mailing list archives
Re: MS Windows Screensaver Privilege Escalation
From: 3APA3A <3APA3A () SECURITY NNOV RU>
Date: Sat, 27 Nov 2004 11:10:45 +0300
Dear Matt Andreko,
If you have permissions to overwrite system files (Power Users group
have this permission) and you can overwrite system file (screensaver is
system file) there is no vulnerability. You should think twice before
adding user to Power Users, because Power User has nearly same
privileges and permissions as local Administrators. Users are never
added to this group automatically, except if updated from previous
Windows version. All users are automatically removed from this group if
you apply "secure workstation" security policy, you can also use
"limited groups" AD policy to limit membership for this and local
--Friday, November 26, 2004, 9:12:50 PM, you wrote to 3APA3A () SECURITY NNOV RU:
MA> I agree that you should restrict the access physically, however if you
MA> can replace that screensaver file, and WindowsXP will execute it as the
MA> system user, is this not a flaw as the original poster intended?
MA> You had stated that replacing the screensaver took special privileges,
MA> however I was showing a way to get around those means. Sure, if I had
MA> physical access to the machine I could do a lot worse, but personally I
MA> feel it's a blended problem. It does need to be restricted physically,
MA> however I don't think Microsoft should be running screensavers which can
MA> easily be replaced as System.
MA> 3APA3A wrote:
Dear Matt Andreko,
Ability to boot machine from bootable to CD is not a problem of Windows
security, it's more problem of physical security. To prevent your
machine from booting from bootable CD reliably you can use certified
BIOS versions (HP and IBM have few), special marks and devices like
Dallas Lock, Secret Net, etc.
--Friday, November 26, 2004, 6:42:34 PM, you wrote to 3APA3A () SECURITY NNOV RU:
MA> Perhaps this is just an amateurish question, but what if I booted off of
MA> a knoppix cd and replaced the current screensaver with my "specially
MA> crafted" screensaver? Or using the bootdisk at
MA> http://home.eunet.no/~pnordahl/ntpasswd/ to edit the registry value?
MA> I know you may think that this is useless, since if you boot off the cd
MA> or disk, you already have better access to the machine, however doing
MA> this method gets you admin access WITHOUT changing the password, correct?
MA> Again, perhaps I'm misunderstanding, but wouldn't this work, and still
MA> show that the vulnerability in the screensaver code is valid, and needs
MA> to be updated? It could allow someone to get local admin access to the
MA> machine without changing the password.
MA> 3APA3A wrote:
Dear Matthew Walker,
Permissions for HKEY_USERS\Control Panel\Desktop allow modification to
only members of Administrators and System.
Power Users can install software, so they can replace any file in
SYSTEM32 directory, including screensaver. It allows to trojan any
system file (for example, one can replace winspool.exe with cmd.exe to
obtain SYSTEM permissions). It's by design and it's documented. Just
never assign users in Power Users group, as Microsoft recommends you. I
see no security vulnerability here.
--Wednesday, November 24, 2004, 8:36:14 PM, you wrote to
full-disclosure () lists netsys com:
MW> To Whom it May Concern;
MW> The Original Post is http://www.securityfocus.com/bid/11711
MW> On Windows XP all releases, when you replace, or change the
MW> screensaver displayed on the login screen with a specially crafted
MW> version designed to execute programs, those programs are launched
MW> under the SYSTEM SID, IE: they are given automatically the highest
MW> access level avalible to Windows. This level is not accessible even
MW> to administrators.
MW> This flaw is important because while one would need Power User
MW> privledges or above to change the Login Screensaver, by default, any
MW> user with the exception of guest can replace the login screensaver
MW> file with a modified version. In theory, any determined user could
MW> execute ANYTHING with SYSTEM privledges. A similar flaw exists in
MW> Win2K, but Microsoft has ignored it.
MW> Matt Walker
MW> Full-Disclosure - We believe in it.
MW> Charter: http://lists.netsys.com/full-disclosure-charter.html
MA> Full-Disclosure - We believe in it.
MA> Charter: http://lists.netsys.com/full-disclosure-charter.html
Сэр Исаак Ньютон открыл, что яблоки падают на землю. (Твен)
Full-Disclosure - We believe in it.