Joe Job and SPF

27 Mar 2007 — eric

First off, get your mind out of the gutter. A joe job has absolutely nothing to do with what you’re thinking about. It’s email related and it can be a pain in the ass to deal with.

What is a Joe Job?
Joe Job is the term used to describe the act of forging bulk email to appear to the recipient as if the email were coming from the victim. Generally speaking, this term is used to describe an attack of this nature. This is to say that when a spambot or botnet sends a massive amount of email to a victim. The named was coined by an attack launched against http://www.joes.com/ in January of 1997. The perpetrator (SPAMMER) sent a flood of emails from spoofed addresses in a (successful) attempt to enrage the recipients to take action against the company.

Why do I care?
There are many reasons, but I will just cover a few until you get the picture. The main victim of a SPAM attack of this nature ends up having an INBOX full of junk. This junk can potentially include malware, virii, and any number of phishing or scam based attacks. Also, since there is so much email traversing the connection, the bandwidth gets sucked up and depending on the actual amount of SPAM coming in, could render the connection unusable until all the mail is filtered through. The problem comes in when there are thousands of messages, that could take days or even weeks. Since the originating address is spoofed, those who don’t know are going to get very upset with who they *believe* to be responsible for sending the email. The last item I am going to touch on is that the person whose email address was spoofed now has to deal with all the auto-responses and whatever else may automatically come their way. (I think you get the idea).

What I can do?
There is nothing that you can do to completely avoid it besides not using the internet or email. There are some steps that you can take. One of the first things is to take a look at SPF (Sender Policy Framework). To set this up in DNS, you need to do the following:

In your DNS zone file for server.com, you should add something like the following:

server.com.  IN TXT    "v=spf1 a mx -all"
  • v – The version of SPF to use
  • a mx – The DNS attributes permitted to send messages for server.com
  • -all – Reject everything else that does match a or mx

This can also get more in depth depending on the number of email accounts you have and from where. For instance, let’s say your mail server’s name is mail.server.com and you also have email accounts on gmail (gmail.com)and your work email (myjob.com). Your line would look something similar to the following:

server.com.   IN   TXT   "v=spf1 mx a:mail.server.com include:gmail.com include:myjob.com -all"

The a line is saying that mail.server.com is authorized to send mail via your mail server. The include statements are basically saying that everything considered legitimate by either gmail.com or myjob.com should also be considered legitimate by you.

There is a lot more information on configuring SPF. The documentation should be read thoroughly as improperly configured SPF can prevent legitimate email from flowing. For more information of SPF and configuring it, check out:

SPF is just one method that can be used to fight against being a victim of a Joe job. You should always be using some method of SPAM filtering in addition to SPF. Layered security needs to be the approach when locking down any type of server or service.

Posted in SPAM, Security. View Comments

Super Security vs. Ease of Use

5 Feb 2007 — eric

I think I am going to get back onto my soapbox about being extraordinarily secure, only this time, I am going to compare it to ease of use. I would once again like to reiterate the fact that I am strongly for security in all its aspects. However, some people get into the presence of a security individual and freak out. They start saying that they know certain things are secure and insecure and then do them anyway.

A great example of this is SSH access to servers. Consider the following physical network layout.

                      ,---------- Server I
Inet---------|F/W|---+----------- Server II
                      `----------- Server III

If you want to leave certain nice to do’s or ease of use functionality available to your self such as leaving SSH open only to root or having a machine with anonymous FTP access available, then take a slightly different approach to securing your environment (or those particular machines): layered security. Without changing the physical layout of your network, change the network layout using iptables and/or tcp wrappers. Make the network look more like this:

                             ,------Server II
Inet-----|F/W|----Server I--<
                             `------Server III

This is essentially saying that all traffic that you want to funnel to Server II or Server III will now go through server I. This can be used in a variety of ways. Let's say that all 3 servers are in the DMZ (De-Militarized Zone) and that Server's II and III are hosting the services available to the outside. Allowing direct access to them via SSH or FTP probably isn't the best idea because these are services that allow files to be changed. So what can we do to change this?

First let's configure TCP wrappers. Starting out with the assumption that you are being paranoid where necessary, let's set SSH up to only allow incoming connections from Server I and deny from everywhere else. In the /etc/hosts.deny file on Server II, add the following lines (everything that goes for Server II goes for Server III to mimic the setup):

sshd1: ALL
sshd2: ALL

Now in the /etc/hosts.allow file on Server II, add the IP address of Server I (IPs are all made up):

sshd1: 167.4.5.23
sshd2: none

This now ensures that the only way SSH traffic will be allowed Server II is through (or from) Server I. But let's say that isn't enough for you. Let's say you want a little more security so you can sleep a little better at night. Enter netfilter and IPTables. On Server II, add the following to your firewall script:

iptables -A INPUT -p tcp --dport 22 -j SSH  # Send to the SSH chain
iptables -A SSH -s 167.4.5.23/32 -j ACCEPT # Allow from Server I
iptables -A SSH -m limit --limit 5/s -j LOG # Log the problems
iptables -A SSH -j DROP # Drop everything else

So what's the point of all this extra configuration? Simple, it allows for a little more flexibility when it comes to your setup. Although I recommend having SSH keys and not allowing direct root access from anywhere, you can get away with a little more. You can allow root access via an SSH key. And if you have enough protections/layers/security in place, you may also even consider using a password-less SSH key depending on what the role of the contacting server is (ie. rsync over SSH).

In the optimal version of a network setup with SSH, you may want to only allow user access to Server II via entry only through Server I. Then to top it off, only allow sudo access to root if the user is in the sudoers file. This throws a lot more protections behind Server II, but makes it somewhat complicated to just do something simple. This is especially true if Server II is on an internal network which isn't very accessible anyway. The advice I generally give is in the form of the question, "What is tradeoff?" More times than not, ease of use the answer. So keeping in mind that ease of use isn't out of the realm of possibility, just remember that layered security can be your friend. It's usually how the tradeoff comes.

Posted in Security. View Comments

10 More Tips Towards Securing Your Linux System

31 Jan 2007 — eric

Since everyone seemed to enjoy my first round of tips and tricks to securing a linux system, I figured I would throw together a few more. Enjoy.

  1. There are files that get changed very infrequently. For instance, if your system won’t have any users added anytime soon then it may be sensible to chattr immutably the /etc/password and /etc/shadow files. Only the superuser or a process possessing the CAP_LINUX_IMMUTABLE capability can set or clear this attribute.
       chattr +i /etc/passwd /etc/shadow
  2. Password protect your linux install with LILO. Edit your /etc/lilo.conf. At the end of each linux image that you want to secure, put the lines:
       read-only
       restricted
       password = MySecurePassword

    Ensure you rereun /sbin/lilo so the changes take effect.

  3. Users who have sudoer (sudo) accounts setup can have the account setup to change to root without a password. To check this, as root use the following command:
       grep NOPASSWD /etc/sudoers

    If there is an entry in the sudoers file, it will look like this:

       eric    ALL=NOPASSWD:ALL

    To get rid of this, type visudo and remove the line in that file.

  4. Use sudo to execute commands as root as a replacement for su. In the /etc/sudoers file, add the following lines by using the visudo command:
       Cmnd_Alias LPCMDS = /usr/sbin/lpc, /usr/bin/lprm
       eric    ALL=LPCMDS

    Now the user ‘eric’ can sudo and use the lpc and lprm commands without having any other root level access.

  5. Turn off PasswordAuthentication and PermitEmptyPasswords in the SSH configuaration file /etc/ssh/sshd_config. This will ensure that users cannot set empty passwords or login without SSH keys.
       PermitEmptyPasswords no
       PasswordAuthentication no
  6. Instead of using “xhost +” to open up access to the X server, be more specific. Use the server name that you are allowing control to:
       xhost +storm:0.0

    Once you are done using it, remember to disallow access to the X server from that host:

       xhost -storm:0.0
  7. To find out the .Xauthority magic cookie looks like and to send it (authorization information) to the remote host, use the following command:
       xauth extract - $DISPLAY | ssh storm xauth merge -

    Now the user who ran this command on the original host can now run xcilents on storm. xauth needs to be present on both hosts.

  8. To turn on spoof protection, run a simple bash script:
       for i in /proc/sys/net/ipv4/conf/*/rp_filter; do echo 1 > $i done;

    Be careful to remember that it drops packets more or less ‘invisibly’.

  9. A SYN-flood attack has the ability to bring the network aspect of your linux box to a snail like crawl. TCP_SYNCookies protection attempts to stop this from taking a heavy toll on the machine. To enable cp_syncookies
    protection, use the following command:

       echo 1 > /proc/sys/net/ipv4/tcp_syncookies
  10. When possible use secure connection methods as opposed to insecure methods. Unless you are required to use telnet, substitute ssh (Secure SHell) in for rsh or telnet. Instead of POP3 or IMAP use SPOP3 or SIMAP (IMAPS). Both SIMAP and SPOP3 are just versions of IMAP and POP3 running over an SSL (Secure Socket Layer) tunnel.
Posted in Linux Security, Security. View Comments

10 Tips To Start Securing Your Linux System

29 Jan 2007 — eric

A while back I had been asked to write a few quick tips that as an administrator, one would find helpful. They published in one form or another and are now available here. There are MANY more, but these are just a few. Enjoy.

  1. Users who may be acting up or aren’t listening can still be controlled. Using a program called ‘skill’ (signal kill) which is part of the ‘procps’ package.
       Halt/Stop User eric: skill -STOP -u eric
       Continue User eric: skill -CONT -u eric
       Kill and Logout User eric: skill -KILL -u eric
       Kill and Logout All Users: skill -KILL -v /dev/pts/*
  2. Make use of security tools out there to test your server’s weaknesses. Nmap is an excellent port scanning tool to test to see what ports you have open. On a remote machine, type the command:
       # nmap -sTU <server_ip>

       Starting nmap 3.70 ( http://www.insecure.org/nmap/ ) at 2006-08-10 13:51 EST
       Interesting ports on eric (172.16.0.1):
       (The 3131 ports scanned but not shown below are in state: closed)
       PORT    STATE         SERVICE
       22/tcp  open          ssh
       113/tcp open          auth
       
       Nmap run completed -- 1 IP address (1 host up) scanned in 221.669 seconds
  3. On a production server that is in a common area (although this should not be the case, some situations are inevidable). To avoid an accidental CTRL-ALT-DEL reboot of the machine, do the following to remove the necessary
    lines from the /etc/inittab file:

       # sed -i 's/ca::ctrlaltdel:/#ca::ctrlaltdel:/g' /etc/inittab
  4. Two SSH configuration options that can be set to improve security should be checked on your production server. UsePrivilegeSeparation is an option, when enabled will allow the OpenSSH server to run a small (necessary) amount of code as root and the of the code in a chroot jail environment. StrictModes checks to ensure that your ssh files and directories have the proper permissions and ownerships before allowing an SSH session to open up. The
    directives should be set in the /etc/ssh/sshd_config as follows:

       UsePrivilegeSeparation yes
       StrcitModes yes
  5. The default umask (usermask) on most systems should be 022 to ensure that files are created with the permissions 0644 (-rw-r–r–). To change the default umask setting for a system, edit /etc/profile to ensure that you umask is appropriate for your setup.
  6. Some users like to have a passwordless account. To check this you need to look at the /etc/shadow account with the following command line:
    awk -F: '$2 == "" { print $1, "has no password!" }' /etc/shadow
  7. Just in case someone else who has access to the superuser account decided to alter the password file and potentially make themselves a superuser. This is a method to check:
       awk -F: '$3 == 0 { print $1, "is a superuser!" }' /etc/passwd
  8. Setuid and Setgid files have the potential to be very hazardous if they are accessilbe by the wrong users on the system. Therefore it is handy to be able to check with files fall into this category.
       find /dir -xdev -type f -perm +ug=s -print
  9. World writable files can be left around by users wanting to make things easier for themselves. It is necessary to be careful about who can write to which files. To find all world writable files:
       find /dir -xdev -perm +o=w ! \( -type d -perm +o=t \) ! -type l -print
  10. Some attackers, prior to attacking a host, (or users nmaping a host) will check to see if the host is alive. They do this by ‘ping’ing the host. In order to check if the host is up, they will use an ICMP echo request packet.
    To disallow these types of packets, use iptables:

       iptables -A INPUT -p icmp --icmp-type echo-request -j DROP
Posted in Linux Security, Security. View Comments
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