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.

III. 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.

IV. 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.

V. WORKAROUND

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

VI. 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.

VII. 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.

VIII. DISCLOSURE TIMELINE

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

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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X. LEGAL NOTICES

Copyright © 2006 iDefense, Inc.

Permission is granted for the redistribution of this alert electronically. It may not be edited in any way without the express written consent of iDefense. If you wish to reprint the whole or any part of this alert in any other medium other than electronically, please e-mail customer service for permission.

Disclaimer: The information in the advisory is believed to be accurate at the time of publishing based on currently available information. Use of the information constitutes acceptance for use in an AS IS condition. There are no warranties with regard to this information. Neither the author nor the publisher accepts any liability for any direct, indirect, or consequential loss or damage arising from use of, or reliance on, this information.

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