Public Vulnerability Reports

GNOME Foundation Display Manager gdmchooser Format String Vulnerability

12.14.06

BACKGROUND

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.

DESCRIPTION

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.

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); 1398 1399 dialog = ve_hig_dialog_new 1400 (GTK_WINDOW (chooser) /* parent */, 1401 GTK_DIALOG_MODAL /* flags */, 1402 GTK_MESSAGE_ERROR, 1403 GTK_BUTTONS_OK, 1404 _("Cannot find host"), 1405 msg); 1406 1407 g_free (msg); 1408 

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.

ANALYSIS

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 could construct a format string which displays the contents of the stack or writes controlled values to arbitrary memory locations. This 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.

DETECTION

This vulnerability has been confirmed to exist in the gdm-2.14.1-1 RPM from Red Hat Fedora Core 5. The vulnerability was introduced into the gdmchooser.c file in version 1.78 of gdm2/gui/gdmchooser.c in the GNOME CVS source code repository.

WORKAROUND

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

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.

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2006-6105 to this issue. This is a candidate for inclusion in the CVE list (http://cve.mitre.org), which standardizes names for security problems.

DISCLOSURE TIMELINE

12/04/2006 Initial vendor notification
12/05/2006 Initial vendor response
12/14/2006 Coordinated public disclosure

CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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LEGAL NOTICES

Copyright © 2006 Verisign, Inc.

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