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Date: Sat, 04 Oct 2014 11:47:41 -0600
From: Kurt Seifried <>
Subject: Re: Re: gnome-shell lockscreen bypass with printscreen

Also note it can use up your disk quota as well. On Fedora 20 I tried
it, held the key down, it continues taking screen shots for some time
after I let go, what was interesting is the screenshot files started out
full size but then got smaller, then full size again, then smaller, so
even on an SSD it appears that it's taking the screenshot, writing it to
disk, but getting interrupted, I never got it to OOM.

On 03/10/14 02:08 PM, wrote:
>> another way to look at it is
> OK, we'll incorporate your suggestion and assign CVE-2014-7300 for:
> "PrtSc is an unauthenticated request that's available to untrusted
> parties. A series of requests can consume a large amount of memory.
> The combination of this PrtSc behavior and the existence of the
> oom-killer allows authentication bypass for command execution.
> Therefore, the product must limit the aggregate memory consumption of
> all active requests, and the lack of this limit is a vulnerability."
> [ the rest of this message has no more CVE assignments ]
>> We agreed on IRC that the best compromise here is to simply only allow
>> screenshots to be saved to the clipboard while the screen is locked.
>> That way taking screenshots is not impossible but someone with access
>> to the machine can't simply abuse it to fill the disk.
> It appears that discussion of this might still be ongoing (e.g., "Can
> an unauthenticated person inject anything else into the clipboard
> while the screen is locked?" "Yes." etc.). However, a second CVE ID
> probably isn't needed. A decision to store PrtSc data in memory,
> rather than on disk, is a solution for the original issue (the
> ultimate cause of the excessive memory consumption is that the system
> can't write PrtSc data to disk as fast as a person can request more
> PrtSc data). This new solution happens to address a second issue: a
> system administrator might want to ensure that an unauthenticated
> person, after going to a locked screen, can't write even one PrtSc
> worth of data to disk. This seems to be essentially a policy change,
> not a fix for behavior that everyone always would've considered wrong.
>> the screencapture (video) feature *is* disabled during lockscreen for
>> precisely the reasons i expected printscreen to be disabled. I'm not
>> sure what to make of that pair of decisions.
> This seems unrelated to the question of CVE assignments, but one
> possibility is that triggering of a video from the locked screen might
> be much less acceptable to customers. For example, there might be high
> support costs or other expenses whenever a legitimate user presses
> Control+Shift+Alt+R once to start a screencast recording, is somehow
> distracted (arguably easier when they can't see the screen), and lets
> the recording continue for a very long time.
>> Turning off the computer is a very different attack from filling up
>> someone's home directory.
> Right. However, unauthenticated triggering of the oom-killer isn't, by
> itself, typically worse than unauthenticated powering off. In the
> former case, the user loses data in one process; in the latter, the
> user loses data in all processes. In other words, unauthenticated
> memory exhaustion wasn't the essence of the original issue. The issue
> required the "special" nature of screen-locking processes, i.e., the
> unusually severe impact when a process dies.
>> Is there a way to make a screenlocking program that is designed to fail
>> closed
> Alan Coopersmith commented on this separately. All we can add is there
> currently isn't a CVE ID for the fail-open behavior of the screen-lock
> functionality in gnome-shell. The design wasn't intended to be a
> fail-closed design. Also, selecting a fail-closed design would've
> required additional research that apparently has never been completed.
> We normally don't have CVE IDs of the form "this product doesn't
> properly address unsolved research problems."

Kurt Seifried -- Red Hat -- Product Security -- Cloud
PGP A90B F995 7350 148F 66BF 7554 160D 4553 5E26 7993

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