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Microsoft Security Bulletin (MS99-026)
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Patch Available for "Malformed Dialer Entry" Vulnerability
Originally Posted: July 29, 1999
Summary
=======
Microsoft has released a patch that eliminates a security vulnerability in
the Phone Dialer accessory in Microsoft(r) Windows NT(r). The vulnerability
could be used to run arbitrary code in a user's security context on Windows
NT systems.
Frequently asked questions regarding this vulnerability can be found
at http://www.microsoft.com/security/bulletins/MS99-026faq.asp
Issue
=====
Dialer.exe has an unchecked buffer in the portion of the program that
processes the dialer.ini file. This vulnerability could be used to run
arbitrary code via a classic buffer overrun technique.
The circumstances of this vulnerability require a fairly complicated attack
scenario that limits its scope. Dialer.exe runs in the security context of
the user, so it would not benefit an attacker to simply modify a dialer.ini
file and run it, as he or she would not gain additional privileges. Instead,
the attacker would need to modify the dialer.ini file of another user who
had higher privileges, then wait for that user to run Dialer.
Although the unchecked buffer is present in all versions of Windows NT 4.0,
the attack scenario would result in workstations that have dial-out
capability being chiefly at risk. The FAQ discusses this in greater detail.
Affected Software Versions
==========================
- Microsoft Windows NT Workstation 4.0
- Microsoft Windows NT Server 4.0
- Microsoft Windows NT Server 4.0, Enterprise Edition
- Microsoft Windows NT Server 4.0, Terminal Server Edition
Patch Availability
==================
- Windows NT Server; Windows NT Server 4.0, Enterprise Edition;
and Windows NT Workstation 4.0:
ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/
NT40/hotfixes-postSP5/Dialer-fix/
- Windows NT Server 4.0, Terminal Server Edition:
ftp://ftp.microsoft.com/bussys/winnt/winnt-public/fixes/usa/
nt40tse/hotfixes-postSP4/Dialer-fix/
NOTE: Line breaks have been inserted into the above URLs for readability.
More Information
================
Please see the following references for more information related to this
issue.
- Microsoft Security Bulletin MS99-026: Frequently Asked Questions,
http://www.microsoft.com/security/bulletins/MS99-026faq.asp.
- Microsoft Knowledge Base (KB) article Q237185,
Dialer.exe Access Violation with Phone Entry more than 128 Bytes,
http://support.microsoft.com/support/kb/articles/q237/1/85.asp.
(Note: It may take 24 hours from the original posting of this
bulletin for the KB article to be visible; however, a copy will
be immediately available in the patch folder.)
- Microsoft Security Advisor web site,
http://www.microsoft.com/security/default.asp.
Obtaining Support on this Issue
===============================
This is a fully supported patch. Information on contacting Microsoft
Technical Support is available at
http://support.microsoft.com/support/contact/default.asp.
Acknowledgments
===============
Microsoft acknowledges David Litchfield of Arca Systems for discovering
this vulnerability and reporting it to us.
Revisions
=========
- July 29, 1999: Bulletin Created.
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