What to Do If after High Sierra Update All Files Are “Damaged and Can’t Be Opened”?

Some Mac users, who had updated to macOS High Sierra by using a clean install from scratch approach and then reverted to Sierra through Migration Assistant, report not being able to open any app. After trying to open even a freshly-downloaded app, they encounter an error message that reads “‘File Name’ is damaged and can’t be opened. You should move it to the Trash.” What makes this error message equally threatening for both technically-savvy and novice users is the fact that in addition to apps, the ostensible ‘damage’ involves all files residing on a hard drive. This makes it virtually impossible to engage in any meaningful interaction with a Mac or MacBook. Furthermore, even disk image files (DMG) downloaded from the Apple support page appear to be corrupted, which cannot be true.

It should be noted that the user files and apps are not actually damaged; therefore, there is no need for moving them to the Trash. Instead, it is recommended to try one out of two possible workarounds for the issue until Apple Support Team fixes it.

Command Line Solution

The first solution involves the use of the command line. It is not the most efficient way of solving the issue because it involves the manual removal of quarantine attributes. Moreover, the attributes have to be disabled for each folder, which takes a lot of users’ time and patience. Even though this workaround is an undeniably unwieldy one, it is the safest way to remove the problem without considerably endangering a Mac. To change the settings for quarantine attributes that get automatically assigned to each file, use the following instructions:

  1. Open the Application folder and proceed to the Utilities folder;
  2. Locate and launch the Terminal application (alternatively, use the Spotlight and search for the Terminal);
  3. Change the working directory to the directory in which ‘corrupted’ files reside by using its path (for example, $ cd /Users/Janice/Documents);
  4. Once in the necessary directory, use the following command: xattr -d com.apple.quarantine *.pdf *.dmg;
  5. Close the Terminal.

This solution requires a bit of technical prowess and has to be repeated multiple times for each involved folder. Therefore, one can try another approach for removing the “damaged app” message.

Gatekeeper Solution

Gatekeeper system of macOS has been specifically designed to protect a Mac from malware and viruses. The solution that follows is based on disabling Gatekeeper; therefore, it is highly recommended to use it as a last resort and only with fully-trusted Apps. If after switching off their Gatekeeper, a user starts downloading and installing random software, they might end up with a severely compromised system. To disable Gatekeeper:

  1. Open the Apple Menu and go to the System Preferences;
  2. Click on the Security and Privacy icon and select the General tab at the top of the window;
  3. Click on the padlock to unlock the settings of the System Preferences;
  4. Select the Anywhere option under the heading that reads “Allow applications downloaded from”;
  5. In a pop-up security window, click on the Allow button;
  6. Close the System Preferences;

Now it is possible to open any app or file without the “damaged app” warning. If there’s a need to return the system to the state of total security, a user can open System Preferences and select the Mac App Store and Identified Developers option.