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Microsoft Security Bulletin MS99-016 - Critical

Patch Available for "Malformed Phonebook Entry" Vulnerability

Published: May 19, 1999 | Updated: March 21, 2003

Version: 2.0

Patch Availability Information Updated: March 21, 2003
Originally Posted: May 20, 1999

Summary

Microsoft has released a patch that eliminates a vulnerability in the Microsoft® Windows NT® remote access service (RAS) client. The vulnerability could allow a denial-of-service attack to be mounted against the client machine or, under certain conditions, could allow arbitrary code to be executed on it.

A fully supported patch is available that eliminates this vulnerability, and Microsoft recommends that affected customers download and install it, if appropriate.

Issue

The component of the RAS client that processes phonebook entries has an unchecked buffer. This results in a vulnerability that poses two threats to safe operation. The first is a denial of service threat; a malformed phonebook entry could overflow the buffer, causing the RAS client service to crash. The second is more esoteric and would be far more difficult to exploit. A carefully-constructed phonebook entry could cause arbitrary code to execute on the client via a classic buffer overrun technique. Neither variant could be exploited accidentally.

It is important to stress that the vulnerability affects RAS client machines, not RAS servers, and that the user must have permission to add or modify phonebook entries in order to mount the attack. (Permissions can be set via the phonebook's ACL). The machines primarily at risk from this vulnerability are workstations that are configured to dial out to other systems, because servers, including terminal servers, are not typically configured to act as RAS clients. It also is important to note that this vulnerability would affect only the local machine; there is no capability to directly attack a remote machine via this vulnerability.

While there are no reports of customers being adversely affected by this vulnerability, Microsoft is proactively releasing this patch to allow customers to take appropriate action to protect themselves against it.

Affected Software Versions

  • Microsoft Windows NT 4.0

Vulnerability Identifier: CVE-1999-0715

What Microsoft is Doing

Microsoft has released patches that fix the problem identified. The patches are available for download from the sites listed below in What Customers Should Do.

Microsoft also has sent this security bulletin to customers subscribing to the Microsoft Product Security Notification Service. See The Microsoft Product Security Notification Service for more information about this free customer service.

Microsoft has published the following Knowledge Base (KB) article on this issue:

What Customers Should Do

Microsoft highly recommends that customers evaluate the degree of risk that this vulnerability poses to their systems and determine whether to download and install the patch. The patch can be found at:

More Information

Please see the following references for more information related to this issue.

Obtaining Support on this Issue

If you require technical assistance with this issue, please contact Microsoft Technical Support. For information on contacting Microsoft Technical Support, please see http://support.microsoft.com/contactussupport/?ws=support.

Acknowledgments

Microsoft acknowledges David Litchfield of Arca Systems for discovering this vulnerability and reporting it to us.

Revisions

  • May 19, 1999: Bulletin Created.
  • V2.0 (March 21, 2003): Introduced versioning and updated patch availability information

For additional security-related information about Microsoft products, please visit http://www.microsoft.com/technet/security

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