Alert: Microsoft Security Bulletin - MS02-063

From: Russ (Russ.Cooper@RC.ON.CA)
Date: 10/31/02

Date:         Thu, 31 Oct 2002 00:10:46 -0500
From: Russ <Russ.Cooper@RC.ON.CA>

Unchecked Buffer in PPTP Implementation Could Enable Denial of Service Attacks (Q329834)

Originally posted: October 30, 2002


Who should read this bulletin: Customers using Microsoft® Windows® 2000 or Windows XP.

Impact of vulnerability: Denial of service.

Maximum Severity Rating: Critical.

Recommendation: Administrators offering PPTP services should install the patch immediately; users who utilize remote access using PPTP should consider installing the patch.

Affected Software:
- Microsoft Windows 2000
- Microsoft Windows XP

Technical description:

Windows 2000 and Windows XP natively support Point-to-Point Tunneling Protocol (PPTP), a Virtual Private Networking technology that is implemented as part of Remote Access Services (RAS). PPTP support is an optional component in Windows NT 4.0, Windows 98, Windows 98SE, and Windows ME.

A security vulnerability results in the Windows 2000 and Windows XP implementations because of an unchecked buffer in a section of code that processes the control data used to establish, maintain and tear down PPTP connections. By delivering specially malformed PPTP control data to an affected server, an attacker could corrupt kernel memory and cause the system to fail, disrupting any work in progress on the system.

The vulnerability could be exploited against any server that offers PPTP. If a workstation had been configured to operate as a RAS server offering PPTP services, it could likewise be attacked. Workstations acting as PPTP clients could only be attacked during active PPTP sessions. Normal operation on any attacked system could be restored by restarting the system.

Mitigating factors:
- As discussed in more detail in the FAQ, Microsoft has only successfully demonstrated denial of service attacks via this vulnerability. Because of how the overrun occurs, it does not appear that that there is any reliable means of using it to gain control over a system.
- Servers would only be at risk from the vulnerability if they had been specifically configured to offer PPTP services. PPTP does not run by default on any Windows system. Likewise, although it is possible to configure a workstation to offer PPTP services, none operate in this capacity by default.
- Exploiting the vulnerability against a PPTP client could be difficult. PPTP is typically used in scenarios in which the client IP address changes frequently (e.g., because the client system is mobile). Not only would an attacker need to learn the IP address, but he or she would also need to mount an attack while the client had an active PPTP session underway.

Vulnerability identifier: CAN-2002-1214

This email is sent to NTBugtraq automatically as a service to my subscribers. Since its programmatically created, and since its been a long time since anyone paid actual money for my programming skills, it may or may not look that good...;-]

I can only hope that the information it does contain can be read well enough to serve its purpose.

Russ - Surgeon General of TruSecure Corporation/NTBugtraq Editor