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 <>

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

Other links for securely erasing drives.

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.

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.

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
  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', '=='))

Harden SSH for AlmaLinux 9 (RHEL, Fedora)

These steps are taken from the following link. They have other guides for hardening Ubuntu, Debian etc.

You will need to become the root user, use either su – or sudo -i

First we need to regenerate the RSA and ED25519 keys

rm /etc/ssh/ssh_host_*
ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""
ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_rsa_key -N ""

Next, remove the small Diffie-Hellman moduli. The moduli file contains prime numbers and generators. Removing the smaller numbers should help increase security as it makes attempting to factor the private keys harder.

awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/
mv /etc/ssh/ /etc/ssh/moduli

We can now specify which key exchange, ciphers, and algorithms to use.

Add the following to “/etc/crypto-policies/back-ends/opensshserver.config”

# Restrict key exchange, cipher, and MAC algorithms, as per
# hardening guide.




RequiredRSASize 3072


GSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-



Finally, restart the ssh server

systemctl restart sshd

Other helpful links

How to Fix OpenVAS “ERROR: The default PostgreSQL version (14) is not 16 that is required libgvmd”

Currently OpenVAS needs PostgreSQL 16 on port 5432. If you have multiple versions of PostgreSQL, the lowest version will typically run on port 5432, and then they’ll increment from there. For example, if you have PostgreSQL 14, 15, and 16, 14 will run on port 5432, 15 on 5433, and 16 on 5434.

The quick fix is to edit the PostgreSQL config files, change the port numbers, restart PostgreSQL, and rerun gvm-setup.

vi /etc/postgresql/16/main/postgresql.conf

Change port number from 5434 to 5432

You will need to remove/disable/change the port for PostgreSQL 14

sudo apt remove postgresql-14
sudo systemctl restart postgresql@16-main.service

We can verify that PostgreSQL is running with netstat.

netstat -tulpn

We can see that the ports 5432 (PostgreSQL 16) and 5433 (PostgreSQL 15) are both running.

Rerun gvm-setup

sudo gvm-setup

Install and Setup OpenVAS on Kali Linux 2023/2024

Notes on installing OpenVAS on Kali Linux in 2023/2024

sudo apt install openvas

Run the setup script. This used to be called openvas-setup, now it is gvm-setup. Note that the script can take a long time to run.


At the end of the script, it will give you a password. Use this password to log into the web interface. You can reset the password if needed.

If you run into issues with PostgreSQL, check out this post

Log into the web interface at


On Kali Linux, you need to run commands as the _gvm user. You can do this by prepending the commands with

sudo runuser -u _gvm -- COMMAND

There are two — dashes, between the _gvm user and the COMMAND. Replace COMMAND with the GVM/OpenVAS command you want to execute.

Example, to list the current users do

sudo runuser -u _gvm -- gvmd --get-users

To create a new user run

sudo runuser -u _gvm -- gvmd --user=newadmin --new-password=longsecurepassword

Failed to find config ‘daba56c8-73ec-11df-a475-002264764cea’

If you receive a `Failed to find config ‘daba56c8-73ec-11df-a475-002264764cea'”` error,

try running the following command

sudo runuser -u _gvm -- greenbone-nvt-sync

This can take awhile, but it should sync all the files needed. Check the following link for more information.

The following link is also helpful for installing OpenVAS

Common Power Issue Terminology

Here is a list of terms associated with power issues.

FaultMomentary loss of power
BlackoutProlonged power outage
SagMomentary low voltage
BrownoutProlonged Low voltage
SpikeMomentary spike in voltage
SurgeExtended spike in voltage
InrushInitial voltage “surge” when a device is plugged in

Here is a visual image.

Table of Types of Law for Cyber Security

There are three types of law. Criminal, civil, and administrative.

Type of LawExamplesStandard of ProofBurden of ProofPenalty
Criminal LawMurder, assault, robbery, arsonBeyond a reasonable doubtInnocent until proven guiltyFines, Jail, Prison, Death penalty
Civil LawProperty Disputes, Personal injuryPreponderance of evidenceClaimant must give proof (most cases)Compensation for injuries/damage
Administrative LawDefine standards of performance and conduct for major industries, organizations and government agencies
Table of Law

List of Laws and Acts

The following is a list of “good to know” legislative acts.

CFAAComputer Fraud and Abuse ActFirst major cyber crime legislation
Federal Sentencing Guidelines (1991)Responsibility on senior management
ECPAElectronic Communications Privacy Act of 1986Made it a crime to invade the electronic privacy of an individual
CALEAComm Assistance for Law Enforcement Act of 1994Amended ECPA. Made wiretaps possible for law enforcement with a court order.
Economic Espionage Act of 1996Made theft no longer tied to something physical
FISMAFederal Information Security Management ActCyber security requirements for government agencies
DMCADigital Millennium Copyright ActCopyright protection is 70 years +
1st major revision added CD/DVD protections
USA PATRIOTUSA PATRIOT Act of 2001Gave law enforcement and intelligence agencies broader wiretapping authorizations
Identity Theft and Assumption Deterrence Act (1998)Made identity theft a crime. Up to 15 years in prison and $250,000 fine.
HIPPAHealth Insurance Portability and Accountability Act (1996)Regulations for security measures for hospitals, physicians, and insurance companies
HITECHealth Information Technology for Economic and Clinical Health Act of 2009Amended HIPPA. Updated privacy/security requirements for Business Associates (BAs), requires a written contract known as a business associate agreement (BAA). BAs are directly subject to HIPPA and enforcement actions like a covered entity.
HITECH also introduced new data breach notifications.
GLBAGramm-Leach-Bliley ActLimits services that banks, lenders, and insurance agencies can provide and information they can share with each other
COPPAChild Online Privacy Protection ActSeeks to protects children (<13 years old) online
FERPAFamily Educational Rights and Privacy ActGives students certain privacy rights. Deals with adults >18, and Children in school <18
ITARInternational Traffic in Arms RegulationRegulates the export of military and defense related technologies
EARExport Administration RegulationsFor commercial use, but may have military applications.
Table of Laws and Acts

Trademark, Patents, Copyright etc.

NameProtection Length
Trademarks10 Years
Patents20 Years
Copyright 70 Years after the death of the author
Trade SecretsUntil they are leaked.
Table of Trademarks, Patents, Copyright, and Trade Secrets

List of Symmetric Encryption Algorithms. Block and Key Size.

List of common symmetric encryption algorithms with their block and key size.

NameBlock SizeKey SizeNotes
AES Advanced Encryption Standard128128, 192, 256Is Rijndael
RijndaelVariable128, 192, 256Is AES
Blowfish 6432-448Often used in SSH
DES Data Encryption Standard6456
Triple DES64112-168DES 3 times
IDEA64128Used in PGP
RC4 Rivest Cipher 4Stream Cipher40-2048Insecure/Not used, used in WEP, WPA, and SSL
RC5 Rivest Cipher 532, 64, 1280-2040
RC6 Rivest Cipher 6128128, 192, 256
Skipjack6480Developed by the NSA and supported key escrow
CAST-256128128, 160, 192, 224, 256
ChaCha20Stream Cipher256
List of Common Symmetric Encryption Algorithms With Block and Key Size