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Denial of Service in Microsoft ISA Server and MS Proxy

04.09.03

BACKGROUND

Microsoft Corp.'s Internet Security and Acceleration Server (ISA) Server integrates an extensible, multi-layer enterprise firewall and a scalable high-performance web cache. It builds on Microsoft Windows 2000 security and directory for policy-based security, acceleration and management of internetworking. More information is available at
http://www.microsoft.com/isaserver/ .  MS Proxy 2.0 is the predecessor to
ISA Server, more information is available at http://www.microsoft.com/isaserver/evaluation/previousversions/default.asp.

DESCRIPTION

A vulnerability exists in ISA Server and MS Proxy 2.0 that allows attackers to cause a denial-of-service condition by spoofing a specially crafted packet to the target system.  Another impact of this vulnerability is the capability of a remote attacker to generate an infinite packet storm between two unpatched systems implementing ISA Server or MS Proxy 2.0 over the Internet.

Both ISA Server and MS Proxy 2.0, by default, install a WinSock Proxy (WSP) service wspsrv.exe, designed for testing and diagnostic purposes. The WSP service creates a User Datagram Protocol socket bound to port 1745. A specially crafted packet can cause WSP to generate a continuous flood of requests and reply requirements.

ANALYSIS

In the case of the attack scenario for an internal LAN attacker causing a denial of service, this malformed packet must meet the following criteria:

*  The source and destination IP are the same as the ISA Server.
*  The source and destination port is 1745.
*  The data field is specially crafted and resembles the request format.

An attacker with access to the LAN can anonymously generate a specially crafted UDP packet that will cause the target ISA Server to fall into a continuous loop of processing request and reply packets. This will cause the ISA Server to consume 100 percent of the underlying system's CPU  usage. It will continue to do so until the system reboots or the WinSock Proxy (WSP) service restarts.

In the case of the attack scenario of a remote attacker causing a packet storm between two systems running ISA Server or MS Proxy 2.0, the malformed packet must meet the following criteria:

*  The source IP is one of the targets
*  The destination IP is the other target
*  The source and destination port is 1745.
*  The data field is specially crafted and resembles the request format.

DETECTION

iDEFENSE has verified that Microsoft ISA Server 2000 and MS Proxy 2.0 are both vulnerable to the same malformed packet characteristics described above.

Wspsrv.exe is enabled by default in Proxy Server 2.0. The Microsoft Firewall server is enabled by default in ISA Server firewall mode and ISA Server integrated mode installations. It is disabled in ISA Server cache mode installations.

WORKAROUND

To prevent the second attack scenario, apply ingress filtering on the Internet router on UDP port 1745 to prevent a malformed packet from reaching the ISA Server and causing a packet storm.

VENDOR RESPONSE

Microsoft has provided fixes for Proxy Server 2.0 and ISA Server at http://www.microsoft.com/technet/security/bulletin/MS03-012.asp .

CVE INFORMATION

The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project has
assigned the identification number CAN-2003-0110 to this issue.

DISCLOSURE TIMELINE

01/23/2003   Issue disclosed to iDEFENSE
02/24/2003   secure@microsoft.com contacted
02/24/2003   Response from Iain Mulholland, MSRC
02/25/2003   iDEFENSE clients notified
03/03/2003   Status request from iDEFENSE
03/11/2003   Status request from iDEFENSE
03/11/2003   Response from Iain Mulholland, MSRC
03/13/2003   Status request from iDEFENSE
03/18/2003   Status request from iDEFENSE
03/18/2003   Response from Iain Mulholland, MSRC
03/24/2003   Status request from iDEFENSE
03/25/2003   Response from Iain Mulholland, MSRC
04/09/2003   Public Disclosure