Re[2]: [Full-Disclosure] MS Windows Screensaver Privilege Escalation

Date: 11/27/04

  • Next message: Gadi Evron: "Re: [Full-Disclosure] To anybody who's offended by my disclosure policy"
    To: Matt Andreko <>
    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
    Administrators groups.

    --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> 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
    >>>>MW> To Whom it May Concern;
    >>>>MW> The Original Post is
    >>>>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> Sincerly;
    >>>>MW> Matt Walker
    >>>>MW> _______________________________________________
    >>>>MW> Full-Disclosure - We believe in it.
    >>>>MW> Charter:

    MA> _______________________________________________
    MA> Full-Disclosure - We believe in it.
    MA> Charter:

    —эр »саак Ќьютон открыл, что €блоки падают на землю. (“вен)
    Full-Disclosure - We believe in it.

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