About XSecureLock

XSecureLock is an X11 screen lock utility designed with the primary goal of security.

Screen lock utilities are widespread. However, in the past they often had security issues regarding authentication bypass (a crashing screen locker would unlock the screen), information disclosure (notifications may appear on top of the screen saver), or sometimes even worse.

In XSecureLock, security is achieved using a modular design to avoid the usual pitfalls of screen locking utility design on X11. Details are available in the Security Design section.

Requirements

The following packages need to be installed; their names depend on your Linux distribution of choice, but will be similar:

• apache2-utils (for the auth_htpasswd module)
• autotools-dev
• binutils
• gcc
• libc6-dev
• libpam-dev (for the authproto_pam module)
• libx11-dev
• libxcomposite-dev
• libxext-dev
• libxfixes-dev
• libxft-dev
• libxmuu-dev
• libxrandr-dev
• libxss-dev
• make
• mplayer (for the saver_mplayer module)
• mpv (for the saver_mpv module)
• pamtester (for the authproto_pamtester module)
• pkg-config
• x11-xserver-utils (for the saver_blank module)
• x11proto-core-dev
• xscreensaver (for the saver_xscreensaver module)

Installation

NOTE: In these instructions, please replace SERVICE-NAME by the name of an appropriate and existing file in /etc/pam.d. If xscreensaver is installed, xscreensaver should always be a good choice; otherwise, on Debian and Ubuntu, common-auth would work. This will be used as default and can be overridden with XSECURELOCK_PAM_SERVICE.

Configuring a broken or missing SERVICE-NAME will render unlocking the screen impossible! If this should happen to you, switch to another terminal (Ctrl-Alt-F1), log in there, and run: killall xsecurelock to force unlocking of the screen.

git clone https://github.com/google/xsecurelock.git
cd xsecurelock
sh autogen.sh
./configure --with-pam-service-name=SERVICE-NAME
make
sudo make install

Special notes for FreeBSD and NetBSD

First of all, on BSD systems, /usr/local is owned by the ports system, so unless you are creating a port, it is recommended to install to a separate location by specifying something like --prefix=/opt/xsecurelock in the ./configure call. You can then run XSecureLock as /opt/xsecurelock/bin/xsecurelock.

Also, in order to authenticate with PAM on FreeBSD and NetBSD, you must be root so you can read the shadow password database. The authproto_pam binary can be made to acquire these required privileges like this:

chmod +s /opt/xsecurelock/libexec/xsecurelock/authproto_pam

Special notes for OpenBSD

First of all, on BSD systems, /usr/local is owned by the ports system, so unless you are creating a port, it is recommended to install to a separate location by specifying something like --prefix=/opt/xsecurelock in the ./configure call. You can then run XSecureLock as /opt/xsecurelock/bin/xsecurelock.

Also, in order to authenticate with PAM on OpenBSD, you must be in the auth group so you can run a setuid helper called login_passwd that can read the shadow password database. The authproto_pam binary can be made to acquire these required privileges like this:

chgrp auth /opt/xsecurelock/libexec/xsecurelock/authproto_pam
chmod g+s /opt/xsecurelock/libexec/xsecurelock/authproto_pam

Note that this adds substantially less attack surface than adding your own user to the auth group, as the login_passwd binary can try out passwords of any user, while authproto_pam is restricted to trying your own user.

Setup

Pick one of the authentication modules and one of the screen saver modules.

Tell your desktop environment to run XSecureLock by using a command line such as one of the following:

xsecurelock
env XSECURELOCK_SAVER=saver_xscreensaver xsecurelock
env XSECURELOCK_SAVER=saver_mplayer XSECURELOCK_DISCARD_FIRST_KEYPRESS=0 xsecurelock
env XSECURELOCK_FONT=`xlsfonts | grep '\<iso10646-1\>' | shuf | head -n 1` xsecurelock

Just kidding about the last one :)

IMPORTANT: Make sure your desktop environment does not launch any other locker, be it via autostart file or its own configuration, as multiple screen lockers may interfere with each other. You have been warned!

Automatic Locking

To automatically lock the screen after some time of inactivity, use xss-lock as follows:

xset s 300 5
xss-lock -n /usr/lib/xsecurelock/dimmer -l -- xsecurelock

The option -l is critical as it makes sure not to allow machine suspend before the screen saver is active - otherwise previous screen content may show up for a short time after wakeup!

NOTE: When using xss-lock, it's recommended to not launch xsecurelock directly for manual locking, but to manually lock using xset s activate. This ensures that xss-lock knows about the locking state and won't try again, which would spam the X11 error log.

WARNING: Never rely on automatic locking for security, for the following reasons:

• An attacker can, of course, use your computer after you leave it alone and before it locks or you return.

• Automatic locking is unreliable by design - for example, it could simply be misconfigured, or a pointer grab (due to open context menu) could prevent the screen lock from ever activating. Media players also often suspend screen saver activation for usability reasons.

Automatic locking should merely be seen as a fallback for the case of the user forgetting to lock explicitly, and not as a security feature. If you really want to use this as a security feature, make sure to kill the session whenever attempts to lock fail (in which case xsecurelock will return a non-zero exit status).

Alternatives

xautolock

xautolock can be used instead of xss-lock as long as you do not care for suspend events (like on laptops):

xautolock -time 10 -notify 5 -notifier '/usr/lib/xsecurelock/until_nonidle /usr/lib/xsecurelock/dimmer' -locker xsecurelock

Possible other tools

Ideally, an environment integrating xsecurelock should provide the following facilities:

1. Wait for one of the following events:
   1. When idle for a sufficient amount of time:
      1. Run dimmer.
      2. When no longer idle while dimmed, kill dimmer and go back to the start.
      3. When dimmer exits, run xsecurelock and wait for it.
   2. When locking was requested, run xsecurelock and wait for it.
   3. When suspending, run xsecurelock while passing along XSS_SLEEP_LOCK_FD and wait for it.
2. Repeat.

This is, of course precisely what xss-lock does, and - apart from the suspend handling - what xautolock does.

As an alternative, we also support this way of integrating:

1. Wait for one of the following events:
   1. When idle for a sufficient amount of time:
      1. Run until_nonidle dimmer || exec xsecurelock and wait for it.
      2. Reset your idle timer (optional when your idle timer is either the X11 Screen Saver extension's idle timer or the X Synchronization extension's "IDLETIME" timer, as this command can never exit without those being reset).
   2. When locking was requested, run xsecurelock and wait for it.
   3. When suspending, run xsecurelock while passing along XSS_SLEEP_LOCK_FD and wait for it.
2. Repeat.

NOTE: When using until_nonidle with other dimming tools than the included dimmer, please set XSECURELOCK_DIM_TIME_MS and XSECURELOCK_WAIT_TIME_MS to match the time your dimming tool takes for dimming, and how long you want to wait in dimmed state before locking.

Options

Options to XSecureLock can be passed by environment variables:

• XSECURELOCK_AUTH: specifies the desired authentication module (the part that displays the authentication prompt).
• XSECURELOCK_AUTHPROTO: specifies the desired authentication protocol module (the part that talks to the system).
• XSECURELOCK_AUTH_BACKGROUND_COLOR: specifies the X11 color (see manpage of XParseColor) for the background of the auth dialog.
• XSECURELOCK_AUTH_SOUNDS: specifies whether to play sounds during authentication to indicate status. Sounds are defined as follows:
  • High-pitch ascending: prompt for user input.
  • High-pitch constant: an info message was displayed.
  • Low-pitch descending: an error message was displayed.
  • Medium-pitch ascending: authentication successful.
• XSECURELOCK_AUTH_FOREGROUND_COLOR: specifies the X11 color (see manpage of XParseColor) for the foreground text of the auth dialog.
• XSECURELOCK_AUTH_TIMEOUT: specifies the time (in seconds) to wait for response to a prompt by auth_x11 before giving up and reverting to the screen saver.
• XSECURELOCK_AUTH_WARNING_COLOR: specifies the X11 color (see manpage of XParseColor) for the warning text of the auth dialog.
• XSECURELOCK_BLANK_TIMEOUT: specifies the time (in seconds) before telling X11 to fully blank the screen; a negative value disables X11 blanking.
• XSECURELOCK_BLANK_DPMS_STATE: specifies which DPMS state to put the screen in when blanking (one of standby, suspend, off and on, where "on" means to not invoke DPMS at all).
• XSECURELOCK_BURNIN_MITIGATION: specifies the number of pixels the prompt of auth_x11 may be moved at startup to mitigate possible burn-in effects due to the auth dialog being displayed all the time (e.g. when spurious mouse events wake up the screen all the time).
• XSECURELOCK_BURNIN_MITIGATION_DYNAMIC: if set to a non-zero value, auth_x11 will move the prompt while it is being displayed, but stay within the bounds of XSECURELOCK_BURNIN_MITIGATION. The value of this variable is the maximum allowed shift per screen refresh. This mitigates short-term burn-in effects but is probably annoying to most users, and thus disabled by default.
• XSECURELOCK_COMPOSITE_OBSCURER: create a second full-screen window to obscure window content in case a running compositor unmaps its own window. Helps with some instances of bad compositor behavior (such as compositor crashes/restarts, but also compton has been caught at drawing notification icons above the screen locker when not using the GLX backend), should prevent compositors from unredirecting as it's 1 pixel smaller than the screen from every side, and should otherwise be harmless, so it's enabled by default.
• XSECURELOCK_DATETIME_FORMAT: the date format to show. Defaults to the locale settings.
• XSECURELOCK_DEBUG_WINDOW_INFO: When complaining about another window misbehaving, print not just the window ID but also some info about it. Uses the xwininfo and xprop tools.
• XSECURELOCK_DIM_ALPHA: Linear-space opacity to fade the screen to.
• XSECURELOCK_DIM_COLOR: X11 color to fade the screen to.
• XSECURELOCK_DIM_MIN_FPS: Minimum framerate to attain during the dimming effect of dimmer. Usually shouldn't need touching.
• XSECURELOCK_DIM_OVERRIDE_COMPOSITOR_DETECTION: When set to 1, always try to use transparency for dimming; when set to 0, always use a dither pattern. Default is to autodetect whether transparency will likely work.
• XSECURELOCK_DIM_TIME_MS: Milliseconds to dim for when above xss-lock command line with dimmer is used; also used by wait_nonidle to know when to assume dimming and waiting has finished and exit.
• XSECURELOCK_DISCARD_FIRST_KEYPRESS: If set to 0, the key pressed to stop the screen saver and spawn the auth child is sent to the auth child (and thus becomes part of the password entry). By default we always discard the key press that started the authentication flow, to prevent users from getting used to type their password on a blank screen (which could be just powered off and have a chat client behind or similar).
• XSECURELOCK_FONT: X11 or FontConfig font name to use for auth_x11. You can get a list of supported font names by running xlsfonts and fc-list.
• XSECURELOCK_FORCE_GRAB: When grabbing fails, try stealing the grab from other windows (a value of 2 steals from all descendants of the root window, while a value of 1 only steals from client windows). This works only sometimes and is incompatible with many window managers, so use with care. See the "Forcing Grabs" section below for details.
• XSECURELOCK_GLOBAL_SAVER: specifies the desired global screen saver module (by default this is a multiplexer that runs XSECURELOCK_SAVER on each screen).
• XSECURELOCK_IDLE_TIMERS: comma-separated list of idle time counters used by until_nonidle. Typical values are either empty (relies on the X Screen Saver extension instead), "IDLETIME" and "DEVICEIDLETIME " where n is an XInput device index (run xinput to see them). If multiple time counters are specified, the idle time is the minimum of them all. All listed timers must have the same unit.
• XSECURELOCK_IMAGE_DURATION_SECONDS: how long to show each still image played by saver_mpv. Defaults to 1.
• XSECURELOCK_KEY_%s_COMMAND where %s is the name of an X11 keysym (find using xev): a shell command to execute when the specified key is pressed. Useful e.g. for media player control. Beware: be cautiuous about what you run with this, as it may yield attackers control over your computer.
• XSECURELOCK_LIST_VIDEOS_COMMAND: shell command to list all video files to potentially play by saver_mpv or saver_mplayer. Defaults to find ~/Videos -type f.
• XSECURELOCK_NO_COMPOSITE: disables covering the composite overlay window. This switches to a more traditional way of locking, but may allow desktop notifications to be visible on top of the screen lock. Not recommended.
• XSECURELOCK_NO_XRANDR: disables multi monitor support using XRandR.
• XSECURELOCK_NO_XRANDR15: disables multi monitor support using XRandR 1.5 and fall back to XRandR 1.2. Not recommended.
• XSECURELOCK_PAM_SERVICE: pam service name. You should have a file with that name in /etc/pam.d.
• XSECURELOCK_PARANOID_PASSWORD: make auth_x11 hide the password length.
• XSECURELOCK_SAVER: specifies the desired screen saver module.
• XSECURELOCK_SHOW_DATETIME: whether to show local date and time on the login. Disabled by default.
• XSECURELOCK_SHOW_HOSTNAME: whether to show the hostname on the login screen of auth_x11. Possible values are 0 for not showing the hostname, 1 for showing the short form, and 2 for showing the long form.
• XSECURELOCK_SHOW_USERNAME: whether to show the username on the login screen of auth_x11.
• XSECURELOCK_SINGLE_AUTH_WINDOW: whether to show only a single auth window from auth_x11, as opposed to one per screen.
• XSECURELOCK_SWITCH_USER_COMMAND: shell command to execute when Win-O or Ctrl-Alt-O are pressed (think "other user"). Typical values could be lxdm -c USER_SWITCH, dm-tool switch-to-greeter, gdmflexiserver or kdmctl reserve, depending on your desktop environment.
• XSECURELOCK_WAIT_TIME_MS: Milliseconds to wait after dimming (and before locking) when above xss-lock command line is used. Should be at least as large as the period time set using "xset s". Also used by wait_nonidle to know when to assume dimming and waiting has finished and exit.
• XSECURELOCK_XSCREENSAVER_PATH: Location where XScreenSaver hacks are installed for use by saver_xscreensaver.

Additionally, command line arguments following a "--" argument will be executed via execvp once locking is successful; this can be used to notify a calling process of successful locking.

Authentication Modules

The following authentication modules are included:

• auth_x11: Authenticates via an authproto module using keyboard input (X11 based; recommended).

Writing Your Own Module

The authentication module is a separate executable, whose name must start with auth_ and be installed together with the included auth_ modules (default location: /usr/local/libexec/xsecurelock/helpers).

• Input: it may receive keystroke input from standard input in a locale-dependent multibyte encoding (usually UTF-8). Use the mb* C functions to act on these.
• Output: it may draw on or create windows below $XSCREENSAVER_WINDOW.
• Exit status: if authentication was successful, it must return with status zero. If it returns with any other status (including e.g. a segfault), XSecureLock assumes failed authentication.
• It is recommended that it shall spawn the configured authentication protocol module and let it do the actual authentication; that way the authentication module can focus on the user interface alone.

Authentication Protocol Modules

The following authentication protocol ("authproto") modules are included:

• authproto_htpasswd: Authenticates via a htpasswd style file stored in ~/.xsecurelock.pw. To generate this file, run: ( umask 077; htpasswd -cB ~/.xsecurelock.pw "$USER" ) Use this only if you for some reason can't use PAM!
• authproto_pam: Authenticates via PAM. Use this.
• authproto_pamtester: Authenticates via PAM using pamtester. Shouldn't be required unless you can't compile authproto_pam. Only supports simple password based conversations.

Writing Your Own Module

The authentication protocol module is a separate executable, whose name must start with authproto_ and be installed together with the included authproto_ modules (default location: /usr/local/libexec/xsecurelock/helpers).

• Input: in response to some output messages, it may receive authproto messages. See helpers/authproto.h for details.
• Output: it should output authproto messages; see helpers/authproto.h for details.
• Exit status: if authentication was successful, it must return with status zero. If it returns with any other status (including e.g. a segfault), XSecureLock assumes failed authentication.

Screen Saver Modules

The following screen saver modules are included:

• saver_blank: Simply blanks the screen.
• saver_mplayer and saver_mpv: Plays a video using mplayer or mpv, respectively. The video to play is selected at random among all files in ~/Videos.
• saver_multiplex: Watches the display configuration and runs another screen saver module once on each screen; used internally.
• saver_xscreensaver: Runs an XScreenSaver hack from an existing XScreenSaver setup. NOTE: some screen savers included by this may display arbitrary pictures from your home directory; if you care about this, either run xscreensaver-demo and disable screen savers that may do this, or stay away from this one!

Writing Your Own Module

The screen saver module is a separate executable, whose name must start with saver_ and be installed together with the included auth_ modules (default location: /usr/local/libexec/xsecurelock/helpers).

• Input: none.
• Output: it may draw on or create windows below $XSCREENSAVER_WINDOW.
• Exit condition: the saver child will receive SIGTERM when the user wishes to unlock the screen. It should exit promptly.
• Reset condition: the saver child will receive SIGUSR1 when the auth dialog is closed. It then shall make sure to blank the screen after a timeout. One way to implent this in shell script based implementations is to simply ignore SIGUSR1 first, start the actual saver in a child processes, and to finally exec saver_blank. Not implementing this will cause the entire saver to reset on this condition.

Security Design

In order to achieve maximum possible security against screen lock bypass exploits, the following measures are taken:

• Authentication dialog, authentication checking and screen saving are done using separate processes. Therefore a crash of these processes will not unlock the screen, which means that these processes are allowed to do "possibly dangerous" things.
• This also means that on operating systems where authentication checking requires special privileges (such as FreeBSD), only that module can be set to run at elevated privileges, unlike most other screen lockers which in this scenario also run graphical user interface code as root.
• The main process is kept minimal and only uses C, POSIX and X11 APIs. This limits the possible influence from bugs in external libraries, and allows for easy auditing.
• The main process regularly refreshes the screen grabs in case they get lost for whatever reason.
• The main process regularly brings its window to the front, to avoid leaking information from notification messages that are OverrideRedirect.
• The main process resizes its window to the size of the root window, should the root window size change, to avoid leaking information by attaching a secondary display.
• The main processes uses only a single buffer - to hold a single keystroke. Therefore it is impossible to exploit a buffer overrun in the main process by e.g. an overlong password entry.
• The only exit conditions of the program is the Authentication Module returning with exit status zero, on which xsecurelock itself will return with status zero; therefore especially security-conscious users might want to run it as sh -c "xsecurelock ... || kill -9 -1" :)

Known Security Issues

• Locking the screen will fail while other applications already have a keyboard or pointer grab open (for example while running a fullscreen game, or after opening a context menu). This will be noticeable as the screen will not turn black and should thus usually not be an issue - however when relying on automatic locking via xss-lock, this could leave a workstation open for days. Above ... || kill -9 -1 workaround would mitigate this issue too by simply killing the entire session if locking it fails.
• As XSecureLock relies on an event notification after a screen configuration change, window content may be visible for a short time after attaching a monitor. No usual interaction with applications should be possible though. On desktop systems where monitors are usually not hotplugged, I'd recommend turning off automatic screen reconfiguration.
• XSecureLock relies on a keyboard and pointer grab in order to prevent other applications from receiving keyboard events (and thus an unauthorized user from controlling the machine). However, there are various other ways for applications - in particular games - to receive input:
  • Polling current keyboard status (XQueryKeymap).
  • Polling current mouse position (XQueryPointer).
  • Receiving input out-of-band (/dev/input), including other input devices than keyboard and mouse, such as gamepads or joysticks.

Most these issues are inherent with X11 and can only really be fixed by migrating to an alternative such as Wayland; some of the issues (in particular the gamepad input issue) will probably persist even with Wayland.

Forcing Grabs

As a workaround to the issue of another window already holding a grab, we offer an XSECURELOCK_FORCE_GRAB option.

This adds a last measure attempt to force grabbing by iterating through all subwindows of the root window, unmapping them (which closes down their grabs), then taking the grab and mapping them again.

This has the following known issues:

• Grabs owned by the root window cannot be closed down this way. However, only screen lockers and fullscreen games should be doing that.
• If the grab was owned by a full screen window (e.g. a game using OverrideRedirect to gain fullscreen mode), the window will become unresponsive, as your actions will be interpreted by another window - which