This is a landing page for linux security.
Ubuntu 18.04 End user Device Guidance
Linux DISA Stigs (Unclassified)
https://public.cyber.mil/stigs/downloads/?_dl_facet_stigs=operating-systems%2Cunix-linux
CIS Benchmarks (Registration Required) PDFs are free to download
https://www.cisecurity.org/cis-benchmarks/
Ubuntu 18.04 Server Security Guide
https://help.ubuntu.com/lts/serverguide/security.html
Raspberry Pi Security
https://www.raspberrypi.org/documentation/configuration/security.md
Cent OS Security
https://wiki.centos.org/HowTos/OS_Protection
Open Suse Security
https://doc.opensuse.org/documentation/leap/security/html/book.security/index.html
Debian Security
https://www.debian.org/doc/manuals/securing-debian-howto/
Fedora Security Lab (This project is useful security auditing, forensics, system rescue and teaching security testing methodologies in universities and other organizations.)
https://labs.fedoraproject.org/en/security/
Kali Linux
https://www.kali.org/downloads/
Parrot Security
https://www.parrotlinux.org/download-security.php
Black Arch Linux
https://blackarch.org/downloads.html
Network Security Toolkit
https://sourceforge.net/projects/nst/files/NST/NST%2030-11210/
BackBox Linux
https://www.backbox.org/download/
Tails
https://tails.boum.org/install/dvd-download/index.en.html
Linux Kodachi
https://www.digi77.com/linux-kodachi/
WhoNix
https://www.whonix.org/download/
Qubes OS (Requires Dedicated Workstation. Do not run this in a VM)
https://www.qubes-os.org/downloads/
Very small list of Linux OS's I've used for sensitive data handling.
Trusted End Node Security
This is a DOD MIL distribution. I use this distro to access sensitive intelligence community sites. This DOD site uses its own internal root ca. So you will get errors about the encrypted website being unsecure. Make sure to use the Encrypt DNS application before accessing sensitive material.
https://www.spi.dod.mil/download.htm
Discrete Linux
Use this distro with caution. It has not been updated since 2016. Its meant for airgapped network testing and accessing sensitive data. Its an interesting project but not really maintained. I do find it noteworthy for what it does.
https://www.privacy-cd.org/index.html
I won't be linking NSA's slide decks of PRISM but you can find it on your own. This is just an interesting website of alternatives to closed source software.
Prism Break