X.Org Security Advisory: CVE-2015-0255: Feb 10, 2015

Information leak in the XkbSetGeometry request of X servers

Description

Olivier Fourdan from Red Hat has discovered a protocol handling issue in the way the X server code base handles the XkbSetGeometry request.

The issue stems from the server trusting the client to send valid string lengths in the request data. A malicious client with string lengths exceeding the request length can cause the server to copy adjacent memory data into the XKB structs. This data is then available to the client via the XkbGetGeometry request. The data length is at least up to 64k, it is possible to obtain more data by chaining strings, each string length is then determined by whatever happens to be in that 16-bit region of memory.

A similarly crafted request can likely cause the X server to crash.

This issue has been assigned CVE-2015-0255

Affected Versions

This bug appears to have been introduced in X11R6.6 (March 1996) and is thus believed to be present in every X server release since, including the current stable release 1.16.3 and 1.17.0.

Fixes

A fix is available via the patch linked below which is intended to be included in xorg-server-1.16.4 and 1.17.1

https://gitlab.freedesktop.org/xorg/xserver/-/commit/20079c36cf7d377938ca5478447d8b9045cb7d43

Note that this patch requires another patch to apply without conflicts:

https://gitlab.freedesktop.org/xorg/xserver/-/commit/81c90dc8f0aae3b65730409b1b615b5fa7280ebd

Thanks

The X.Org Foundation thanks Olivier for bringing this issue to our attention and providing the fixes.