the current process token.
    Return True if the assignment is successful.
    """
    # create a space in memory for a TOKEN_PRIVILEGES structure
    #  with one element
    size = ctypes.sizeof(TOKEN_PRIVILEGES)
    size += ctypes.sizeof(LUID_AND_ATTRIBUTES)
    buffer = ctypes.create_string_buffer(size)
    tp = ctypes.cast(buffer, ctypes.POINTER(TOKEN_PRIVILEGES)).contents
    tp.count = 1
    tp.get_array()[0].enable()
    tp.get_array()[0].LUID = get_symlink_luid()
    token = get_process_token()
    res = AdjustTokenPrivileges(token, False, tp, 0, None, None)
    if res == 0:
        raise RuntimeError("Error in AdjustTokenPrivileges")

    ERROR_NOT_ALL_ASSIGNED = 1300
    return ctypes.windll.kernel32.GetLastError() != ERROR_NOT_ALL_ASSIGNED

def main():
    assigned = enable_symlink_privilege()
    msg = ['failure', 'success'][assigned]

    print("Symlink privilege assignment completed with {0}".format(msg))

if __name__ == '__main__': main()