full-disclosure - MS Windows Screensaver Privilege Escalation

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From: 3APA3A at SECURITY.NNOV.RU (3APA3A)
Subject: MS Windows Screensaver Privilege Escalation

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@...URITY.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@...ts.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> Sincerly;
>> MW> Matt Walker
>> 
>> MW> _______________________________________________
>> MW> Full-Disclosure - We believe in it.
>> MW> Charter: http://lists.netsys.com/full-disclosure-charter.html
>> 
>> 

-- 
~/ZARAZA
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