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Subject: Claimed vulnerability in GTK_MODULES
From: Owen Taylor (otaylorREDHAT.COM)
Date: Wed Jan 03 2001 - 09:40:33 CST
What follows is the official GTK+ team position on this matter. (It
can be found at http://www.gtk.org/setuid.html as well.) The summary
is that we don't consider it a problem because writing set[ug]id
programs with a GUI toolkit is simply a bad idea and not supported for
We are aware of no commonly distributed GTK+ programs that run
set[ug]id and would consider any such to be security holes. (Note that
GNOME games drop setgid games privileges before initializing GTK+.)
Why GTK_MODULES is not a security hole
GTK+ supports the environment variable GTK_MODULES which specifies
arbitrary dynamic modules to be loaded and executed when GTK+ is
initialized. It is somewhat similar to the LD_PRELOAD environment
variable. However, this (and similar functionality such as specifying
theme engines) is not disabled when running setuid or setgid. Is this
a security hole? No. Writing setuid and setgid programs using GTK+ is
bad idea and will never be supported by the GTK+ team.
You should not write setuid GTK+ programs because:
* GTK+ is too big. GTK+-1.2 and its dependent libraries (ignoring
Xlib) total over 200,000 lines of code. For GTK+-2.0 (ignoring
Xlib and image loading libraries), this figure will be around
500,000 lines of code.
* GTK+ is too complex. GTK+ takes input from dozens of sources, from
drag-and-drop, to root-window properties, to keyboard input, to
configuration files. This is a much broader scope for compromises
than a typical server and makes auditing GTK+ especially tricky.
* Security of GTK+ requires the security of Xlib. The GTK+ team is
not prepared to make that guarantee. Security bugs have been found
in the recent past in such areas of Xlib as the input method code.
* You should not make your GUI setuid at all. Why run the risk of
security bugs in code that does not need to be running with
In the opinion of the GTK+ team, the only correct way to write a
setuid program with a graphical user interface is to have a setuid
backend that communicates with the non-setuid graphical user interface
via a mechanism such as a pipe and that considers the input it
receives to be untrusted.
For this reason, no effort is made in GTK+ to disable the obvious ways
that you could compromise a setuid GTK+ program - GTK_MODULES and the
ability for the user to specify theme engines, because we consider
this to be only papering over the fundamental problems of writing
setuid programs with any GUI toolkit. GTK+ may be modified in the
future to simply refuse to run with elevated privileges, though it
does not do this currently.
Does this mean that there are no security considerations for GTK+? No.
In particular image loaders have been and will continue to be an area
of special care, since users may load images from untrusted sources.
And in addition to the possibility of this variety of exploit, most
potential security holes are essentially bugs and even as mere bugs,
must be squashed. To help accomplish this goal, GTK+ extensively uses
high-level data structure abstractions which minimize the risk of most
traditional buffer overflows.
However, the secure setuid program is a 500 line program that does
only what it needs to, rather than a 500,000 line library whose
essential task is user interfaces.
By Owen Taylor <otaylorredhat.com>
2 January 2000