If you for example download Skim on a new OS X installation, set Skim as the default application for PDF files, and open a PDF file, a dialog like this is shown:

You are opening the application Skim for the first time. Are you sure you want to open this application?

The application is in a folder named Applications. To see the application in the Finder without opening it, click Show Application.

The dialog is not shown if you open an application directly.

The dialog is shown again for each application if you reset the Launch Services database. The information about what applications have been opened before might be stored in /var/folders/*/*/C/com.apple.LaunchServices-*.csstore, because it's modified when I allow opening an application.

defaults write com.apple.LaunchServices LSQuarantine -bool false only disables the dialogs about opening quarantined files.

  • It seems you need to killall Finder after changing this setting. – Daniel Serodio May 7 '12 at 14:13
  • @DanielSerodio Or just log out and back in or whatever. It still only disables the dialogs about opening files, not applications. – Lri May 7 '12 at 14:19

You can simply open a Terminal window (/Applications/Utilities/Terminal.app) and run this command:

defaults write com.apple.LaunchServices LSQuarantine -bool NO

This will prevent downloaded files from being quarantined. Note that this needs to be run on a per-user basis. If you want it to apply to the whole system, run it with sudo and type your password:

sudo defaults write com.apple.LaunchServices LSQuarantine -bool NO
Password: <your password>
  • @Lri I don't believe so, no. I have that enabled and haven't seen one of either message in a while. – squircle Apr 3 '11 at 22:11
  • I've also set LSQuarantine to false, but I'm only getting these warnings. Try for example downloading iChm, associating chm files with it, and opening a .chm from Finder. – Lri Apr 3 '11 at 22:41
  • 1
    These two commands had no effect in Mountain Lion. I am still getting Stuffit Expander warnings, for example, every time I open an archive. This did not occur prior to the upgrade to ML – user152300 Aug 14 '12 at 20:47
  • Not working on Sierra – Shai Jul 17 '17 at 12:31

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