X.Org Security Advisory: CVE-2015-0255: Information leak in the XkbSetGeometry request of X servers

Peter Hutterer peter.hutterer at redhat.com
Tue Feb 10 14:48:35 PST 2015

X.Org Security Advisory: Feb 10, 2015 - CVE-2015-0255
Information leak in the XkbSetGeometry request of X servers
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 X11R6.1 (March 1996) and is 
thus believed to be present in every X server release since, including 
the current stable releases 1.16.3 and 1.17.0

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

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

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