[UNIX] GNOME Foundation Display Manager gdmchooser Format String Vulnerability

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GNOME Foundation Display Manager gdmchooser Format String Vulnerability


The gdmchooser program provides XDMCP (X Display Manager Control Protocol)
functionality to the GNOME Display Manager. This protocol allows a user to
interact remote systems via the local X11 display. Local exploitation of a
format string vulnerability in GNOME Foundation's GNOME Display Manager
host chooser window (gdmchooser) could allow an unauthenticated attacker
to execute arbitrary code on the affected system.


Vulnerable Systems:
* gdm version 2.14.1-1

Immune Systems:
* gdm version 2.14.11
* gdm version 2.16.4
* gdm version 2.17.4

The vulnerability specifically exists in the handling of values entered
when choosing a remote host to connect to from the current system. The
following snippet from gdmchooser.c shows the vulnerable code:

1395 msg = g_strdup_printf (_("Cannot find the host \"%s\". "
1396 "Perhaps you have mistyped it."),
1397 name);
1399 dialog = ve_hig_dialog_new
1400 (GTK_WINDOW (chooser) /* parent */,
1401 GTK_DIALOG_MODAL /* flags */,
1404 _("Cannot find host"),
1405 msg);
1407 g_free (msg);

The error dialog uses the temporary value 'msg', constructed from 'name'
when the host is not found. By supplying a sequence of formatting
operations which modify memory values, an unauthenticated attacker can
execute code with the permissions under the gdm account.

Successful local exploitation of this vulnerability would allow an
unauthenticated attacker to execute arbitrary code with the privileges of
the gdm service.

In order to exploit this vulnerability, the attacker would need to have
access to the system running gdmchooser. Connecting remotely to this
service will not work as the functionality is designed to connect to a
remote server already, and proxying is not allowed.

The attacker selects the option to run the gdmchooser, and then types the
format string into the dialog box. They can send a string which displays
the contents of the stack, and use there construct a format string which
will write controlled values to arbitrary memory locations, which allows
execution of code with the privileges of the gdmchooser, typically the
user 'gdm'. Although this user does not have a high privilege level, once
this account has been gained, it would be relatively simple to insert a
logger into gdm processes to capture other users passwords.

The gdmchooser is not accessible from the default login screens in any of
the tested Red Hat systems, however the preferences utility does contain
some preset screens which do contain it.

If XDMCP functionality is not required, setting the permissions on the
gdmchooser to not allow execution will prevent any attempts at

Vendor response:
The GNOME maintainers have addressed this problem by releasing versions
2.14.11, 2.16.4, and 2.17.4 of the GNOME Display Manager.

Disclosure Timeline:
12/04/2006 - Initial vendor notification
12/05/2006 - Initial vendor response
12/14/2006 - Coordinated public disclosure


The information has been provided by
<mailto:idlabs-advisories@xxxxxxxxxxxx> iDefense Labs Security Advisories.
The original article can be found at:


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