Tracing the attack – Part I

Intro

In this post I will talk about some of the techniques used to attack systems, and some solutions that can reduce much the number of attacks that a system may suffer. A vision to the attack from scanning to gain root in one system.
This work is a continuation of my work on honeypots. The attacks presented here could be seen in a honeypot of high activity, real systems that have known vulnerabilities. Unfortunately I used one low activity honeypot in previous posts, emulation of vulnerabilities, so I just could not get to identify some of the attacks mentioned herein.
This post continues in Part II.

Starting

First let me talk about the profile of the principal threat that we face from the moment
we use the Internet, the Script Kiddie.
To a Script Kiddie not interest who is he attacking, its only purpose is to gain root access in a machine. The problem is that for helping him he has several tools that automate all the process, the only thing that usually has to do is put the tool to scan the entire Internet.
Sooner or later it is guaranteed that he will get access in some machine.
The fact that these tools are becoming more known ally to randomness searches, this threat is extremely dangerous because it is not “if” but “when” we will be victims of a search.

I introduced the “who”, now let me introduce the “how”. Before starting any type of attack, the enemy must find that machines are online and their ports (services) that are open to the outside.
This technique is called Port Scanning

Port Scanning

Port Scanning is used both by system administrators and by attackers, the first thing to determine the level of security of their machines; seconds to find the vulnerabilities as I said before.
I will give special focus to this technique because it domain is essential in both situations.

Basically a Port Scan is send a message to each port of a machine and depending on what kind of response we get determine the status of the port.

These possible state ports are:

• Open or Accepted: The machine has sent a response to indicate that a service that is listening port
• Closed, Denied or Not Listening: The machine has sent a response to indicate that any connection the port will be denied
• Filtered, Dropped or Blocked: There was no response from the machine

Just a note on the legality of this technique: in fact Port Scanning can be seen as a bell to ring to confirm if someone is home. Nobody can be accused of nothing just for doing a Port Scan. Now if we are playing constantly on the bell, could be alleged the similarity to a denial of service (for more about legal issues related to Port Scanning).

Now we are ready to see the most popular techniques in Port Scanning.

TCP SYN scan

The SYN scan is the most popular because it can be done in a very quick way, searching thousands
of ports per second on a network (fast network and without firewalls). This kind of scan is relatively discreet because they never complete TCP connections. It also allows a distinct differentiation between the states of ports with a good confidence.
This technique is also known as semi-open scan because it never gets to open a complete TCP connection. It is sent a SYN packet (the beginning of a normal complete connection) and expects a answer. If it receives a SYN/ACK, the port is Open, if it receives a RST, the port is Closed, and if there is no response after several retransmissions of the SYN, the port is marked as Filtered.
This last scenario repeats itself in the event of an ICMP Unreachable Error.

Normal scenario (client send a SYN, server reply with SYN/ACK, and client send an ACK):

TCP SYN attack (client send a SYN, server reply with SYN/ACK, and client do nothing):

This technique is used when the user does not have privileges to create crude packets (in nmap for example, you must have root privileges to this option.) Rather than be sent in “custom” packets as in all other techniques mentioned here, it established a complete TCP connection in the target machine port. This causes a considerable increase of time for giving same information as the previous technique (most packets are produced). In addition, most operative systems log these connections, because it is not a quick or silent scan. On Unix, for example, is added an entry in syslog.

UDP scan

A UDP scanis much slower than a TCP one, one of the reasons why many systems administrators ignore the security of these ports, but not the attackers.
This technique consists in sending an empty UDP header (no data) to all target ports. If an ICMP Unreachable Error is returned, depending on type and code of error is given the port state (Closed or Filtered). However if returned another UDP header, then the port is Open, if not received any response after the retransmissions of the header, the port is classified as a combination of Open or Filtered because may be in a state or in the other.

But as I said the greatest disadvantage of this technique is the time spent on scan; Open or Filtered ports rarely respond which can lead to several retransmissions (because it is assumed that the packet could have been lost). Closed ports are an even bigger problem because normally respond with ICMP Unreachable Errors and many operative systems limit the frequency of ICMP packets transmission, as a example in Linux that normally limited to a maximum of one package per second.

Faced with such constraints, if we were to the full range of ports of a machine (65536 ports) it will take more than 18 hours. Some optimizations can pass through the more common ports.

There are many others techniques used in Port Scanning (more information: here) but this ones come to show the behavior of this type of scan. It is easy to see why it is a critical process for an attacker methodology. For the correct identification of the target, only one technique can not be the most appropriate/enough.
So the Port Scanning area is not only making TCP SYN scans.
The tool I recommend to Port Scanning is the Nmap because it supports all these techniques that I said, and a lot more. This tool is being actively developed, and the author Gordon “Fyodor” Lyon is a guru in this area and highly responsible for the constant development of it.

For more information about Port scanning go to Nmap page explaining Port Scanning Techniques.

Gain access to the machine

At this time the attacker already has a huge list of reachable machines and services. Now to get remote access on a machine, in essence, we can use two techniques: brute-force/password dictionaries (see the Infinite monkey theorem).

It may seem silly but it is still heavily used in services such as SSH. As you can see in your /var/log/auth.log file…

Dec 24 01:24:46 kubos sshd[23906]: Invalid user oracle from 89.235.152.18
Dec 24 01:24:46 kubos sshd[23906]: pam_unix(ssh:auth): check pass; user unknown
Dec 24 01:24:46 kubos sshd[23906]: pam_unix(ssh:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=89.235.152.18
Dec 24 01:24:48 kubos sshd[23906]: Failed password for invalid user oracle from 89.235.152.18 port 48785 ssh2
Dec 24 01:24:49 kubos sshd[23908]: reverse mapping checking getaddrinfo for 89-235-152-18.adsl.sta.mcn.ru [89.235.152.18] failed - POSSIBLE BREAK-IN ATTEMPT!

Dec 24 01:26:01 kubos sshd[23963]: Invalid user test from 89.235.152.18
Dec 24 01:26:01 kubos sshd[23963]: pam_unix(ssh:auth): check pass; user unknown
Dec 24 01:26:01 kubos sshd[23963]: pam_unix(ssh:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=89.235.152.18
Dec 24 01:26:04 kubos sshd[23963]: Failed password for invalid user test from 89.235.152.18 port 57886 ssh2
Dec 24 01:26:05 kubos sshd[23965]: reverse mapping checking getaddrinfo for 89-235-152-18.adsl.sta.mcn.ru [89.235.152.18] failed - POSSIBLE BREAK-IN ATTEMPT!

Dec 24 01:26:21 kubos sshd[23975]: Invalid user cvsuser from 89.235.152.18
Dec 24 01:26:21 kubos sshd[23975]: pam_unix(ssh:auth): check pass; user unknown
Dec 24 01:26:21 kubos sshd[23975]: pam_unix(ssh:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=89.235.152.18
Dec 24 01:26:22 kubos sshd[23975]: Failed password for invalid user cvsuser from 89.235.152.18 port 59883 ssh2
Dec 24 01:26:24 kubos sshd[23977]: reverse mapping checking getaddrinfo for 89-235-152-18.adsl.sta.mcn.ru [89.235.152.18] failed - POSSIBLE BREAK-IN ATTEMPT!

And these are just some of the hundreds of attempts that it recorded.
I will then present some strategies for protection against these attacks:

Use strong passwords

This section may seem ridiculous but the fact that this type of attack is used demonstrates the lack of choice of passwords culture.
Here are some requirements to define a strong password in today times:

• A password should have, a minimum of 8 characters
• If the password can be found in a dictionary is trivial and is not good. Attackers have large dictionaries of words in different languages (via IP, for example, can determine the dictionary to use)
• As trivial variations of trivial passwords, such as “p4ssword” is almost as bad as “password” in a dictionary attack but is substantially better in an brute-force attack
• The password should ideally be a combination of symbols, numbers, uppercase and lowercase
• Mnemonic are easy to remember (even with special symbols) and this is the best kind of passwords, such as “Whiskey-Cola is EUR 3 in Academic Bar!” = “W-CiE3iAB!” (this password is Very String according to The Password Meter)

The fact is, using strong passwords can prevent the success of the attempt but not prevent the numerous attempts that are made consistently. And in an extreme situation might be suffering Denial of Service attacks (it is imperative to avoid this on a machine whose purpose is to offer the service to the outside).

Not mentioned the fact that we can limit the number of connections established in the port 22 (SSH) through iptables.

Using iptables to mitigating the attacks

We can limiting the access to a single source:

iptables -A INPUT -m state --state NEW -m tcp -p tcp -s 192.168.1.100 --dport 22 -j ACCEPT

The -s flag is used to indicate the source host ip.
We can also restrict the access to some sub-net, or some IP class address with this flag:

iptables -A INPUT -m state --state NEW -m tcp -p tcp -s 192.168.1.0/24 -dport 22 -j ACCEPT

If we want access our machine from everyware we might want to limit our server to accept, for example, 2 connections per minute:

iptables -N SSH_CONNECTION -A INPUT -m state --state NEW -p tcp --dport 22 -j SSH_CONNECTION -A SSH_CONNECTION -m recent --set --name SSH -A SSH_CONNECTION -m recent --update --seconds 60 --hitcount 3 --name SSH -j DROP

We can create a new chain called SSH_CONNECTION. The chain uses the recent module to allow at maximum two connection attempts per minute per IP.

RSA authentication

To avoid the use of passwords, we can use a pair of RSA keys completely avoiding brute-force/dictionaries attacks.
To do this we will do the following steps:
Generate the key pair with the command

ssh-keygen-t rsa

This command create the files:
~/.ssh/id_rsa (private key) and ~/.ssh/id_rsa.pub (public key).
In each machine where we want to connect (target), put the “id_rsa.pub” generated in ~/.ssh/authorized_keys concatenate the contents of this form for example:

cat id_rsa.pub >> ~/.ssh/authorized_keys

In each machine where we want to call (home), put the “id_rsa” in ~/.ssh/
Only missing off the password-based login to add the line “PasswordAuthentication no” in /etc/ssh/sshd_config and then restart the daemon “sshd” through:

/etc/init.d/sshd restart

Exploitation of vulnerabilities

Now that we reduce the chances of being attacked, let’s see another way that attackers use to gain access into a system.
Exploit is the name given to a piece of code used to exploit flaws in applications in order to cause a behavior not previously anticipated in them. Thus is common, for example, gaining control of a machine or spread privileges.
A widely used type of exploit is stack smashing which occurs when a program writes a memory address outside their allocated space for the structure of data in the stack, usually a buffer of fixed size. Take a very simple example of local implementation of this exploit:

# include
# include 

int main(int argc , char *argv []) {
         char buffer [10];
         strcpy (buffer ,argv [1]);
         printf ( buffer );
         return 0;
}

When we try to execute the above code, we get:

user@honeypot :~$ gcc exploit .c -o exploit
user@honeypot :~$ ./ exploit thisisanexploit
*** stack smashing detected ***: ./ exploit terminated
thisisanexploitAborted

As we can see, GCC has introduced mechanisms for blocking implementation of code potentially malicious. But this example is as simple as possible. More sophisticated attacks against systems can avoid this mechanisms.

The stack

When a function is called, the return value must be addressed, and it address must be somewhere saved in the stack. Saving the return address into the stack is one advantage, each task has its own stack, so each must have its return address. Another thing, is that recursion is completely supported. In case that a function call itself, a return address must be created for each recursive phase, in each stack function call.
For example, the following code:

/** lengthOf  returns the length of list  l  */
public int lengthOf(Cell l) {
  int length;

  if ( l == null ) {
    length = 0;
  }
  else {
    length = 1 + lengthOf(l.getNext());
  }
  return length;
}

Will produce the following stacks:

The stack also contain local data storage. If any function declare local variables, they are stored there also. And may also contain parameters passed to the function, for more information about that.

GCC and Stack canary’s

If you wondering why a canary.
GCC, and other compilers insert in the stack known values to monitor buffer overflows. In the case that the stack buffer overflows, the first field to be corrupted will be the canary. Forward, the sub-routines inserted into the program by GCC verify the canary field and verify that this as changed, sending a message “*** stack smashing detected ***”.
For more information about this subject.

Stack corruption

If you still thinking what stack buffer overflow is god for? I give you a simple example (from Wikipedia article).
Imagine you have the following code:

void foo (char *bar)
{
   char  c[12];

   memcpy(c, bar, strlen(bar));  // no bounds checking...
}

int main (int argc, char **argv)
{
   foo(argv[1]);
}

As you can see, there are no verification about the input of the function foo, about the *bar variable.
This is the stack that are created at some point when this function is called by another one:

When you call the foo, like:

void function() {
  char *newBar = (char *)malloc(sizeof(char)*6);
  strcpy(newBar,"hello");
  foo(newBar);
}

This is what happens to the stack:

Now, if you try this:

void function() {
  char *newBar = (char *)malloc(sizeof(char)*24);
  strcpy(newBar,"A​A​A​A​A​A​A​A​A​A​A​A​A​A​A​A​A​A​A​A​x08​x35​xC0​x80");
  foo(newBar);
}

This is what happens to the stack:

So, now, when the foo function terminates, it will pop the return address and jump to that address (0x08C03508, in this case), and not to the expected one. In this iamge, the address 0x08C03508 is the beginning of the char c[12] code. Where the attacker can previously put shellcode, and not AAAAA string…
Now imagine that this program is SUID bit on to run as root, you can instantly scale to root…

Fortunately this kind of attacks is being to reduce, since the introduction of stack canary’s. This kind of protection is possible because the stack is not dynamic, but as we gone see later (in part II), the heap is.

References

• Stack buffer overflow Wikipedia entry
• Stack smaching protection Wikipedia entry