Re: registry hacked under XP limited account
From: Stefan Kanthak (postmaster_at_18.104.22.168.in-addr.arpa)
Date: Sun, 29 May 2005 07:32:12 +0200
"Karl Levinson, mvp" <email@example.com> wrote:
> "lecter" <firstname.lastname@example.org> wrote in message
> > I have a computer run under winxp system. And one day I found that
> > the registry was modified and I couldn't run any .exe file! (the
> > problem have been solved by input a registry key.)
> > The thing I want to know is that the registry can be modified
> > under winXP limited account?
> Very very easily. Running as limited account does VERY LITTLE to stop
> viruses. Anyone who tells you any different is mistaken. Even well-known
> people at Microsoft have this misconception.
Right so far: EVERY piece of code that comes to execution (intentionally
or not) has exactly the same rights/privileges as you. It can trash ALL
your files (remember: on NTFS the owner has full access; on FAT: forget
ANYTHING about security then) and write garbage everywhere you are
allowed to write to. This specifically includes your userprofile (your
registry hive beeing a part of) and your home directory.
BUT: as long as your account has no administrative rights and NO debug
privilege logging out will terminate all processes you started.
AND: running with administrative rights is a VERY BAD HABIT.
Multiuser operating systems are about 50 years old, Unix about 30 years,
and one of the first rules a novice system administrator will learn is:
NEVER work with administrative rights if you don't do administrative
tasks! You get your own limited account and use it for your daily work.
This will LIMIT any damage: just try an RMDIR /S /Q %SystemDrive% with
your limited account and then with administrative rights.
> Running as limited user does prevent much spyware and adware today, but only
> because the authors of that malware see no need to make their programs work
> as limited users. This tactic will NOT be effective against future malware.
WRONG: running as limited or restricted user on a properly setup XP or
2K system prevents malware from infecting or compromising the system
itself or other user accounts.
Malware can do anything you are allowed to do on your account, but cant
compromise other accounts or write itself to %ProgramFiles%, %SystemDrive%
or %SystemRoot% and beyond. It can do anything with [HKCU], but nothing
in [HKLM] and the registry hives of other users.
... except when using a (not yet fixed) security hole.
Up to now I don't know malware that used a (remote) exploit before the
fix was available.
If you're in doubt how to setup a system properly: Microsoft, the
No Such Agency, the NIST and some others published detailed guides how
to "harden" a system. Have a look at the (high) security templates in
%SystemRoot%\System32\Security\Templates\ and use them (carefully).
If you have XP home: turn OFF that dumb "simple file sharing" and
answer the question whether the user profiles should be secured from
other access with YES!
If you don't know how to properly setup a system: go and hire someone
who is able to do this right (but beware).
BUT: when you have a window displayed on your desktop that runs in a
higher privileged process (MOST, if not ALL of those pseudo^Wpersonal
firewalls and some virus scanners do so) then it's possible to attack
that process and perform a privilege escalation.
That's a PRINCIPAL problem of Windows and well known as shatter attack
and should BY ALL MEANS be avoided (don't use such software, and don't
buy such crap).
> Malware running as limited user can do anything that you can do. If you
> were able to change the registry and fix the problem while logged in as a
> limited user, then malware would have the same permissions. You can see the
> permissions of that registry value by clicking Start, Run and typing
Correct. But since you are owner of [HKCU] you have full access to any
of your registry entries (or can get it), so this advice ain't so very
> Also, many viruses use buffer overflows or could theoretically
> use other exploits like local privilege escalation to gain full System
> privileges, regardless of the permissions of the currently logged-in user.
> If the registry value you fixed did not give Write permission to your
> limited account [or to the Users or Everyone groups], then I would go to
> http://windowsupdate.microsoft.com to check to make sure your system has all
> its critical Windows patches to prevent remote buffer overflow viruses.
The least you can and should do is to patch your system timely. Up to now
the exploits came all after the fixes...
> If you have multiple user accounts sharing one machine, logging in as a
> limited user may prevent malware from loading and running when other people
> log in. If you are the only user of your machine, however, that limitation
> means absolutely nothing. Even if multiple people use the same system, they
> can all become infected if they all happen to run a shared infected file,
> for example.
But then that infected file must have been written (itself?) to a
location where all other users will execute it. In a properly setup XP
(Home: turn off "simple file sharing") or 2K the ACLs prevent this.
> What running as limited user does primarily is prevent the user from
> changing the system configuration too much, mainly to implement change
> control within an enterprise. It also makes it harder for malware running
> under your account to do some things like create new login accounts. It's
> also a security best practice, but not really because of viruses or malware.
> Running as limited user does not prevent you from becoming infected, sending
> out infected emails or packets, infecting other systems, deleting all your
> data, searching your data for credit card numbers and passwords, running a
> listening service, etc.
> Note also that "Power User" is really not a very limited user. It is easy
> to escalate privileges to Administrator. Also, most accounts in the Guests
> group are not as limited by default as you might think.
> RUNNING AS LIMITED USER DOES LITTLE OR NOTHING AGAINST VIRUSES. Spread the
But it limits the damage to your own user profile and home directory!
It's therefore possible to clean the infection without reinstallation
of the system: login as another user with administrative rights (you
might prefer "secure mode" so that most autostart mechanisms wont be
triggered) and erase the user profile and the home directory of the
user account where the malware was executed.
Here the typical home user with just one PC has an advantage above the
office user in a companies' network: the latter must be cleaned at all
places where the compromised user account had write access!
AND: if you really do it right then use software restriction policies
and deny the execution of ANY file except beneath %SystemRoot% and
%ProgramFiles%. Since restricted users aren't allowed to write there
they can't run arbitrary code, but only the programs the administrator
(or a power user) installed.
If that's to restrictive: you should AT LEAST deny execution in %TEMP%,
?:\RECYCLE?\, ?:\System Volume Information\, the caches of your browser
and mailer, all removable drives.
ALSO: if you use your PC standalone at home you SHOULD turn off the
whole "Windows network", i.e. file and printer sharing, NetBIOS, RPC,
DirectSMB and so on. You'll need TCP/IP and nothing more to surf the
net and communicate per mail and news and "ICQ" and whatever you like.
Have a look at http://home.arcor.de/skanthak/harden.html and see the
HARDEN2K.INF linked there: this will lock down 2K as far as possible.