Barracuda WAF Early Boot Root Shell

Vulnerability Description

Firmware reversing of the Barracuda Web Application Firewall uncovered debug features that should have been removed on the production images. Appending a debugging statement onto a grub configuration line leads to an early boot root shell.

Technical Description

After the initramfs image has been decrypted (see https://www.reddit.com/r/netsec/comments/5fonrm/rooting_an_appliance_for_fun/dandnx2/), some extra kernel command-line options are checked for by init.

  # cat init|more
  [snip]
  Some interesting things you can do at the kernel command line:

     * oldscheme - Choose the "old" partitioning scheme.
     * decrypt - Specify that the main partitions are encrypted.
     * shell - Run /sbin/init when we're done setting things up.
     * wyldstallyns - (soon) better debugging
  [snip]

Investigating them further, we find a specific comparison looking for wyldstallyns=69dude.

  # grep -ri "wyldstallyns" *
  lib/Barracuda/Preboot/Utils.pm:        $self->{WYLDSTALLYNS} = 1 if $cmd eq "wyldstallyns=69dude";

This perl module is imported and used during system init.

  # grep "Barracuda::Preboot::Utils" init
  use Barracuda::Preboot::Utils qw/try catch/;

If the kernel line contains the debugging statement, a DEBUG variable is set to one.

  # cat lib/Barracuda/Preboot/Utils.pm
  [snip]
  $self->{DEBUG}                  = 1 if $self->{WYLDSTALLYNS};
  [snip]

This variables determines whether or not the bash shell is executed during early boot.

  # cat init
  [snip]
  eval {
      parse_args($init);
      init( $dialog, $init, $iso );
      main( $dialog, $init, $iso, $tools );
      boot( $dialog, $init, $iso );
  } or do {
      if ( defined $init->{DEBUG} ) {
          print $@;
          exec "/bin/bash";
      }
      else {
          $dialog->hang($@);
      }
  };
  [snip]

Mitigation and Remediation Recommendation

The vendor has patched this vulnerability in the lastest virtual appliance release.

Credit

This vulnerability was discovered by Matt Bergin (@thatguylevel) of KoreLogic, Inc. and Joshua Hardin.

Proof of Concept

Start by rebooting, hit ‘e’ during the grub boot menu. When prompted for credentials, provide: root / bimg. Now, on the linux line append wyldstallyns=69dude

Hit F10 when finished.

During the boot process you will be prompted with: Paused (continue|exit)

When you type exit, you are dropped into a root shell.

root@(none):~#

The contents of this advisory are copyright(c) 2017 KoreLogic, Inc. and are licensed under a Creative Commons Attribution Share-Alike 4.0 (United States) License: http://creativecommons.org/licenses/by-sa/4.0/

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