The first bug in Vista UAC?

Posted by AB
Jul 27 2008

(This post is moved here from the TweakUAC web site, were it appeared first on January 1st, 2007).

I believe I’ve stumbled upon the first bug in Vista UAC (in the final release of Vista, not in a beta version).

It’s very easy to see the bug in action:

  • Login to your computer with the Guest account. (You may need to enable the Guest account first, using the Control Panel).
  • Download any digitally signed program (such as TweakUAC), save it to the default download folder (C:\Users\Guest\Downloads).
  • Now run the file you’ve just downloaded, and take a look at the elevation prompt displayed:

As you can see, UAC cannot recognize that the file contains a valid digital signature, and it warns you that the program is “unidentified”. This is a bug, because you can check that the digital signature of the file is actually valid:

This problem is not limited to the TweakUAC file, any other digitally signed executable (such as the installation utilities of most software packages) will produce the same effect. All you need to do to reproduce this bug is login to Vista with the Guest account and run a digitally signed file from the Guest\Downloads folder. Note that if you copy the executable into the C:\Program Files folder, and run the file from there, its digital signature would magically become recognizable by UAC! Move the file to the root folder C:\, and the file again becomes unidentified to UAC.

Is this bug dangerous? Yes, it is! The whole idea behind UAC is to shift the responsibility of distinguishing the bad programs from the good ones to the end user (you!). The only tool that UAC gives you in this regard is the digital signature information, and it turns out it’s broken! How are you supposed to make the decision whether to trust a certain program or not if UAC does not provide you with the correct information? (Nevermind, it’s a rhetorical question).

Andrei Belogortseff

WinAbility Software Corp.

Comments:

21 Responses to “The first bug in Vista UAC?”

  1. Myria Says:

    I believe that this is because there is an NTFS fork on the directory that says that anything in that directory shouldn’t be trusted. This is similar to XP in how it knows that a file was downloaded recently by IE.

  2. Soumitra Says:

    Hi Andrei,

    Have a quick question about TweakUAC. Can I suppress UAC messages only for a single application using TWeak? Or does it suppress all UAC messages, system wide?

    Thanks.

    Regards,

    Soumitra

  3. Andrei Belogortseff Says:

    Hi Soumitra,

    > Can I suppress UAC messages only for a single application using TWeak?

    No, it’s impossible.

    > Or does it suppress all UAC messages, system wide?

    Yes, that’s how it works.

    Andrei.

  4. Andrei Belogortseff Says:

    Hi Myria,

    > I believe that this is because there is an NTFS fork on the directory that says that anything in that directory shouldn’t be trusted. This is similar to XP in how it knows that a file was downloaded recently by IE.

    It may very well be so, but it does not make it any less of a bug. If a file contains a valid digital signature, Windows should not misrepresent it as coming from an unidentified publisher.

    Andrei.

  5. Chris Says:

    How did you take a screenshot of the UAC? I can’t get Print Screen to copy it to the clipboard, and the snipping tool isn’t working either.

  6. Andrei Belogortseff Says:

    Hi Chris,

    > How did you take a screenshot of the UAC? I can’t get Print Screen to copy it to the clipboard, and the snipping tool isn’t working either.

    Those tools don’t work because UAC displays its messages on the secure desktop, to which the “normal” user tools have no access. To solve this problem, I’ve changed the local security policy to make the UAC prompts to appear on the user’s desktop. After that, I used the regular Print Screen key to capture the screenshots.

    Hope this helps,

    Andrei.

  7. Matthew Bragg Says:

    Hi Andrei,

    I sell software to a *very* non-technical customer base. My setup procedure includes installation of an .ocx file into the \windows\system32 folder and registration of it using regsvr32. In order to copy anything into the \windows\system32 folder under Vista I have to turn off UAC. I would like to be able to do this automatically, programmatically, so I don’t have to make my users mess with UAC. I’d like to be able to turn off UAC for a second or two programmatically, then turn it back on. Will your software enable me to do that?

    Thanks

  8. Andrei Belogortseff Says:

    Hi Matthew,

    > I’d like to be able to turn off UAC for a second or two programmatically, then turn it back on.

    Unfortunately it’s impossible: if you enable or disable the UAC, Windows must be restarted before the change would have take effect.

    To solve your problem:

    > In order to copy anything into the \windows\system32 folder under Vista I have to turn off UAC.

    It looks like your setup process is executing non-elevated, that’s why it cannot do that. You may want to try to start it elevated and see if it would have solved the problem without turning off the UAC.

    HTH

    Andrei.

  9. caz Says:

    don’t use TweakUAC because this program makes your Vista unsafe!

  10. Herbys Says:

    > How are you supposed to make the decision whether to trust a certain program or not if UAC does not provide you with the correct information? (Nevermind, it’s a rhetorical question).

    The answer is you are not. A guest should not be allowed to make any decision about installing software. If you log on as a valid user, the prompt works just fine. If you log on as a guest, you shouldn’t be installing software, so any dire warning is fine.

    Yes, this might be unintended behavior (or perhaps it is not), but its impact is null.

  11. Andrei Belogortseff Says:

    Hi Herbys, you wrote:

    > The answer is you are not. A guest should not be allowed to make any decision about installing software.

    Sorry, but you are missing the point: the UAC displays this information for the _administrator_ to use and to make a decision, not for the guest user. The administrator is supposed to review the information and enter his or her password to approve the action. Take a look at the screenshot and see for yourself.

    > If you log on as a guest, you shouldn’t be installing software

    Why shouldn’t I? What if I want to install a program for use by the guests only? For example, I use only one web browser (IE), but I never know what browser a guest may want to use. So, being a good host :-) I want to install also Firefox and Opera, but I don’t want them to clutter my desktop, etc., I want them to be used by the guests only. To achieve that, I would log in to the guest account and install the additional browsers from there.

    > so any dire warning is fine.

    Wrong.

    > Yes, this might be unintended behavior (or perhaps it is not), but its impact is null.

    May be, may be not. In any case, it does not make it any less of a bug!

  12. Timothy Conner Says:

    Is there any plan to adapt your program into a Control Panel Applet? I think that would be very clever.

  13. Andrei Belogortseff Says:

    Hi Timothy, you wrote:

    > Is there any plan to adapt your program into a Control Panel Applet?

    No, we don’t have such plans at this time, sorry.

  14. Mike Says:

    Anyone know why Vista won’t let me rename any new folder?

    The permissions are all checked for me as administrator, still I get an error message, “folder does not exist”. I can put things in folder and move it, but can’t rename it?

  15. Bob Says:

    To be honest, I have always thought that digitally signing was merely a way of generating more revenue. It doesn’t offer you any more security and windows will always moan at you regardless of an application having a signature or not.

    Even if your application has the “all powerful” and completely unnecessary Windows Logo certification, it still offers nothing to you as a user other than the reassurance that the person/s developing the software has allot of spare cash.

  16. Thomas Says:

    Bob Said:

    > To be honest, I have always thought that digitally signing was merely a way of generating more revenue.

    I have to agree. I’ve heard the argument of how it’s all designed to protect users from malicious software, and that’s all well and good as far as that goes — but since Vista, and most mobile OSes, don’t offer a way for users to say “okay, I understand the risk, I accept full responsibility, please go ahead and run this unsigned application without restrictions, and never bother me again when I try to run this application”… That makes it pretty clear it’s just a racket initiated by VeriSign and the like, and happily endorsed by Microsoft.

  17. Andrew Says:

    This is not a bug.

    The first screen shot shows that Windows doesn’t trust the identity contained in the certificate. In other words, “I can read this, but I don’t know if I should trust the person who wrote it.”

    The second screen shot just shows that the certificate is well-formed, that Windows can understand the information contained within it. It says nothing about what Windows will do with that information.

    Who did you did you pay to sign the certificate for you? If they’re not someone with a well-established reputation, then I don’t WANT my computer to automatically trust them.

    It’s just like how web browsers automatically trust SSL certificates signed by Thawte or Verisign, but will ask you before accepting a certificate from Andy’s Shady Overnight Certificate Company. As always, it’s a balancing act between usability and security.

  18. Andrei Belogortseff Says:

    Andrew, you wrote:

    > This is not a bug.

    OK, there is a fine line between a bug and a feature, let’s assume for a moment that it’s a feature rather than a bug. If so, what benefit is this feature supposed to provide? As the second screen shows, the file is digitally signed, and Vista can detect that. Yet, it shows the publisher as “unidentified” on the first screen. Note also (as I mentioned in the post), that if you move the file to one a few specific folders (such as C:/Program files), Vista would magically begin to recognize the publisher. Move the file to some other folder, and it’s unidentified again.

    If you can explain why they designed it that way, I would agree with you. Until then, it’s a bug. Guilty until proven innocent!

    > Who did you pay to sign the certificate for you?

    That particular file was signed with a Verisign certificate, but the same problem occurs with _any_ file, signed with _any_ certificate. Try it yourself and you will see.

  19. Farthen Says:

    I think that it occurs because of IE7 protected mode – see http://victor-youngun.blogspot.com/2008/03/internet-explorer-7-protected-mode-vs.html, it’s a guide to run firefox in protected mode, and this explains very good how the protected mode works… the prompt is because the “Download” Folder is a protected folder (level “low”) and I think it only displays at the guest, because Windows forces UAC to display certificate in normal user mode in “low” level folders, but NOT for the MUCH MORE RESTRICTED “guest” account. This would be my explanaition.

    It doesn’t mean that I like it how Microsoft handels this but this would eventually explain WHY the warning appears sometimes and sometimes not.

  20. Farthen Says:

    sorry, in my last post there is a comma in the link, the correct link is:

    http://victor-youngun.blogspot.com/2008/03/internet-explorer-7-protected-mode-vs.html

  21. Andrei Belogortseff Says:

    Farthen: thank you for the information and explanation!

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