Differences between RTO, RPO, MTBF, and MTFF

Here is a quick overview of the differences between, RTO, RPO, MTBF, and MTFF.

RTO (Recovery Time Objective)Time it takes to recover from a disruption, system failure, data loss etc.
RPO (Recovery Point Objective)How much data can you afford to loose? If RPO is 24 hours, then backups need to be performed daily.
MTBF (Mean Time Between Failures)Time between failures. Use for repairable systems
MTTF (Mean Time to Failure)Time before system fails. Use for nor repairable systems.




Securely Delete Files on Linux

We can use srm to securely delete files on Linux.

Install srm with

sudo apt install secure-delete

We can now securely delete files by running

srm filetodelete.txt

# srm --help
srm v3.1 (c) 1997-2003 by van Hauser / THC <vh@thc.org>

Syntax: srm [-dflrvz] file1 file2 etc.

-d ignore the two dot special files "." and "..".
-f fast (and insecure mode): no /dev/urandom, no synchronize mode.
-l lessens the security (use twice for total insecure mode).
-r recursive mode, deletes all subdirectories.
-v is verbose mode.
-z last wipe writes zeros instead of random data.

srm does a secure overwrite/rename/delete of the target file(s).
Default is secure mode (38 writes).
You can find updates at http://www.thc.org

Other links for securely erasing drives.

Top 8 Nmap options

Here are 8 excellent Nmap options, what they do, and why you would use them.

Most of the options can be run together. You will normally want to perform scans with administrator or root privileges.

OptionWhat is doesWhy you would use
1.-snNo port scanHelpful for quickly discovering hosts that are up
2.-iL file.lstScan IP addresses in file.lstHelpful if you already have a list IP addresses to scan
3. -nSkip reverse DNS lookupThis can help speed up scanning
4.-PnPretend host is upUse when hosts have Ping disabled. e.g. Windows
5. -OOS detectionUse to detect OS version
6.-T4Speed up scanIncreases scan speed (Default is -T3)
7.-AAggressive scan optionsShorthand option. Enables OS detection (-O), version Scanning (-sV), script scanning (-sC), and runs a traceroute
8.-oA filenameSave output to ALL formatsThis saves the output to separate files for XML and grepable formats
Nmap table: 8 common options.

A Quick Overview of SAML

SAML stands for Security Assertion Markup Language. It allows for Single Sign On or SSO to a service.

There are three entities or roles involved when using SAML to sign into a service.

  1. Principal or Subject: a.k.a. you, or the person or service logging in.
  2. Service Provider (SP): This is the service you are accessing. It could be email, a website, etc.
  3. Identity Provider (IdP): This is the entity response for authenticating the Principal.

As an example, let’s say you want to log into a new website utilizing your email SSO credentials. You click the SSO login button, you are redirected to the IdP to login. Once authenticated, your device will receive a token which is then passed back to the Service Provider and allows you access to the new website.

This is a very simplified version of what happens when you login using SAML. It may be helpful to know that the Service Provider and the Identify Provider will have needed to be configured to work together before the user attempts to log in.



Locate large files on Linux

Show size of directories. The -h option prints the size in human readable format.

du -h --max=1 ./

We can use sort and tail to filter and only show the 10 largest files and directories. The -a option shows all files and directories.

du -ah ./ | sort -h | tail -n10

We can use the find command to show all files over xMB. In this case 100MB

fine . -type f -size +100M -print


Hacking Hashed SSH known_hosts file

On Ubuntu, by default, the hosts in .ssh/known_hosts are hashed. This can theoretically help with security. If an attacker compromises a host, they will not be able to tell the IP addresses of other hosts in the known_hosts file.


Anatomy of the hashed known_hosts

Here is an example of a hashed entry in the known_hosts file.

|1|ma8KL2XrNYkNnknf68N4IuZ+c+I=|PmR+n2i0/epUGZZh2S+LB6OaowQ= ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEjqG8/el8c669FxcvEw5mMfDRTDxsjgLiz44dCTtchs

There are three main parts.

The first part ma8KL2XrNYkNnknf68N4IuZ+c+I= is the salt to use.

PmR+n2i0/epUGZZh2S+LB6OaowQ= This is our hashed IP address/hostname

ssh-ed25519 is the key type

AAAAC3NzaC1lZDI1NTE5AAAAIEjqG8/el8c669FxcvEw5mMfDRTDxsjgLiz44dCTtchs Is the public SSH key of the remote host.


We can use ssh-keyscan to check the keys of hosts. The -t ssh-ed25519 option only shows ed25519 keys. Remove or change to show all key types e.g. RSA/DSA

For example:

└─$ ssh-keyscan -t ssh-ed25519 localhost
# localhost:22 SSH-2.0-OpenSSH_9.6p1 Debian-3
localhost ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEjqG8/el8c669FxcvEw5mMfDRTDxsjgLiz44dCTtchs

We can compare the SSH public key with the one in our known_hosts file to verify we have the correct host.

As a side note, we can also use the -H option to show us a hashed version. The salt changes each time it is run, so it is not useful for comparing the hashed IP address.


└─$ ssh-keyscan -H -t ssh-ed25519 localhost
# localhost:22 SSH-2.0-OpenSSH_9.6p1 Debian-3
|1|j2j9iv/GkPfnG9Yv4WzJsy/L1pc=|wethKgsGBH0Mi+rFW3zSNSWiGso= ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEjqG8/el8c669FxcvEw5mMfDRTDxsjgLiz44dCTtchs

Hacking known_hosts hashes

There are a few different techniques that can be used to identify known hosts IP addresses even if they are hashed.


Search through bash history

history | egrep '([0-9]{1,3}\.){3}[0-9]{1,3}'

Example output

└─$ history | egrep '([0-9]{1,3}\.){3}[0-9]{1,3}' | head -n2
1 ssh kali@

Check if SSH Public Key is on Shodan

Since the SSH public key is um, well, public, we can search for it on Shodan to see if it’s a known public server. https://www.shodan.io

Copy the public ssh key from the known_hosts file. It is the last portion of the line i.e.

|1|ma8KL2XrNYkNnknf68N4IuZ+c+I=|PmR+n2i0/epUGZZh2S+LB6OaowQ= ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEjqG8/el8c669FxcvEw5mMfDRTDxsjgLiz44dCTtchs

Brute force

Since the address space for IPv4 is fairly small, and the private IP address space even smaller, brute forcing all the addresses is perfectly feasible.

Here is a quick example on how you would hash an IP address. Commands are taken from the above Stack Exchange link.

Take the salt and put it into a variable

key=`echo j2j9iv/GkPfnG9Yv4WzJsy/L1pc= | base64 -d | xxd -p`

Next we can run the following command to hash the result. The IP ( is where we would want to enumerate the IP address.

echo -n "" | openssl sha1 -mac HMAC -macopt hexkey:$key|awk '{print $2}' | xxd -r -p | base64

The output is PmR+n2i0/epUGZZh2S+LB6OaowQ= which is the correct hash.

Automating should be fairly simple.

A note on SSH ports. If the host is using a non standard ssh port, you will need to update the above command with the port, but the address needs to be wrapped in square brackets []

echo -n "[]:2222" | openssl sha1 -mac HMAC -macopt hexkey:$key|awk '{print $2}' | xxd -r -p | base64

Use ssh-keyscan

A final way we can discover known-hosts, is by using ssh-keyscan. The man page says the following

ssh-keyscan is a utility for gathering the public SSH host keys of a number of hosts. It was designed to aid in building and verifying ssh_known_hosts files

ssh-keyscan uses non-blocking socket I/O to contact as many hosts as possible in parallel, so it is very efficient. The keys from a domain of 1,000 hosts can be collected in tens of seconds, even when some of those hosts are down or do not run sshd(8). For scanning, one does not need login access to the machines that are being scanned, nor does the scanning process involve any encryption.

Hosts to be scanned may be specified by hostname, address or by CIDR network range (e.g. 192.168.16/28). If a network range is specified, then all addresses in that range will be scanned.

This makes it super convenient to do a network scan using ssh-keyscan and then compare the public ssh keys with those in the known_hosts file.



To scan all private IP ranges (RFC1912), we just run the scan with all three IP ranges


Check for backdoored version of xz (CVE-2024-3094) (Ansible/Bash)

Info on the xc backdoor




Kostas on Twitter posted a helpful one-liner to check the xz version without running the actual command.


Versions 5.6.0 and 5.6.1 are backdoored.

Bash one liner

The following Bash commands were taken and modified from the above Twitter link

Here is a one liner that will check the version of xz binaries and return if they are safe or vulnerable. You’ll need to run this in a Bash shell. May have issues in sh.

for xz_p in $(type -a xz | awk '{print $NF}' ); do  if ( strings "$xz_p" | grep "xz (XZ Utils)" | grep '5.6.0\|5.6.1' ); then echo $xz_p Vulnerable; else echo $xz_p Safe ; fi ; done 

Ansible Playbooks

Here are two different Ansible Playbooks to check if the xz package(s) are backdoored.

This one uses the above Bash commands to check the xz binaries.

- name: Check if XZ tools are compromised
# https://twitter.com/kostastsale/status/1773890846250926445
  hosts: all

    - name: Run Bash command
      shell : 
        for xz_p in $(type -a xz | awk '{print $NF}' ); do 
          if ( strings "$xz_p" | grep "xz (XZ Utils)" | grep '5.6.0\|5.6.1' ); 
            then echo $xz_p Vulnerable!; 
            echo $xz_p Safe ; 
          fi ; 
        executable: /bin/bash
      register: result

    - name: Show output
        msg: "{{ result.stdout_lines }}"

The following playbook uses the package manager to check the xz version. On RHEL/Fedora this is the xc package. On Debian/Ubuntu, it is part of the liblzma5 package.

- name: Check if XZ tools are compromised
  hosts: all

    - name: Collect package info
        manager: auto

    - name: Check if liblzma5 is vulnerable (Ubuntu/Debian)
        msg: "Installed version of liblzma5/xz: {{ ansible_facts.packages['liblzma5'] | map(attribute='version') | join(', ') }} Vulnerable!"
      when: ('liblzma5' in ansible_facts.packages) and (ansible_facts.packages['liblzma5'][0].version.split('-')[0] is version('5.6.0', '==') or ansible_facts.packages['liblzma5'][0].version.split('-')[0] is version('5.6.1', '=='))

    - name: Check if xz is vulnerable (RHEL/Fedora/Rocky/Alma)
        msg: "Installed version of xz: {{ ansible_facts.packages['xz'] | map(attribute='version') | join(', ') }} is vulnerable"
      when: ('xz' in ansible_facts.packages) and (ansible_facts.packages['xz'][0].version is version('5.6.0', '==') or ansible_facts.packages['xz'][0].version is version('5.6.1', '=='))

How to Disable the Bandwidth Server on Mikrotik/RouterOS

The Bandwidth test tool can be helpful to test speed between Mikrotik routers. But you can disable it if you don’t need it.

From Winbox

From Winbox click on Tools > BTest Server > Disable > OK

From Command Line

From the command line you can disable the bandwidth server by running the following command.

/tool/bandwidth-server/set enabled=no

If you are still on RouterOS 6.x use

/tool bandwidth-server set enabled=no

Enable Bandwidth test

If you need to enable the bandwidth server again, just change enabled=no to enabled=yes

/tool bandwidth-server set enabled=yes


Handling Spaces in File Names on Linux

Using ls to parse file names is not recommended for multiple reasons


Let’s say we have a directory with two files in it.

Hello, world.txt

Now we want to loop over the files. If we use ls in our for loop,

for file in $(ls); do echo "$file" ; done

We receive the following output


The space in “Hello, world.txt” is translated as a new line. This could break our script.

Here is a better way

for file in * ; do echo "$file" ; done

Helpful links