X.Org security advisory: October 25, 2018

[Matthieu Herrb]

X.Org security advisory: October 25, 2018

Privilege escalation and file overwrite in X.Org X server 1.19 and later
========================================================================

Incorrect command-line parameter validation in the Xorg X server can
lead to privilege elevation and/or arbitrary files overwrite, when the
X server is running with elevated privileges (ie when Xorg is
installed with the setuid bit set and started by a non-root user).

The -modulepath argument can be used to specify an insecure path to
modules that are going to be loaded in the X server, allowing to
execute unprivileged code in the privileged process.

The -logfile argument can be used to overwrite arbitrary files in the
file system, due to incorrect checks in the parsing of the option.

This issue has been assigned CVE-2018-14665

Background
==========

The commit
https://gitlab.freedesktop.org/xorg/xserver/commit/032b1d79b7 which
first appeared in xorg-server 1.19.0 introduced a regression in the
security checks performed for potentially dangerous options, enabling
the vulnerabilities listed above.

Overwriting /etc/shadow with -logfile can also lead to privilege
elevation since it's possible to control some part of the written log
file, for example using the -fp option to set the font search path
(which is logged) and thus inject a line that will be considered as
valid by some systems.

Patches
=======

A patch for the issue was added to the xserver repository on
October 25, 2018.

https://gitlab.freedesktop.org/xorg/xserver/commit/50c0cf885a6e91c0ea71fb49fa8f1b7c86fe330e

Workaround
==========

If a patched version of the X server is not available, X.Org
recommends to remove the setuid bit (ie chmod 755) of the installed
Xorg binary.  Note that this can cause issues if people are starting
the X window system using the 'startx', 'xinit' commands or variations
thereof.

X.Org recommends the use of a display manager to start X sessions,
which does not require Xorg to be installed setuid.

Thanks
======

X.Org thanks Narendra Shinde who discovered and reported the issue,
and the Red Hat Product Security Team who helped understand all
impacts.

-- 
Matthieu Herrb
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