A "critical" flaw in software code that implements a protocol for
virtual private networks (VPN) makes Windows 2000 and Windows XP
systems vulnerable to denial of service attacks, according to
Microsoft.
An unchecked buffer exists in the code that implements the
Point-to-Point Tunnelling Protocol (PPTP), a protocol that enables
users to create and use VPNs that is natively supported by Windows
2000 and Windows XP, Microsoft said in security bulletin.
The announcement is an embarrassment, to the company, coming the
day after it announced Windows 2000 had received a prestigious
global security award, the Common Criteria for Information
Technology Security Evaluation (CCITSE) certification, also known
as ISO-IEC 15408.
PPTP is an option in Routing and Remote Access Services in server
versions of Windows 2000 and Windows XP and part of the Remote
Access Client in workstation versions. Systems are only at risk if
PPTP has been enabled, Microsoft said.
Both server and client systems are at risk, though an attack on a
client would be more difficult as it typically changes its IP
(Internet Protocol) address every time a connection is setup,
Microsoft said. An attacker could cause a vulnerable system to fail
by sending malformed PPTP control data to the system, Microsoft
said.
A patch to fix the problem is available from Microsoft's TechNet
Web site. Administrators offering PPTP services should install it
immediately, and users of remote access using PPTP should consider
installing the patch, the software maker said.
Microsoft has also warned in a separate security bulletin of
another, "moderate" security issue affecting Windows 2000.
The default permission settings for the software provide users in
the "Everyone" group with full access to the system root folder. An
attacker could mount a Trojan horse attack against users on the
same system by placing a program in the root and having that run
when another user signs on, Microsoft said. Administrators should
consider changing access permissions on the Windows 2000 root
directory, Microsoft said.