Linux Hardening - Made Easy

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Hardening Linux
Made Easy
Guy Barnhart-Magen, Melior Security
@barnhartguy

Who am I?
Father of two, hacker
BSidesTLV chairman and CTF Lead
(Lucky to speak at many conferences)
Today: Cyber Security Consultant
Before: Intel, Cisco and a couple of Startups
OS Hardening, Crypto, Embedded Security, Security of ML
@barnhartguy

@barnhartguy
Why Hardening?
Assumption - the attacker has a foothold in your server
At least shell access (or something that will give him access)
Can we limit what he can do?

@barnhartguy
Threat Model
Consider the following:
● Is the VM compromised?
● Do we need a scalable solution?
● Is it open to the internet?
● How do we do patch management?
● If an attacker gets a shell, what is compromised?
● If an attacker gets root access, what is compromised?
● Do we have someone to look at reports?

@barnhartguy
Threat Model
Attacker model
● Is this a targeted or opportunistic attack?
● Do I have vital business value on this VM?
● Is the system old? Any security concerns? Something signaling to attackers?

@barnhartguy
Passive vs. Active
Passive - build defenses, but an attacker is not present in the system allowing for more
ﬂexibility
Active - need to remove an attacker (or suspicion) from the system, before deploying
defenses

@barnhartguy
Shopping list?
CIS Benchmarks
Lynis
NIST - SP800-123
Other standards

@barnhartguy
Hardening the System
● Passive vs. Active
● Firewall
● Updates
○ Repo, security, patches/upgrades
○ Remove unneeded packages
● SSH
○ 2FA
○ fail2ban
● User Accounts
○ Credentials, ACL
● Remote Logging
● Sensitive Files/Directories
● Remove unneeded TTY
● Secure Shared Memory/tmp folder
● Remove uncommon ﬁlesystems
● Disable compilers
● Set UMASK
● Disable core dumps

@barnhartguy
● Wrapper for iptables
● Enable Firewall
Firewall
$ sudo ufw allow ssh
$ sudo ufw enable

@barnhartguy
We would like to keep all our repositories up to
date
● Also, we would like to automate this
● Be careful - updates can break stuﬀ!
● Rebooting is also a concern
Updating the System
$ sudo apt-get update
$ sudo apt-get upgrade -y

@barnhartguy
We would like to keep all our repositories up to
date
● Also, we would like to automate this
● Be careful - updates can break stuﬀ!
● Rebooting is also a concern
Updating the System
$ sudo apt-get install unattended-upgrades
apt-listchanges
$ sudo dpkg-reconfigure -plow
unattended-upgrades
$ sudo nano
/etc/apt/apt.conf.d/50unattended-upgrades
Unattended-Upgrade::Mail "user@example.com";
Unattended-Upgrade::Automatic-Reboot "true";
$ sudo unattended-upgrades --dry-run

@barnhartguy
● Reduce attack surface
● We should remove old/unneeded packages
Examples:
Ipv6, irqbalance, Bluetooth, USB storage driver,
Anacron, Apport, Atd, Autofs, Avahi, CUPS,
Dovecot, Modemmanager, Nfs, Snmp, Telnet,
Whoopsie, Zeitgeist
Updating the System
$ dpkg --list
$ dpkg --list packageName
$ apt-get remove packageName
$ sudo apt-get --purge ntfs-3g

@barnhartguy
● We should limit the number of users that are
allowed to login (never root)
● We should better protect these account
● If you can, use PKI keys
○ If you cannot, use 2FA
SSH Hardening
$ ssh-keygen -t ed25519
$ nano /etc/ssh/sshd.conf
PermitRootLogin no
ChallengeResponseAuthentication no
PasswordAuthentication no
UsePAM no
AuthenticationMethods publickey
PubkeyAuthentication yes
AllowUsers user1 user2
PermitEmptyPasswords no
ClientAliveInterval 300
ClientAliveCountMax 0
IgnoreRhosts yes

@barnhartguy
● Use TOTP
● Try to limit the number of users who have
access, or share TOTP values
SSH Hardening - 2FA
$ sudo apt-get install
libpam-google-authenticator
$ google-authenticator -td --rate-limit=3
--rate-time=120
$ nano /etc/pam.d/sshd
auth required pam_google_authenticator.so
nullok
sudo nano /etc/ssh/sshd_config
ChallengeResponseAuthentication yes
$ sudo systemctl restart sshd.service
$ sudo service ssh restart
$ sudo apt-get install oathtool
$ oathtool -b --totp `head -n 1
~/.google_authenticator`

@barnhartguy
● Fail2Ban and Rate Limiting
● Future updates can overwrite ﬁles, make
copies
SSH Hardening - Brute Force Attacks
$ sudo ufw limit ssh comment “rate limit ssh”
$ sudo apt-get install fail2ban
$ sudo cp /etc/fail2ban/fail2ban.conf
/etc/fail2ban/fail2ban.local
$ sudo cp /etc/fail2ban/jail.conf
/etc/fail2ban/jail.local
$ sudo systemctl start fail2ban
$ sudo systemctl enable fail2ban

@barnhartguy
● Separate user and admin accounts
● Limit “root” access
○ Root account shouldn’t have a login
● Verifying/setting that all world writable directories have their sticky bit set
User Accounts, ACL and special ﬁles/directories
$ sudo passwd -l root
$ sudo chown root:root /etc/passwd /etc/shadow /etc/group /etc/gshadow
$ sudo chmod 644 /etc/passwd /etc/group
$ sudo chmod 500 /etc/shadow /etc/gshadow
$ sudo find / -xdev -type d ( -perm -0002 -a ! -perm -1000 ) -print | while read
directory; do
echo "$FUNCNAME: ${GREEN} Making sticky on ${directory}..."
chmod +t ${directory}
done

@barnhartguy
● Verifying/setting that there are no world-writable files on the system
● Verifying/setting that there are no unauthorized SETUID/SETGID files on the system
User Accounts, ACL and special files/directories
$ sudo find / -xdev -type f -perm -0002 -print | while read file; do
chmod o-w ${file}
done
$ sudo find / -xdev ( -perm -4000 -o -perm -2000 ) -type f -print| while read file; do
if grep -Fxq "$file" "allowed_suid_list.txt"
then
echo “${file} - This program is allowed; leave it alone.”
else
chmod -s ${file}
fi
done

@barnhartguy
● Use RSysLog
Remote Logging
$ sudo apt-get update && apt-get install rsyslog
$ sudo systemctl enable rsyslog
$ sudo systemctl start rsyslog
$ sudo nano /etc/rsyslog.d/01-server.conf
*.* @@distant-server-ip:514
$ sudo systemctl restart rsyslog
$ journalctl -f -u rsyslog

@barnhartguy
● Several tools: CIS Benchmark, Lynis
Audit
$ git clone https://github.com/CISOfy/lynis
$ lynis/lynis audit system

@barnhartguy
● You mostly pay attention to a single TTY, an attacker can work in a diﬀerent one
Allow Single TTY
$ cat <<EOF > /etc/securetty
Console
Tty1
EOF
$ sudo nano /etc/default/console-setup
ACTIVE_CONSOLES=”/dev/tty1”
Reboot
$ dmesg | grep tty

@barnhartguy
●
Secure Shared Memory
$ sudo nano /etc/fstab
tmpfs /run/shm tmpfs defaults,noexec,nosuid 0 0

@barnhartguy
● Backup the /tmp dir, replace with new one (which is secure)
Secure Temporary Directories
dd if=/dev/zero of=/usr/tmpDSK bs=1024 count=1024000
mkdir /tmpbackup && cp -Rpf /tmp /tmpbackup
mount -t tmpfs -o loop,noexec,nosuid,rw /usr/tmpDSK /tmp
chmod 1777 /tmp
cp -Rpf /tmpbackup/* /tmp/ && rm -rf /tmpbackup/*
echo "/usr/tmpDSK /tmp tmpfs loop,nosuid,noexec,rw 0 0" >> /etc/fstab
mount -o remount /tmp
mkdir /var/tmpold
mv /var/tmp /var/tmpold
ln -s /tmp /var/tmp
cp -prf /var/tmpold/* /tmp/

@barnhartguy
● Prevent attackers from mounting ﬁlesystems that you don’t need and might beneﬁt them
Disable Uncommon File Systems
$ ls -1 /lib/modules/$(uname -r)/kernel/fs | sort | uniq > avail_fs
$ mount | column -t | cut -c 82-90 | sort | uniq > used_fs
$ for fs in $(comm -1 used_fs avail_fs); do echo "blacklist $fs"; done
>> /etc/modprobe.d/blacklist.conf

@barnhartguy
● Prevent attackers from compiling code to get
higher order abilities
Disable Compilers
>>
COMPILERS=(
"/usr/bin/byacc"
"/usr/bin/yacc"
"/usr/bin/bcc"
"/usr/bin/kgcc"
"/usr/bin/cc"
"/usr/bin/gcc"
"/usr/bin/c++"
"/usr/bin/g++"
)
for compiler in ${COMPILERS[@]}; do
if [ -f ${compiler} ]; then
echo "removing ${compiler}
chmod 000 ${compiler}
else
echo "missing ${compiler}
fi
done

Thank You!
@barnhartguy
I’ll be happy to answer more questions after
the talk (outside)

Linux Hardening - Made Easy

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