macOS 'Quick Look' Bug Can Leak Encrypted Data Through Thumbnail Caches

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Apr 12, 2001
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A long-standing bug in macOS's Quick Look feature has the potential to expose sensitive user files like photo thumbnails and the text of documents, even on encrypted drives, according to security researchers.

Details on the Quick Look flaw were shared earlier this month by security researcher Wojciech Regula and over the weekend on security researcher Patrick Wardle's blog (via The Hacker News).

Image via Wojciech Regula

Quick Look in macOS is a convenient Finder feature that's designed to present a zoomed-in view when you press the space bar on a photo or document that's selected.

To provide this preview functionality, Quick Look creates an unencrypted thumbnail database where thumbnails of files are kept, with the database storing file previews from a Mac's storage and any attached USB drives whenever a folder is opened. These thumbnails, which provide previews of content on an encrypted drive, can be accessed by someone with the technical know how and there's no automatic cache clearing that deletes them. As Regula explains:
It means that all photos that you have previewed using space (or Quicklook cached them independently) are stored in that directory as a miniature and its path. They stay there even if you delete these files or if you have previewed them in encrypted HDD or TrueCrypt/VeraCrypt container.
This is an issue that's existed for at least eight years and concerns have been raised about it in the past, but Apple has made no changes in macOS to address it. "The fact that behavior is still present in the latest version of macOS, and (though potentially having serious privacy implications), is not widely known by Mac users, warrants additional discussion," writes Wardle.

As Wardle points out, this information is valuable in law enforcement investigations, but most users are not going to be happy to learn that their Mac records file paths and thumbnails of documents from every storage device that's been attached to it.
For a forensics investigation or surveillance implant, this information could prove invaluable. Imagine having a historic record of the USB devices, files on the devices, and even thumbnails of the files...all stored persistently in an unencrypted database, long after the USB devices have been removed (and perhaps destroyed). For users, the question is: "Do you really want your Mac recording the file paths and 'previews' thumbnails of the files on any/all USB sticks that you've ever inserted into your Mac?" Me thinks not...
It's worth noting that if the main drive on the Mac is encrypted, the Quick Look cache that's created is too. Wardle says that data "may be safe" on a machine that's powered off, but on a Mac that's running, even if encrypted containers are unmounted, the caching feature can reveal their contents.

"In other words, the increased security encrypted containers were thought to provide, may be completely undermined by QuickLook," writes Wardle.

Wardle recommends that users concerned about unencrypted data storage clear the Quick Look cache manually whenever a container is unmounted, with instructions for this available on Wardle's website. It's also worth checking out Wardle's site for full details on the Quick Look bug.

Article Link: macOS 'Quick Look' Bug Can Leak Encrypted Data Through Thumbnail Caches
 

magicschoolbus

macrumors 68000
May 27, 2014
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This is an issue that's existed for at least eight years and concerns have been raised about it in the past, but Apple has made no changes in macOS to address it. "The fact that behavior is still present in the latest version of macOS, and (though potentially having serious privacy implications), is not widely known by Mac users, warrants additional discussion," writes Wardle.
Apple does not care about the Mac. The hardware and this proves it. You guys should seriously consider naming this site iosrumors.com (that's not a shot at you either.. Apple is all about iOS)
 

syntax

Suspended
May 8, 2002
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Just to be clear: the current implementation of FileVault encrypts the entire hard drive, effectively neutralizing this vulnerability, correct?
 
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Dave-Z

macrumors 6502a
Jun 26, 2012
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Just to be clear: the current implementation of FileVault encrypts the entire hard drive, effectively neutralizing this vulnerability, correct?
It's encrypted at rest. So if the computer is booted someone could walk up and access the files. Even the lock screen is not enough for a determined person. If the computer is powered off it should be fine with a sufficiently strong password.
 

iapplelove

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Nov 22, 2011
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Apple does not care about the Mac. The hardware and this proves it. You guys should seriously consider naming this site iosrumors.com (that's not a shot at you either.. Apple is all about iOS)
Apple is all about where the revenue comes from. You would too. We all would.

iPhone is their main source of revenue.

Still though it’s a damn shame they can’t at least keep a healthy Mac lineup going.
 

jchap

macrumors member
Sep 25, 2009
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It's a one line command (in terminal) to clear the cache. You need to be an "admin" user, but you don't need to be root:

qlmanage -r cache
Great tip—very good to know! Thank you for sharing this.
 

ignatius345

macrumors 68030
Aug 20, 2015
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Your welcome, although this info came from the blog post that's linked in the MR article above. I knew most people would rather whine, criticize, and/or blame Tim Cook, than read the blog, so I posted the meaningful bit.
I personally won't bother clearing my QL caches because the convenience of the preview outweighs some very, very implausible attack on my low-value data -- but this does seem like a very sensible and quick fix for those with greater security needs than myself.

For anyone who uses Alfred, by the way, you can enter terminal commands right into the search window by starting with a ">" character and then entering the terminal command you want to run. It'll call up a Terminal window and enter the string for you right away. Makes this command even faster to run, until such time as the bug properly fixed.
 
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Baymowe335

macrumors 603
Oct 6, 2017
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Apple does not care about the Mac. The hardware and this proves it. You guys should seriously consider naming this site iosrumors.com (that's not a shot at you either.. Apple is all about iOS)
This is simply not true. Software has bugs and Apple will fix them, better than others. A tiny software bug that will be fixed isn’t a referendum on the state of the Mac every single time.

Apple doesn’t update hardware based on what you think is appropriate. They have all the data and facts to back up their strategy, which is working.

Mac revenue is still holding strong, up slightly y/y. Mac is <10% of their revenue but still a large ~$25B/yr business. They do care, but Apple understands what they’re doing. The vocal minority isn’t reality. If it were a major emergency, you’d see them update the various Mac lines. The reality is, it’s just not an issue.
 

luvbug

macrumors 6502
Aug 11, 2017
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Getting closer every day!
This is simply not true. Software has bugs and Apple will fix them, better than others. A tiny software bug that will be fixed isn’t a referendum on the state of the Mac every single time.

Apple doesn’t update hardware based on what you think is appropriate. They have all the data and facts to back up their strategy, which is working.

Mac revenue is still holding strong, up slightly y/y. Mac is <10% of their revenue but still a large ~$25B/yr business. They do care, but Apple understands what they’re doing. The vocal minority isn’t reality. If it were a major emergency, you’d see them update the various Mac lines. The reality is, it’s just not an issue.
Thank you! That was refreshing!
 

Acidsplat

macrumors regular
Aug 12, 2011
209
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So, you get the prize for first whiner! I guess assigning blame is more important to you than addressing the problem in the first person using readily available information.
Ordinary people wouldn’t know to input a terminal command, or even know that Quick Look is leaking their data.

The bug lies with Apple’s code. How is this the fault of the consumer? The consumer is certainly not the party to blame in this situation.
 
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