JNCIS-SEC, Security

JNCIS-SEC – Custom IDP policies on the Juniper SRX

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Activating Intrusion Detection and Prevention (or IPS, as all the cool kids call it) on the SRX is quite straightforward in itself, but turning it on and relying solely on the default IDP policies is not going to cut it. While doing my own tests, running some scriptkiddie attacks against a virtual machine, I was expecting lots of sirens but unfortunately, the SRX stayed silent. No detection and definitely no prevention. As I quickly gathered, you actually need to put some thought in the services you are exposing and the kind of attacks you can expect, and then craft your own policies with the specific signatures.

For this one, I’ll be creating a custom IDP rule on the vSRX. I’m assuming you have already set up the vSRX, activated the advanced features license and set up IDP. If not, here are some links that will put you on the right path.

Scenario

  • We are hosting an SSH/SFTP server on my public IP address of 1.1.1.10/32. Every day, there are hundreds of brute-force login attempts. Obviously, the server admin has not implemented protective measures so we will try to reduce the impact by enabling IDP.
  • Destination NAT has been configured and is translating 1.1.1.10 to internal DMZ server 10.0.4.10.

Here is the firewall configuration. IDP has already been enabled on this policy.

[edit security policies from-zone untrust to-zone dmz policy FW-SFTP-Servers]
root@NP-vSRX-01# show
match {
    source-address any;
    destination-address Grp-SFTP-Servers;
    application junos-ssh;
}
then {
    permit {
        application-services {
            idp;
        }
    }
    log {
        session-init;
    }
}

Server setup and brute-force script

I’m using this script which runs a txt file of common passwords against the username specified.

On the linux server, let’s create a user, aptly named “admin”, with one of the passwords from the text files

lab@V104-10:~$ sudo useradd admin
lab@V104-10:~$ sudo passwd admin
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

First, running the script with the standard IDP policy of Server-Protection-1G

lab@V120-10:/tmp/brutessh$ python brutessh.py -h 1.1.1.10 -u admin -d passlist.txt

*************************************
*SSH Bruteforcer Ver. 0.2           *
*Coded by Christian Martorella      *
*Edge-Security Research             *
*laramies@gmail.com                 *
*************************************

HOST: 1.1.1.10 Username: admin Password file: passlist.txt
===========================================================================
Trying password...
miller


Auth OK ---> Password Found: changeme
tiger

Times -- > Init: 0.032772 End: 6.861204

The SRX was not paying attention during the attack -nothing in the attack table- and the server got owned. 

root@NP-vSRX-01> show security idp attack table
root@NP-vSRX-01>

Rolling your own IDP rules

Unfortunately, the server guy had to be let go and we are now looking for a way to be prevent these kinds of attacks. Of course, the first thing to do would be to harden the server and enforce strong credentials, but it does help if you are notified of an ongoing attack. Even better if the SRX could also prevent or slow down the attack.

For our scenario, we are specifically looking for SSH Brute-force attacks. A good way to find out if the SRX has an on-board signature for this is to browse the attack database. You can find all these signatures in operational mode. 

root@NP-vSRX-01# run show security idp attack description SSH?
Possible completions:
          Attack name
  SSH:AUDIT:SSH-V1
  SSH:AUDIT:UNEXPECTED-HEADER
  SSH:BRUTE-LOGIN
  SSH:ERROR:COOKIE-MISMATCH
  SSH:ERROR:INVALID-HEADER
  SSH:ERROR:INVALID-PKT-TYPE
  SSH:ERROR:MSG-TOO-LONG
  SSH:ERROR:MSG-TOO-SHORT
  SSH:MISC:EXPLOIT-CMDS-UNIX
  SSH:MISC:MAL-VERSION
  SSH:MISC:UNIX-ID-RESP
  SSH:NON-STD-PORT
  SSH:OPENSSH-MAXSTARTUP-DOS
  SSH:OPENSSH:BLOCK-DOS
  SSH:OPENSSH:GOODTECH-SFTP-BOF
  SSH:OPENSSH:NOVEL-NETWARE
  SSH:OVERFLOW:FREESSHD-KEY-OF
  SSH:OVERFLOW:PUTTY-VER
  SSH:OVERFLOW:SECURECRT-BOF
  SSH:PRAGMAFORT-KEY-OF
  SSH:SYSAX-MULTI-SERVER-DOS
  SSH:SYSAX-SERVER-DOS

root@NP-vSRX-01# run show security idp attack description SSH:BRUTE-LOGIN
Description: This signature detects attempts by remote attackers to log in to an SSH server by brute-forcing the password.

Another great resource for finding the right signature is this page.

If you are using one of the predefined IDP policy sets, navigate down and edit the configuration. I am using Server-Protection-1G on mine.

The policy below will detect SSH brute-force attempts and, by using the ip-action, block the source IP for 30 minutes. That should slow ’em down. 

[edit security idp idp-policy Server-Protection-1G rulebase-ips]
root@NP-vSRX-01# show rule SSH-SFTP-Services
match {
    from-zone any;
    source-address any;
    to-zone any;
    destination-address any;
    application default;
    attacks {
        predefined-attacks SSH:BRUTE-LOGIN;
    }
}
then {
    action {
        close-client;
    }
    ip-action {
        ip-block;
        target source-address;
        timeout 1800;
    }
    notification {
        log-attacks;
    }
}

You can rearrange the rules by using the insert policy before

Testing the policy

Running the script for a second time, it only took a small number of brute-force attempts until the alarm triggered. After a minute or two of time-outs, the script gave up and started spawning python errors. 

lab@V120-10:/tmp/brutessh$ python brutessh.py -h 1.1.1.10 -u admin -d passlist.txt

*************************************
*SSH Bruteforcer Ver. 0.2           *
*Coded by Christian Martorella      *
*Edge-Security Research             *
*laramies@gmail.com                 *
*************************************

HOST: 1.1.1.10 Username: admin Password file: passlist.txt
===========================================================================
Trying password...
abc123
Exception in thread carmen
:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "brutessh.py", line 44, in run
    t = paramiko.Transport(hostname)
  File "/tmp/brutessh/paramiko/transport.py", line 235, in __init__
    sock.connect((hostname, port))
  File "/usr/lib/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
error: [Errno 110] Connection timed out
Exception in thread test
:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "brutessh.py", line 44, in run
    t = paramiko.Transport(hostname)
  File "/tmp/brutessh/paramiko/transport.py", line 235, in __init__
    sock.connect((hostname, port))
  File "/usr/lib/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
error: [Errno 110] Connection timed out
Exception in thread password1
:

Some verification commands on the SRX itself. 

root@NP-vSRX-01> show security log | match IDP:
2015-09-17 19:22:45 UTC  IDP: at 1442517764, SIG Attack log <2.2.2.1/6768->1.1.1.10/22> for TCP protocol and service SERVICE_IDP application NONE by rule 6 of rulebase IPS in policy Server-Protection-1G. attack: repeat=0, action=CLOSE_CLIENT, threat-severity=MEDIUM, name=SSH:BRUTE-LOGIN, NAT <0.0.0.0:0->10.0.4.10:0>, time-elapsed=0, inbytes=0, outbytes=0, inpackets=0, outpackets=0, intf:untrust:ge-0/0/0.0->dmz:ge-0/0/2.0, packet-log-id: 0, alert=no, username=N/A, roles=N/A and misc-message -

The SRX detected three login attempts through before shunning the remote IP 

root@NP-vSRX-01> show security idp attack table
IDP attack statistics:

  Attack name                                  #Hits
  SSH:BRUTE-LOGIN                              3

root@NP-vSRX-01> show security idp attack detail SSH:BRUTE-LOGIN
Display Name: SSH: Brute Force Login Attempt
Severity: Minor
Category: SSH
Recommended: true
Recommended Action: Drop
Type: signature
Direction: CTS
False Positives: rarely
Service: SSH
Shellcode: no
Flow: control
Context: first-data-packet
Negate: false
TimeBinding:
        Scope: peer
        Count: 10
Hidden Pattern: False
Pattern: \[SSH\].*

root@NP-vSRX-01> show security idp counters action
IDP counters:

  IDP counter type                                                      Value
 None                                                                    0
 Recommended                                                             0
 Ignore                                                                  0
 Diffserv                                                                0
 Drop packet                                                             0
 Drop                                                                    0
 Close                                                                   0
 Close server                                                            0
 Close client                                                            3
 IP action rate limit                                                    0
 IP action drop                                                          3
 IP action close                                                         0
 IP action nofity                                                        0
 IP action failed                                                        0

Wrapping up

Although very basic, this example demonstrated that it absolutely pays off to invest some time in crafting your own IDP rules. Keeping a detailed inventory of the services you are hosting and matching it against the associated application-level attack signatures will greatly increase your security posture. Combined with the Screen Options, which protect against some of the L3/L4 attacks, this makes the SRX well worth considering 🙂

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