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SETTINGS.md

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General Settings

ignore_user

Default: 0.

If set to 1, the script will consider a system already scanned when one user has been accessed. For example, if user1@host has been scanned and the script makes its way to user2@host somehow, a repeated scan will not occur. This may be useful in an environment where sudo is accessible by every user (because user1@host can already access the keys that user2@host can access).

use_sudo

Default: 1

If set to 1, the script will attempt to use sudo to elevate its privileges. If it is successful, the script runs as root; if it isn't, the script runs as the normal user.

ssh_timeout

Default: 3

The connection timeout (in seconds) for SSH connections and DNS resolution. See ssh_config(5)'s ConnectTimeout.

retry_count

Default: 3

In some cases, a recoverable error may be encountered while using SSH. This number corresponds to the maximum amount of times a destination with a recoverable error should be tried again. It is advised to be at least 1.

ignored_users

Default: ()

A list of usernames that are always ignored when attempting to SSH to another system. For example, if "root" is ignored, there will be no attempts to SSH into the root user of any host.

ignored_hosts

Default: ()

A list of hosts (ip addresses or hostnames) that are always ignored/skipped when attempting to SSH to another system. It is generally safest to specify the hostname in ip address form.

ignored_dests

Default: ()

A list of destinations in the form of username@hostname that are always ignored. It is generally safest to specify the hostname in ip address form.

ignored_key_files

Default: ("*badcert.pem*" "*badkey.pem*")

A list of locations that are ignored when searching for SSH private keys. This setting supports globbing/wildcards using the standard asterisk as in any other bash script. For example, to reject just the filename "file.name", you would include "*/file.name" in the list. Also note that for example, "/dir/*/file" will also match "/dir/dir2/dir3/file".

custom_cmds

Default: ()

A list of commands that should be run when the script has been initialized on each system. The output of these commands are printed as base64 strings (including stderr). Note: these commands will only ever run once as they are executed right after checking that the system has not already been scanned.

This list also supports sudo (if available), and can be used by using ${s} as a literal. For example, custom_cmds=('${s} cat /etc/shadow'). If ${s} is not specified, the command will not run as sudo even if use_sudo=1.

Configurable Discovery Strategies

SSH-Snake comes with various configurable strategies (functions) which can be used to discover SSH private keys on a system and discover hosts and destinations to attempt to connect to. Sane defaults have been provided, however if you want to perform a scan as thoroughly as possible, then enabling more discovery techniques can help. With the exception of one strategy (find_ssh_keys), each of these strategies can be toggled off/on. Below are details of each of the strategies.

Detailed technical explanations of how some of these strategies work can be found in this blog post.

SSH Private Key Discovery Strategies

The following strategies are used to discover any private keys on the system. We use these strategies over, say, checking every single file on the system, because it would be prohibitively expensive to open and parse every file on the system. Instead, the following strategies can be used to provide hints towards the location of private keys.

find_ssh_keys

Checks every readable file within the .ssh/ folder in every home folder of the system (identified using getent passwd and /home/*/. This strategy is not optional.

scan_paths / scan_paths_depth

Using the scan_paths and scan_paths_depth user-setting, the script uses find(1) to search each of the locations specified by scan_paths for keys. A maximum depth for searching directories is specified using scan_paths_depth (equivalent to find $location -maxdepth $scan_paths_depth). Multiple locations may be specified.

For example, scan_paths=("/root/" "/var/*/") and scan_paths_depth=3 will look for look for keys in /root/*/*/* and /var/*/*/*/*.

If scan_paths is left empty (scan_paths=()) or scan_paths_depth is less than 1, this strategy does nothing. The default value is () and 3 respectively.

SSH Host Discovery Strategies

The following strategies are used to discover potential hosts which the script will SSH to. Hosts correspond to either a hostname or an ip address: they do not constitute a destination that SSH will use in of itself, since SSH is missing a username that corresponds to the host.

Since hosts do not correspond to destinations, it is necessary for hosts to somehow be combined with usernames to actually be useful. Therefore, all "host discovery strategies" are disabled unless either:

  1. use_combinate_interesting_users_hosts is enabled and there is at least one entry in interesting_users,
  2. use_combinate_users_hosts_aggressive is enabled.

use_combinate_interesting_users_hosts will combine all of the interesting_users with all of the hosts discovered to create destinations, and use_combinate_users_hosts_aggressive will combine all users discovered with all of the hosts discovered to creation destinations. That process is described in combinatorial_destination_discovery_strategies.

use_find_from_hosts

By parsing /etc/hosts (or more correctly, running getent ahostsv4), extra hosts may be discovered.

This strategy may be toggled off/on with 0/1. The default value is 1.

use_find_arp_neighbours

This function adds all of the neighbour hosts from the arp table to the list of hosts.

This strategy may be toggled off/on with 0/1. The default value is 1.

use_find_d_block

This function takes the current host's address(es) and adds the all d-block addresses into the list of hosts. For example, if the current host has the address 10.0.0.5, this function will add the 10.0.0.0-10.0.0.255 hosts.

This strategy may be toggled off/on with 0/1. The default value is 0.

SSH Destination Discovery Strategies

In the context of this script, a "destination" (or "dest") means a username and host combination; the destination that ssh will attempt to connect to (ssh user@host). Without destinations, there is nowhere for the script to attempt to SSH to. These strategies are used to discover potential destinations that will be SSH'd to.

Note that for each of these destinations, the username is also loaded into a collection of usernames, and the hosts are loaded into a collection of hosts. If use_combinate_users_hosts_aggressive or use_combinate_interesting_users_hosts are enabled, they will use these usernames and hosts to create destinations.

use_find_from_authorized_keys

authorized_keys files may include ip-based restrictions for key usage such as from="10.0.0.1,10.2.3.3" ssh-rsa ....

This strategy adds any hosts discovered in the from directive of authorized_keys file to our list of destinations using $user@$host, where $user is the username corresponding to the location where the authorized_keys file was found.

This strategy may be toggled off/on with 0/1. The default value is 1.

use_find_from_last

This strategy checks the last logins to the system by using last, and extracts the addresses the logins came from. The strategy then naively assumes that the destination username of that SSH access also corresponds to the source username, adding the $destination_username@$source_address to the list of destinations.

This strategy may be toggled off/on with 0/1. The default value is 1.

use_find_from_prev_dest

This strategy takes the destination that we are connecting from and adds it to the list of destinations.

For example, consider the chain user@system_a->user@system_b->user@system_c. Using this strategy, user@system_b will try to connect to user@system_a. user@system_c will then try to connect to user@system_b.

This strategy may be toggled off/on with 0/1. The default value is 1.

use_find_from_known_hosts

known_hosts files contain a list of the hosts a user has previously SSH'd into, and can be a wealth of knowledge for discovering hosts. For example: # 2048 MD5:32:41:b4:e7:3e:d7:ee:a4:3a:c3:a8:44:40:45:16:04 10.0.0.1 (RSA).

This function extracts the 10.0.0.1 host. Likewise, we add $user@$host to the list of destinations, where $user is the username corresponding to the location where the known_hosts file was found.

Incidentally, this function also extracts users, hosts, and destinations from authorized_keys files which may share a similar format like # 2048 MD5:62:38:9a:f0:6d:e7:57:57:25:09:71:4d:c7:bb:4b:b0 root@system (RSA). This function will extract root@system.

This strategy may be toggled off/on with 0/1. The default value is 1.

use_find_from_hashed_known_hosts

known_hosts files contain a list of hosts that a user has previously connected to with SSH. However, SSH offers an option HashKnownHosts which is used to hash the entries of the hosts in this file such that they cannot easily be read. This is the default on most Debian-based operating systems.

For example, a user that has has HashKnownHosts enabled when running ssh 192.168.1.1 will have an entry in their known_hosts file similar to this: |1|e77JRypO4qWElXpIaBGiFLOJBXg=|HBu6N6IGFeOz5wt0HFXz9/hp/wY= ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIClRF2LjL1EJNfEYAKpr9rENgZVFlH7jtEPla0x3hakB

Here, |1|e77JRypO4qWElXpIaBGiFLOJBXg=|HBu6N6IGFeOz5wt0HFXz9/hp/wY= corresponds to a hash of the host 192.168.1.1. Hashing is a one-way procedure, however it is possible to brute-force this hash using ssh-keygen:

$ ssh-keygen -F 192.168.1.1
# Host 192.168.1.1 found: line 58 
|1|96KUSpeaZrkYrbQhhjFLF/jJ15w=|xMX7qNROy8SwPZK1zEjrlEeYU24= ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIClRF2LjL1EJNfEYAKpr9rENgZVFlH7jtEPla0x3hakB

When use_find_from_hashed_known_hosts is enabled, we attempt to brute-force all of the ip addresses of the c and d blocks of the current system. For example, if we are currently on user@85.82.81.80 and we discover hashed known hosts, we will attempt to discover them by checking whether the hash corresponds to any of the hosts 85.82.0.0-85.82.255.255. If any of the hashes are cracked, we add $user@$host to the list of destinations, where $user is the username corresponding to the location where the known_hosts file was found.

Note that this strategy is extremely slow and for each known_hosts file may take many minutes. Some performance testing yielded the following time taken per known_hosts file:

# xargs available: 2m42.820s
# no xargs available: 7m52.145s
# xargs available using sudo: 3m56.659s
# no args available using sudo for-loop: 15m36.738s

Unlike the rest of the script, if use_sudo is enabled, sudo will not actually be used unless it is completely necessary (i.e. the known_hosts file cannot be read by the current user).

This strategy may be toggled off/on with 0/1. The default value is 0.

use_find_from_ignore_list

At any point in time, the so-called "ignore list" contains a list of destinations which have previously been successfully accessed and scanned. This strategy takes this list and adds all of those destinations to the list of destinations to connect to. There are two different ways this strategy can be used.

When use_find_from_ignore_list=1, the script will parse the ignore list when the script starts and connect to any destinations which have previously been connected to.

When use_find_from_ignore_list=2, the script will do the same as above, but also parse the ignore list when the script finishes, re-trying with any newly added destinations on the ignore list.

use_find_from_ignore_list=0 disables this function completely. The default value is 0.

use_retry_all_dests

The 2nd option from the use_find_from_ignore_list strategy is slightly flawed. Consider the following:

A->B->C ; Normal scan
A->D->C ; A->D discovered naturally, D->C discovered using use_find_from_ignore_list=1 or use_find_from_ignore_list=2.
A->C ; A->C discovered using use_find_from_ignore_list=2.

It is possible that C->D could also exist. However, this link will not be discovered because destination D was first discovered after C. Since C has already been scanned, the script will not scan it again, losing the valuable data of C->D.

In order to discover every possible combination of connected systems, use_retry_all_hosts takes a different approach. Once the whole scan is completely finished (i.e. the script the user is running), the scan simply repeats itself but adds all of the previously discovered destinations to interesting_dests, ensuring that every single destination will try every other destination.

On the re-run, we do NOT attempt to discover any NEW users/hosts/destinations, only discover keys. This means that although new chains may be discovered, no new destinations will be discovered. In fact, the function to add new users, hosts, and destinations is noop'd during the re-run, so it is not possible for new destinations to be discovered at all.

This strategy may be toggled off/on with 0/1. The default value is 1.

Both SSH Private Key and Destination Discovery Strategies

Some of these strategies can be used to find both private keys and destinations. Both of these strategies are generally advised to be kept enabled, as they are the most natural resource for finding how SSH has been used on the system (and therefore where we can find keys and where they connect to easily).

use_find_from_bash_history

.bash_history files are a jackpot/wealth of information when it comes to SSH private keys and especially destinations. By parsing .bash_history files, we can see exactly how users are utilizing the ssh, scp, and rsync binaries, and determine where they have been connecting to and with which keys.

Parsing each bash_history line, we look at all calls to ssh, scp, and rsync and extract the following:

  1. Usernames,
  2. Hosts,
  3. Destinations,
  4. Key Files.

For ssh, scp, and rsync, we extract any destinations in the form user@host.

For ssh and scp specifically, we parse and extract:

  1. The location of any key files which have been passed with the -i parameter: -i /absolute/key_file, -i ~/expanded/key_file, or -i relative/key_file. In the middle case, we expand the ~ to the home directory of the user corresponding to where the .bash_history file was found. In the latter case, we also assume the location corresponds to the relative location based on the home directory of the user corresponding to the .bash_history file. Likewise, all cases -i/absolute/key_file, -i~/expanded/key_file, and -irelative/key_file are also extracted.
  2. Any username of any remote destinations that have been passed with the -l parameter (see man(1)).
  3. Any hosts of any remote destination
  4. Any destinations.

For example, this strategy is able to extract the following (note: this list is not exhaustive and these are just some examples. the use_find_from_bash_history function is complicated and can hopefully handle all legitimate usage of ssh and scp):

ssh user@host ; extract user@host as a destination
scp file user@host:~/ ; extract user@host as a destination 
scp user@host:~/file ./ ; extract user@host as a destination
rsync -a * user@host:~/ ; extract user@host as a destination 
scp file host:~/ ; extract host, and assume $user@host as a destination
scp host:~/file ./ ; extract host, and assume $user@host as a destination
ssh -i.ssh/id_rsa host ; extract .ssh/id_rsa and host, assume $user@host as a destination
ssh -i .ssh/id_rsa host ; extract .ssh/id_rsa and host, assume $user@host as a destination
ssh -luser host ; extract user, extract user@host as a destination
ssh -i /tmp/key host -luser ps ; extract /tmp/key and user@host as a destination
ssh host -v 'bash -c ls' ; extract host, assume $user@host as a destination
sssh -D 9000 localhost -luser ; extract user@localhost as a destination
ssh -i key_file -v -luser host ps ; extract key_file and extract user@host as a destination

This strategy may be toggled off/on with 0/1. The default value is 1.

use_find_from_ssh_config

By parsing every readable file in.ssh/ folders, we may encounter ssh_config files. These files generally look something like this:

 Host example.com
   Hostname example.com
   User your_username
   IdentityFile ~/.ssh/id_rsa

We parse this file and extract the IdentityFile for the location of a private key. We also extract the Host, Hostname, and User.

Unfortunately at the moment, this function is unable to generate individual destinations as it naively parses the file line-by-line, rather than block-by-block. Future versions will be able to identify the destination that the block corresponds to.

This strategy may be toggled off/on with 0/1. The default value is 1.

Combinatorial Destination Discovery Strategies

Each time a system is scanned, various individual usernames, hosts, and destinations will be discovered which can point towards other destinations. However, if usernames and hosts are discovered in a context that doesn't necessarily result in a definitive full destination being discovered, this data is effectively unused. That's where combinatorial destination discovery strategies come in.

combinate_interesting_users_hosts

This strategy takes pre-defined usernames, hosts, and destinations, and combinates them with their respective others to form destinations.

All interesting_users will be combined with all of the hosts that are discovered on the system to form destinations.

All interesting_hosts will be combined with all of the usernames that are discovered on the system to form destinations.

All interesting_users will be combined with all of the interesting_hosts to form destinations.

All interesting_dests will be used to form destinations, and can be used to force every system to attempt to SSH into specific destinations with every key.

This strategy may be toggled off/on with 0/1, or with empty interesting_users=(), interesting_hosts=(), interesting_dests() values. The default values are 1, (), ("$USER" "root"), and () respectively.

use_combinate_users_hosts_aggressive

Every single username discovered is combined with every single host discovered to form destinations.

This strategy will result in massive growth of attempted destinations; for example, 100 usernames discovered on a system along with 100 hosts will result in 10,000 destinations. Use this with caution.

This strategy may be toggled off/on with 0/1. The default value is 0.

Alternative Settings

In some parts of the world, it is common to simply look for any SSH private keys in .ssh/ folders and try to SSH into every address in the d-block of the system which the key was found. The below settings imitate a similar strategy. That is to say, the below settings ensure that the script only looks for nearby hosts in the d-block, arp neighbours, and /etc/hosts, and attempts to SSH into each of those hosts with the username root and the username which the system started with ($USER evaluates to the username that has been SSH'd into on each destination).

interesting_users=("$USER" "root")
interesting_hosts=("127.0.0.1")
interesting_dests=()
use_combinate_interesting_users_hosts=1
use_combinate_users_hosts_aggressive=0
use_retry_all_dests=1
scan_paths_depth=3
use_find_from_hosts=1
use_find_arp_neighbours=1
use_find_d_block=1
ignored_key_files=("*badcert.pem*" "*badkey.pem*")
use_sudo=1
ssh_timeout=3
retry_count=3
ignore_user=0
ignored_users=()
ignored_hosts=()
ignored_dests=()
custom_cmds=()
scan_paths=()
use_find_from_authorized_keys=0
use_find_from_last=0
use_find_from_prev_dest=0
use_find_from_known_hosts=0
use_find_from_hashed_known_hosts=0
use_find_from_ignore_list=0
use_find_from_ssh_config=0
use_find_from_bash_history=0