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Title: Unchecked Buffer in PPTP Implementation Could Enable
Denial of Service Attacks (Q329834)
Date: 30 October 2002
Software: Windows 2000, Windows XP
Impact: Denial of Service
Max Risk: Critical
Bulletin: MS02-063

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS02-063.asp.

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Issue:

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.

Risk Rating:

• Internet systems: Critical
• Intranet systems: Low
• Client systems: Low

Patch Availability:

• A patch is available to fix this vulnerability. Please read the
  Security Bulletin at
  http://www.microsoft.com/technet/security/bulletin/ms02-063.asp
  for information on obtaining this patch.

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