Public Vulnerability Reports

XFree86 Font Information File Buffer Overflow



In short, XFree86 is an open source X11-based desktop infrastructure.

XFree86, provides a client/server interface between display hardware
(the mouse, keyboard, and video displays) and the desktop environment
while also providing both the windowing infrastructure and a
standardized application interface (API). XFree86 is platform
independent, network-transparent and extensible.


Exploitation of a buffer overflow in The XFree86 Project Inc.'s XFree86
X Window System allows local attackers to gain root privileges.

The problem specifically exists in the parsing of the 'font.alias' file.
The X server (running as root) fails to check the length of user
provided input. A malicious user may craft a malformed 'font.alias'
file causing a buffer overflow upon parsing, eventually leading to the
execution of arbitrary code.

From XFree86-4.2.1/xc/lib/font/fontfile/dirfile.c:

ReadFontAlias(char *directory, Bool isFile, FontDirectoryPtr *pdir)

The above code sets up the buffer that will be exploited directly in
front of the frame pointer and return address.

while (status == Successful) {
token = lexAlias(file, &lexToken);

lexAlias() reads an arbitrary length token from file, and returns a
pointer to it in &lexToken, without performing any bounds checking.
It then returns NAME when it reaches whitespace.

switch (token) {
case NAME:
strcpy(alias, lexToken);

If lexToken is longer than MAXFONTNAMELEN (1024 chars) an overflow

To reproduce the overflow on the command line:

# cat > fonts.dir <<EOF
word.bdf -misc-fixed-medium-r-semicondensed--13-120-75-75-c-60-iso8859-1
# perl -e 'print "0" x 1024 . "A" x 96 . "n"' > fonts.alias
# X :0 -fp $PWD

{Some output removed}

Caught signal 11.

Server aborting...

eip: 41414141 eflags: 00003282

{Some output removed}

Code: Segmentation fault (core dumped)


Successful exploitation requires that an attacker be able to execute
commands in the X11 subsystem. This can be done either by having console
access to the target or through a remote exploit against any X client
program such as a web-browser, mail-reader or game. Successful
exploitation yields root access.


iDEFENSE has confirmed the existence of this vulnerability in XFree86
versions 4.1.0 to the current version 4.3.0. It is suspected that
earlier versions are vulnerable as well.


The patch for the problem is at and
it is applicable to all affected XFree86 versions.

The change log entry is:

   794. Fix font alias overrun.

See also for changelog extracts for
the trunk and several branches.  The patch has been applied to the
trunk and all of the 4.x release branches.


The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project
has assigned CAN-2004-0083 to this issue.


Greg MacManus (iDEFENSE Labs) is credited with the discovery of this