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iDEFENSE Security Advisory 04.09.03:
http://www.idefense.com/advisory/04.09.03.txt
Denial of Service in Microsoft Proxy Server 2.0 and Internet Security and
Acceleration Server 2000
April 9, 2003

I. 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


II. 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.

III. 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.

IV. 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.

V. 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.

VI. RECOVERY

Restart either the WinSock Proxy Service or the affected system to resume
normal operation.

VII. VENDOR FIX/RESPONSE

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

VIII. CVE INFORMATION

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

IX. DISCLOSURE TIMELINE

01/23/2003  Issue disclosed to iDEFENSE
02/24/2003  security@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



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About iDEFENSE:

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vulnerabilities and hacker profiling to the global spread of viruses
and other malicious code. Our security intelligence services provide 
decision-makers, frontline security professionals and network 
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visit http://www.idefense.com .


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