A flaw in software code that implements a protocol for VPN makes Windows 2000 and Windows XP systems vulnerable to denial-of-service attacks, Microsoft warned late Wednesday.

An unchecked buffer exists in the code that implements the Point-to-Point Tunneling 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 MS02-063. The software maker deems the issue "critical."

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 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 Redmond, Wash., software maker said.

Microsoft warned of another, "moderate," security issue affecting Windows 2000 in a separate security bulletin issued Wednesday.

The default permission settings for the software provide users in the "Everyone" group 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.

The IDG News Service is a Network World affiliate.