Q: Failed to verify the preflight file. It is not signed by Apple.

Try repairing permissions in Applications -> Utilities -> Disk Utility -> First Aid.

Also do not use any third party utilities for system maintenance, as they may actually cause more problems than they solve.

Yes please, because I have the same issue with all the updates and I did try the repair permissions.

same issue for me after updating to Maverick(10.9).

same here. can't update iMovie or iPhoto

Here's what got me the iWorks updates: I created a temporary user with Administrator priviledges, logged out then back in as the temp. Next, I opened AppStore, logged in with my usual Apple ID, did the updates, logged out of the temp account then logged back in as "me" and deleted the temp user.

This would indicated something wonky in my User folder (Library > Preferences, perhaps?)

However, this did not get me updates for iBooks Author or Xcode as they didn't show up as updatable when I was logged in as the "temp" user and am still getting the preflight error for them.

I am having the same issues: Failed to verify the preflight file. It is not signed by Apple. Have repaired permissions did a safe reboot. Still an issue.

If you are behind a corporate firewall or proxy server this can mess up signatures.  If you were a beta tester, contact Apple's beta testing to determine if an erase is needed, as sometimes beta versions are not fully signed either.

As user Rusto mentioned, the workaround for me was to login as a different user with Admin rights.

In order to force any updates, you can go to the Purchases section of the App Store and then click on Download, Install or Update (according to your situation and needs). After the apps have downloaded and installed, you can log out and go back to your usual id, but I would really recommend restarting the machine to make sure all the changes take and propagate to all users.

I'm having the same problems. I was able to update iTunes, but otherwise I get the Failed to verify the preflight... whenever I try to update Aperture, iWork, iPhoto and iMovie. I even tried purchasing Pages in the App Store and got the same error message.

It sounds like some software you got was forged somewhere, and is not original:

http://support.apple.com/kb/HT3949

No, this is a widespread problem. I purchased my software from an Apple Store, so it definitely wasn't forged.