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Bug#895149: cinnamon-screensaver: Fails to lock the screen if a menu is selected

Package: cinnamon-screensaver
Version: 3.6.1-2
Severity: grave
Tags: security
Justification: user security hole

Dear Maintainer,

I found that cinnamon-screensaver does not start if a window menu is clicked.
This can be a security problem as users unaware of this can leave their
computer unlocked unwillingly if they clicked a menu before abandoning or
trusting that their computer will be locked.

To reproduce this, just click the "File" menu in a window. For example, gnome-
terminal's "File" menu.

Starting cinnamon-screensaver in a terminal and looking at its log, I see the
following when the screensaver tries to start:


couldn't grab keyboard
couldn't grab keyboard
couldn't grab keyboard
couldn't grab keyboard
couldn't grab keyboard
couldn't grab keyboard
couldn't grab keyboard
couldn't grab keyboard
couldn't grab mouse
couldn't grab mouse
couldn't grab mouse
couldn't grab mouse
Can't fade in screensaver, unable to grab the keyboard


If I unselect the "File" menu and wait for the screensaver to trigger again, it
is then able to do it.

I'm reporting this as a security issue but I understand having this exploited
is somewhat unlikely: it would require the attacker to somehow make (or wait
for) the victim to click a menu and ensuring that he leaves his computer
unlocked and unattended without noticing the screen lock was not triggered.

Thanks and best regards,
Luís Picciochi Oliveira

-- System Information:
Debian Release: buster/sid
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.15.0-2-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE=en_GB:en (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages cinnamon-screensaver depends on:
ii  cinnamon-desktop-data       3.6.2-2
ii  gir1.2-accountsservice-1.0  0.6.45-1
ii  gir1.2-cinnamondesktop-3.0  3.6.2-2
ii  gir1.2-gkbd-3.0             3.26.0-3
ii  gir1.2-glib-2.0             1.56.0-2
ii  gir1.2-gtk-3.0              3.22.29-2
ii  gir1.2-xapp-1.0             1.0.4-2
ii  iso-flags-png-320x240       1.0.1-1
ii  libc6                       2.27-3
ii  libcscreensaver0            3.6.1-2
ii  libglib2.0-0                2.56.0-4
ii  libgtk-3-0                  3.22.29-2
ii  python3                     3.6.4-1
ii  python3-gi                  3.28.1-1
ii  python3-gi-cairo            3.28.1-1
ii  python3-setproctitle        1.1.10-1+b1
ii  python3-xapp                1.0.1-1
ii  python3-xlib                0.20-3

Versions of packages cinnamon-screensaver recommends:
pn  cinnamon-screensaver-x-plugin  <none>

Versions of packages cinnamon-screensaver suggests:
pn  cinnamon-screensaver-webkit-plugin  <none>

-- no debconf information

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