[tor-bugs] #18101 [Applications/Tor Browser]: IP leak from Windows UI dialog with URI

Tor Bug Tracker & Wiki blackhole at torproject.org
Thu Nov 9 19:12:59 UTC 2017

#18101: IP leak from Windows UI dialog with URI
 Reporter:  uileak                               |          Owner:
                                                 |  arthuredelstein
     Type:  defect                               |         Status:
                                                 |  needs_revision
 Priority:  Very High                            |      Milestone:
Component:  Applications/Tor Browser             |        Version:
 Severity:  Major                                |     Resolution:
 Keywords:  tbb-disk-leak, tbb-proxy-bypass,     |  Actual Points:
  TorBrowserTeam201711                           |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:

Comment (by arthuredelstein):

 Replying to [comment:54 gk]:
 > With the patch for Windows I still see DNS leaks. Here is what I did:

 I was able to reproduce this on Windows 7, but not on Windows 8 Server or
 Windows 10.

 To attempt to fix this on Windows 7, I tried using both IFileDialog and
 GetOpenFileName. Both show the same modern file dialog. It's possible to
 hook some UI events in both, using IFileDialogEvents and
 OPENFILENAME.lpfnHook respectively. But, unfortunately, there is no event
 that occurs after the user clicks the "Open" button (or presses enter),
 and before the DNS leak occurs, that could allow us to cancel the event
 and prevent the leak.

 As an experiment, I tried using the Windows Firewall to block internet
 connections from firefox.exe. While browsing the web is blocked as
 expected, the file dialog still causes DNS leaks! So it seems the leak may
 be happening outside the browser process.

 I also looked into `IFileDialogCustomize::RemoveControlItem`, but as far
 as I could tell, this function only applies to custom control items added
 to the dialog by the application; default control items cannot be removed.

 So, finally, I did find one method that works. It involves:
 1. Use GetOpenFileName
 2. Exclude `OFN_EXPLORER` from OPENFileName.Flags;
 3. Use an "Old-Style" hook function

 The result is an "old-style" file dialog. No DNS leaks occur when the user
 enters a remote URL. And the dialog says "The file name is not valid."

 The main drawback is the dialog is the design from Windows 3.1 (no joke).
 Here it is:


 I guess this dialog is so primitive it doesn't know how to process URLs.

 So we could force Windows 7 and earlier to use this dialog. It's a rather
 stark usability/privacy tradeoff. What do people think? I'll post a patch
 if people think this UI is acceptable. I'm also very open to other
 suggestions for fixing this.

Ticket URL: <https://trac.torproject.org/projects/tor/ticket/18101#comment:62>
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