Security

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This article contains recommendations and best practices for hardening an Arch Linux system.

Concepts

  • It is possible to tighten the security so much as to make your system unusable. The trick is to secure it without overdoing it.
  • There are many other things that can be done to heighten the security, but the biggest threat is, and will always be, the user. When you think security, you have to think layers. When one layer is breached, another should stop the attack. But you can never make the system 100% secure unless you unplug the machine from all networks, lock it in a safe and never use it.
  • Be a little paranoid. It helps. And be suspicious. If anything sounds too good to be true, it probably is!
  • The principle of least privilege: each part of a system should only be able to access what is required to use it, and nothing more.

Passwords

Passwords are key to a secure linux system. They secure your user accounts, encrypted filesystems, and SSH/GPG keys. They are the main way a computer chooses to trust the person using it, so a big part of security is just about picking secure passwords and protecting them.

Choosing secure passwords

When relying on a passphrase, it must be complex enough to not be easily guessed from e.g. personal information, or cracked using e.g. brute-force attacks. The tenets of strong passphrases are based on length and randomness. In cryptography the quality of a passphrase is referred to as its Wikipedia:Entropic security.

Insecure passwords include those containing:

  • Personally identifiable information (e.g., your dog's name, date of birth, area code, favorite video game)
  • Simple character substitutions on words (e.g., k1araj0hns0n)
  • Root "words" or common strings followed or preceded by added numbers, symbols, or characters (e.g., DG091101%)
  • Common phrases or short phrases of grammatically related words (e.g. all of the lights), and even with character substitution.

The right choice for a password is something long (8-20 characters, depending on importance) and seemingly completely random. A good technique for building secure, seemingly random passwords is to base them on characters from every word in a sentence. Take for instance “the girl is walking down the rainy street” could be translated to t6!WdtR5 or, less simply, t&6!RrlW@dtR,57. This approach could make it easier to remember a password, but note that the the various letters have very different probabilities of being found at the start of words (Wikipedia:Letter frequency).

A better approach is to generate pseudo-random passwords with tools like pwgen or apg: for memorizing them, one technique (for ones typed often) is to generate a long password and memorize a minimally secure number of characters, temporarily writing down the full generated string. Over time, increase the number of characters typed - until the password is ingrained in muscle memory and need not be remembered. This technique is more difficult, but can provide confidence that a password will not turn up in wordlists or "intelligent" brute force attacks that combine words and substitute characters.

It is also very effective to combine these two techniques by saving long, complex random passwords with a password manager, which will be in turn accessed with a mnemonic password that will have to be used only for that purpose, especially avoiding to ever transmit it over any kind of network. This method of course limits the use of the stored passwords to the terminals where the database is available for reading (which on the other hand could be seen as an added security feature).

Also consider the Diceware Passphrase method, using a sufficient number of words.

See Bruce Schneier's article Choosing Secure Passwords or The passphrase FAQ for some additional background. Also, you can check entropy level of your chosen passphrase here, or consulting Wikipedia:Password strength.

Maintaining passwords

Once you pick a strong password, be sure to keep it safe. Watch out for manipulation, shoulder surfing, and avoid reusing passwords so insecure servers cannot leak more information than necessary. Password managers can help manage large numbers of complex passwords: if you are copy-pasting the stored passwords from the manager to the applications that need them, make sure to clear the copy buffer every time, and ensure they are not saved in any kind of log (e.g. do not paste them in plain terminal commands, which would store them in files like .bash_history). Lastpass is a service that stores encrypted passwords online for synchronization between devices, but requires that you trust both closed-source code and an external corporation.

As a rule, do not pick insecure passwords just because secure ones are harder to remember. Passwords are a balancing act. It is better to have an encrypted database of secure passwords, guarded behind a key and one strong master password, than it is to have many similar weak passwords. Writing passwords down is perhaps equally effective[1], avoiding potential vulnerabilities in software solutions while requiring physical security.

Another aspect of the strength of the passphrase is that it must not be easily recoverable from other places. If you use the same passphrase for disk encryption as you use for your login password (useful e.g. to auto-mount the encrypted partition or folder on login), make sure that /etc/shadow either also ends up on an encrypted partition, or uses a strong hash algorithm (i.e. sha512/bcrypt, not md5) for the stored password hash (see SHA password hashes for more info).

Password hashes

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Mention key derivation functions, in particular PBKDF2, bcrypt and scrypt, how to use them, advantages and disadvantages, especially regarding custom-hardware-based brute-force attacks. (Discuss in Talk:Security#Removal of incorrect warning)

By default, Arch stores the hashed user passwords in the root-only-readable /etc/shadow file, separated from the other user parameters stored in the world-readable /etc/passwd file, see Users and groups#User database. See also #Restricting root.

Passwords are set with the passwd command, which stretches them with the crypt function and then saves them in /etc/shadow. See also SHA password hashes. The passwords are also salted in order to defend them against rainbow table attacks.

See also How are passwords stored in Linux (Understanding hashing with shadow utils).

Enforcing strong passwords using pam_cracklib

pam_cracklib provides protection against Dictionary attacks and helps configure a password policy that can be enforced throughout the system.

Warning: The root account is not affected by this policy.
Note: You can use the root account to set a password for a user that bypasses the desired/configured policy. This is useful when setting temporary passwords.

If for example you want to enforce this policy:

  • prompt 2 times for password in case of an error
  • 10 characters minimum length (minlen option)
  • at least 6 characters should be different from old password when entering a new one (difok option)
  • at least 1 digit (dcredit option)
  • at least 1 uppercase (ucredit option)
  • at least 1 other character (ocredit option)
  • at least 1 lowercase (lcredit option)

Edit the /etc/pam.d/passwd file to read as:

#%PAM-1.0
password required pam_cracklib.so retry=2 minlen=10 difok=6 dcredit=-1 ucredit=-1 ocredit=-1 lcredit=-1
password required pam_unix.so use_authtok sha512 shadow

The password required pam_unix.so use_authtok instructs the pam_unix module to not prompt for a password but rather to use the one provided by pam_cracklib.

You can refer to the pam_cracklib(8) and pam_unix(8) man pages for more information.

Storage

Disk encryption

Disk encryption, preferably full disk encryption with a strong passphrase, is the only way to guard data against physical recovery. This provides complete security when the computer is turned off or the disks in question are unmounted.

Once the computer is powered on and the drive is mounted, however, its data becomes just as vulnerable as an unencrypted drive. It is therefore best practice to unmount data partitions as soon as they are no longer needed.

Certain programs, like Dm-crypt, allow the user to encrypt a loop file as a virtual volume. This is a reasonable alternative to full disk encryption when only certain parts of the system need be secure.

File systems

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: You don't mount a partition, but a file system; i.e. this is not related to partitioning. In fs.protected_hardlinks etc., the "fs" stands for "file system". (Discuss in Talk:Security#)

The kernel now prevents security issues related to hardlinks and symlinks if the fs.protected_hardlinks and fs.protected_symlinks sysctl switches are enabled, so there is no longer a major security benefit from separating out world-writable directories.

Partitions containing world-writable directories can still be kept separate as a coarse way of limiting the damage from disk space exhaustion. However, filling a partition like /var or /tmp is enough to take down services. More flexible mechanisms for dealing with this concern exist (like quotas), and some filesystems include related features themselves (btrfs has quotas on subvolumes).

Mount options

Following the principle of least privilege, filesystems should be mounted with the most restrictive mount options possible (without losing functionality).

Relevant mount options are:

  • nodev: Do not interpret character or block special devices on the file system.
  • nosuid: Do not allow set-user-identifier or set-group-identifier bits to take effect.
  • noexec: Do not allow direct execution of any binaries on the mounted filesystem.

Partitions used for data should always be mounted with nodev, nosuid, noexec. Potential usage is presented in the table below.

Partition nodev nosuid noexec
/var yes yes yes [1]
/home yes yes yes, if you do not code, use wine or steam
/dev/shm yes yes yes
/tmp yes yes maybe, breaks compiling packages and various other things
/boot yes yes yes

[1] Note that some packages (building nvidia-dkms for example) may require exec on /var.

File access permissions

The default file permissions allow read access to almost everything and changing the permissions can hide valuable information from an attacker who gains access to a non-root account such as the http or nobody users.

For example:

# chmod 700 /boot /etc/{iptables,arptables}

The default Umask can be changed to improve security for newly created files. The NSA RHEL5 Security Guide suggests a umask of 077 for maximum security, which makes new files not readable by users other than the owner. To change this, see Umask#Set the mask value.

User setup

After installation make a normal user for daily use. Do not use the root user for daily use.

Lockout user after three failed login attempts

To further heighten the security it is possible to lockout a user after a specified number of failed login attempts. The user account can either be locked until the root user unlocks it, or automatically be unlocked after a set time. To lockout a user for ten minutes after three failed login attempts you have to modify /etc/pam.d/system-login:

/etc/pam.d/system-login
auth required pam_tally.so deny=2 unlock_time=600 onerr=succeed file=/var/log/faillog
#auth required pam_tally.so onerr=succeed file=/var/log/faillog

If you do not comment the second line every failed login attempt will be counted twice. That is all there is to it. If you feel adventurous, make three failed login attempts. Then you can see for yourself what happens. To unlock a user manually do:

# pam_tally --user username --reset

If you want to permanently lockout a user after 3 failed login attempts remove the unlock_time part of the line. The user can then not login until root unlocks the account.

Limit amount of processes

On systems with many, or untrusted users, it is important to limit the number of processes each can run at once, therefore preventing fork bombs and other denial of service attacks. /etc/security/limits.conf determines how many processes each user, or group can have open, and is empty (except for useful comments) by default. adding the following lines to this file will limit all users to 100 active processes, unless they use the prlimit command to explicitly raise their maximum to 200 for that session. These values can be changed according to the appropriate number of processes a user should have running, or the hardware of the box you are administrating.

* soft nproc 100
* hard nproc 200

Restricting root

The root user is, by definition, the most powerful user on a system. Because of this, there are a number of ways to keep the power of the root user while limiting its ability to cause harm, or at least to make root user actions more traceable.

Use sudo instead of su

Using sudo for privileged access is preferable to su for a number of reasons.

  • It keeps a log of which normal privilege user has run each privileged command.
  • The root user password need not be given out to each user who requires root access.
  • sudo prevents users from accidentally running commands as root that do not need root access, because a full root terminal is not created. This aligns with the principle of least privilege.
  • Individual programs may be enabled per user, instead of offering complete root access just to run one command. For example, to give the user alice access to a particular program:
# visudo
/etc/sudoers
alice ALL = NOPASSWD: /path/to/program

Or, individual commands can be allowed for all users. To mount Samba shares from a server as a regular user:

%users ALL=/sbin/mount.cifs,/sbin/umount.cifs

This allows all users who are members of the group users to run the commands /sbin/mount.cifs and /sbin/umount.cifs from any machine (ALL).

Tip:

To use nano instead of vi with visudo,

/etc/sudoers
Defaults editor=/usr/bin/rnano

Exporting # EDITOR=nano visudo is regarded as a severe security risk since everything can be used as an EDITOR.

Editing files using sudo

Running a text editor as root can be a security vulnerability as many editors can run arbitrary shell commands or affect files other than the one you intend to edit. To avoid this, use sudoedit filename (equivalently, sudo -e filename) to edit files. This edits a copy of the file using your normal user privileges and then overwrites the original using sudo only after the editor is closed. You can change the editor this uses by setting the SUDO_EDITOR environment variable:

export SUDO_EDITOR=vim

Alternatively, use an editor like rvim which has restricted capabilities in order to be safe to run as root.

Restricting root login

Once sudo is properly configured, full root access can be heavily restricted or denied without losing much usability. To disable root, but still allowing to use sudo, you can use passwd -l root.

Allow only certain users

The PAM pam_wheel.so lets you allow only users in the group wheel to login using su. Edit both /etc/pam.d/su and /etc/pam.d/su-l, then uncomment the line:

# Uncomment the following line to require a user to be in the "wheel" group.
auth		required	pam_wheel.so use_uid

This means only users who are already able to run privileged commands may login as root.

Denying ssh login

Even if you do not wish to deny root login for local users, it is always good practice to deny root login via SSH. The purpose of this is to add an additional layer of security before a user can completely compromise your system remotely.

Mandatory access control

Mandatory access control (MAC) is a type of security policy that differs significantly from the discretionary access control (DAC) used by default in Arch and most Linux distributions. MAC essentially means that every action a program could perform that affects the system in any way is checked against a security ruleset. This ruleset, in contrast to DAC methods, cannot be modified by users. Using virtually any mandatory access control system will significantly improve the security of your computer, although there are differences in how it can be implemented.

Pathname MAC

Pathname-based access control is a simple form of access control that offers permissions based on the path of a given file. The downside to this style of access control is that permissions are not carried with files if they are moved about the system. On the positive side, pathname-based MAC can be implemented on a much wider range of filesystems, unlike labels-based alternatives.

  • AppArmor is a Canonical-maintained MAC implementation seen as an "easier" alternative to SELinux.
  • Tomoyo is another simple, easy-to-use system offering mandatory access control. It is designed to be both simple in usage and in implementation, requiring very few dependencies.

Role-based access control

The MAC implementation grsecurity supports is called role-based access control. RBAC associates roles with each user. Each role defines what operations can be performed on certain objects. Given a well-written collection of roles and operations your users will be restricted to perform only those tasks that you tell them they can do. The default "deny-all" ensures you that a user cannot perform an action you have not thought of.

  • Grsecurity RBAC has a learning mode like AppArmor for easy configuration
  • Grsecurity RBAC does not rely on extra meta-data like SELinux. RBAC is significantly faster then SELinux.

Labels MAC

Labels-based access control means the extended attributes of a file are used to govern its security permissions. While this system is arguably more flexible in its security offerings than pathname-based MAC, it only works on filesystems that support these extended attributes.

  • SELinux, based on a NSA project to improve Linux security, implements MAC completely separate from system users and roles. It offers an extremely robust multi-level MAC policy implementation that can easily maintain control of a system that grows and changes past its original configuration.

Access Control Lists

Access Control Lists (ACLs) are an alternative to attaching rules directly to the filesystem in some way. ACLs implement access control by checking program actions against a list of permitted behavior.

  • grsecurity implements ACL access control, as well as a complete kernel patchset focused on improving security. Its changes extend to control of memory allocation, improved chroot restrictions, and rules involving specific network behavior.

Kernel hardening

Restricting access to kernel logs

The kernel logs contain useful information for an attacker trying to exploit kernel vulnerabilities, such as sensitive memory addresses. The kernel.dmesg_restrict flag was to forbid access to the logs without the CAP_SYS_ADMIN capability (which only processes running as root have by default).

/etc/sysctl.d/50-dmesg-restrict.conf
kernel.dmesg_restrict = 1

Restricting access to kernel pointers in the proc filesystem

Enabling kernel.kptr_restrict will hide kernel symbol addresses in /proc/kallsyms from regular users without CAP_SYSLOG, making it more difficult for kernel exploits to resolve addresses/symbols dynamically. This will not help that much on a pre-compiled Arch Linux kernel, since a determined attacker could just download the kernel package and get the symbols manually from there, but if you're compiling your own kernel, this can help mitigating local root exploits. This will break some perf commands when used by non-root users (but many perf features require root access anyway). See FS#34323 for more information.

/etc/sysctl.d/50-kptr-restrict.conf
kernel.kptr_restrict = 1

Keep BPF JIT compiler disabled

The Linux kernel includes the ability to compile BPF/Seccomp rule sets to native code as a performance optimization. The net.core.bpf_jit_enable flag should be left at 0 for a maximum level of security.

This can be helpful in specific domains, but is not usually useful. A JIT compiler opens up the possibility for an attacker to perform a heap spraying attack, where they fill the kernel's heap with malicious code. This code can then potentially be executed via another exploit, like an incorrect function pointer dereference.

Note: grsecurity includes JIT hardening for the BPF JIT compiler, greatly reducing the risk of exploitation.

ptrace scope

Arch enables the Yama LSM by default, providing a kernel.yama.ptrace_scope flag. This flag is enabled by default and prevents processes from performing a ptrace call on other processes outside of their scope without CAP_SYS_PTRACE. While many debugging tools require this for some of their functionality, it is a significant improvement in security. Without this feature, there is essentially no separation between processes running as the same user without applying extra layers like namespaces. The ability to attach a debugger to an existing process is a demonstration of this weakness.

Note: In grsecurity, this protection is toggled via the kernel.grsecurity.harden_ptrace flag instead.

Examples of broken functionality

Note: You can still execute these commands as root (such as allowing them through sudo for certain users, with or without a password), or enable ptrace selectively through setcap cap_sys_ptrace=eip /path/to/program.
  • gdb -p $PID
  • strace -p $PID
  • perf trace -p $PID
  • reptyr $PID

hidepid

The kernel has the ability to hide other users' processes from unprivileged users by mounting the proc filesystem with hidepid=1 or hidepid=2. This can be automated with the hidepid package. It will install polkit and systemd-logind exceptions and make sure /proc is mounted with hidepid=2,gid=26.

grsecurity

The grsecurity kernel provides many security-related improvements. It hardens both the kernel and userspace against common memory corruption vulnerabilities, along with providing many miscellaneous features and a role-based access control system. It is the only way to secure the kernel itself against exploitation, which is the most important improvement for a system already making good use of user isolation, containers/chroots and sandboxes.

Sandboxing applications

Firejail

Firejail is an easy to use and simple tool for sandboxing applications and servers alike. Firejail is suggested for browsers and internet facing applications, as well as any servers you may be running. Firejail is further enhanced when used with Grsecurity.

Tip: If you do not require to sandbox graphical applications, an alternative is playpen.

chroots

Manual chroot jails can also be constructed.

Linux containers

Linux Containers are another good option when you need more separation than the other options (short of KVM and Virtualbox) provide. LXC's run on top of the existing kernel in a pseudo-chroot with their own virtual hardware.

Other virtualization options

Using other, more full virtualization options such as VirtualBox, KVM, or Xen can also improve security in the event you plan on running risky applications or browsing dangerous websites.

Network and firewalls

Firewalls

While the stock Arch kernel is capable of using Netfilter's iptables, it is not enabled by default. It is highly recommended to set up some form of firewall to protect the services running on the system. Many resources (including ArchWiki) do not state explicitly which services are worth protecting, so enabling a firewall is a good precaution.

  • See iptables for general info.
  • See Simple stateful firewall for a guide on setting up an iptables firewall.
  • See Firewalls for other ways of setting up netfilter.
  • See Ipset for blocking lists of ip addresses, such as those from Bluetack.

Kernel parameters

Kernel parameters which affect networking can be set using Sysctl. For how to do this, see Sysctl#TCP/IP stack hardening.

SSH

Avoid using Secure Shell without requiring SSH keys. This prevents brute-force attacks. Alternatively Fail2ban or Sshguard offer lesser forms of protection by monitoring logs and writing iptables rules but open up the potential for a denial of serving, since an attacker can spoof packets as if they came from the administrator after identifying their address.

You may want to harden authentication even more by using two-factor authentication. Google Authenticator provides a two-step authentication procedure using one-time passcodes (OTP).

Denying root login is good practice, both for tracing intrusions and adding an additional layer of security before root access.

DNS

See DNSSEC and DNSCrypt.

Proxies

Proxies are commonly used as an extra layer between applications and the network, sanitizing data from untrusted sources. The attack surface of a small proxy running with lower privileges is significantly smaller than a complex application running with the end user privileges.

For example the DNS resolver is implemented in glibc, that is linked with the application (that may be running as root), so a bug in the DNS resolver might lead to a remote code execution. This can be prevented by installing a DNS caching server, such as Dnsmasq, which acts as a proxy. [2]

Managing SSL certificates

See OpenSSL and Network Security Services (NSS) for managing custom server-side SSL certificates. Notably, the related Let’s Encrypt project is also supported.

The default internet SSL certificate trustchains are provided by the ca-certificates package and its dependencies. Note that Arch relies on trust-sources (e.g. ca-certificates-cacert, ca-certificates-mozilla) providing the certificates to be trusted per default by the system.

There may be occasions when you want to deviate from the default. For example, you may read some news and want to distrust a certificate rather than wait until the trust-source providers do. The Arch infrastructure makes such easy:

  1. Obtain the respective certificate in .crt format (Example: view, download; in case of an existing trusted root certificate authority, you may also find it extracted in the system path),
  2. Copy it to /etc/ca-certificates/trust-source/blacklist/ and
  3. Run update-ca-trust as root.

To check the blacklisting works as intended, you may re-open your preferred browser and do so via its GUI, which should show it as untrusted now.

Authenticating packages

Attacks on package managers are possible without proper use of package signing, and can affect even package managers with proper signature systems. Arch uses package signing by default and relies on a web of trust from 5 trusted master keys. See Pacman-key for details.

Follow NVD/CVE alerts

Subscribe to the Common Vulnerabilities and Exposure Security Alert updates, made available by National Vulnerability Database, and found on the NVD Download webpage. See also Arch CVE Monitoring Team and CVE.

Warning: Do not be tempted to perform partial upgrades, as they are not supported by Arch Linux and may cause instability: the whole system should be upgraded when upgrading a component. Also note that infrequent system updates can complicate the update process.

Physical security

Note: You can ignore this section if you just want to secure your computer against remote threats.

Physical access to a computer is root access given enough time and resources. However, a high practical level of security can be obtained by putting up enough barriers.

An attacker can gain full control of your computer on the next boot by simply attaching a malicious IEEE 1394 (FireWire), Thunderbolt or PCI Express device as they are given full memory access.[3] There is little you can do from preventing this, or modification of the hardware itself - such as flashing malicious firmware onto a drive. However, the vast majority of attackers will not be this knowledgeable and determined.

#Disk encryption will prevent access to your data if the computer is stolen, but malicious firmware can be installed to obtain this data upon your next log in by a resourceful attacker.

Locking down BIOS

Adding a password to the BIOS prevents someone from booting into removable media, which is basically the same as having root access to your computer. You should make sure your drive is first in the boot order and disable the other drives from being bootable if you can.

Bootloaders

It is highly important to protect your bootloader. There is a magic kernel parameter called init=/bin/sh. This makes any user/login restrictions totally useless.

Syslinux

Syslinux supports password-protecting your bootloader. It allows you to set either a per-menu-item password or a global bootloader password.

GRUB

GRUB supports bootloader passwords as well. See GRUB/Tips and tricks#Password protection of GRUB menu for details. It also has support for encrypted boot partitions, which only leaves some parts of the bootloader code unencrypted. GRUB's configuration, kernel and initramfs are encrypted.

Denying console login as root

Changing the configuration to disallow root to login from the console makes it harder for an intruder to gain access to the system. The intruder would have to guess both a username that exists on the system and that user's password. When root is allowed to log in via the console, an intruder only needs to guess a password. Blocking root login at the console is done by commenting out the tty lines in /etc/securetty.

/etc/securetty
#tty1

Repeat for any tty you wish to block. To check the effect of this change, start by commenting out only one line and go to that particular console and try to login as root. You will be greeted by the message Login incorrect. Now that we are sure it works, go back and comment out the rest of the tty lines.

Note: If you see ttyS0 this is for a serial console. Similarly, on Xen virtualized systems hvc0 is for the administrator.

Automatic logout

If you are using Bash or Zsh, you can set TMOUT for an automatic logout from shells after a timeout.

For example, the following will automatically log out from virtual consoles (but not terminal emulators in X11):

/etc/profile.d/shell-timeout.sh
TMOUT="$(( 60*10 ))";
[ -z "$DISPLAY" ] && export TMOUT;
case $( /usr/bin/tty ) in
	/dev/tty[0-9]*) export TMOUT;;
esac

If you really want EVERY Bash/Zsh prompt (even within X) to timeout, use:

$ export TMOUT="$(( 60*10 ))";

Note that this will not work if there is some command running in the shell (eg.: an SSH session or other shell without TMOUT support). But if you are using VC mostly for restarting frozen GDM/Xorg as root, then this is very useful.

See also