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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