This article contains recommendations and best practices for hardening an Arch Linux system.
- It is possible to tighten security to the point where the system is unusable. Security and convenience must be balanced. The trick is to create a secure and useful system.
- The biggest threat is, and will always be, the user.
- The principle of least privilege: Each part of a system should only be able to access what is strictly required, and nothing more.
- Defense in depth: Security works better in independent layers. When one layer is breached, another should stop the attack.
- Be a little paranoid. And be suspicious. If anything sounds too good to be true, it probably is!
- You can never make a system 100% secure unless you unplug the machine from all networks, turn it off, lock it in a safe, smother it in concrete and never use it.
- Prepare for failure. Create a plan ahead of time to follow when your security is broken.
Passwords are key to a secure 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
Passwords must be complex enough to not be easily guessed from e.g. personal information, or cracked using methods like social engineering or brute-force attacks. The tenets of strong passwords are based on length and randomness. In cryptography the quality of a password is often referred to as its entropy.
Insecure passwords include those containing or those using as a base before substitution/variation:
- 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), as modern dictionary attacks can easily work with these
- Root "words" or common strings followed or preceded by added numbers, symbols, or characters (e.g.,
- Common phrases or short strings of common dictionary words (e.g.
photocopyhauntbranchexpose) including with character substitution (e.g.
Ph0toc0pyh4uN7br@nch3xp*se) (See Diceware below for when a combination of dictionary words can be secure)
- Any of the most common passwords
The best choice for a password is something long (the longer, the better) and generated from a random source. It is important to use a long password. Weak hash algorithms allow an 8-character password hash to be compromised in just a few hours.
Tools likeor AUR can generate random passwords. However, these passwords can be difficult to memorize. One memorization technique (for ones often typed) 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.
Apart from password management,offers password/passphrase generation. It is possible to customize the generation in a GUI. Dictionary based passphrases are also supported.
One technique for memorizing a password is to use a mnemonic phrase, where each word in the phrase reminds you of the next character in the password.
Take for instance “the girl is walking down the rainy street” could be translated to
t6!WdtR5 or, less simply,
This approach could make it easier to remember a password, but note that the various letters have very different probabilities of being found at the start of words (Wikipedia:Letter frequency).
Another effective technique can be to write randomly generated passwords down and store them in a safe place, such as in a wallet, purse, or document safe. Most people do a generally good job of protecting their physical valuables from attack, and it is easier for most people to understand physical security best practices compared to digital security practices.
It is also very effective to combine the mnemonic and random technique by saving long randomly generated passwords with a password manager, which will be in turn accessed with a memorable "master password"/primary password that must be used only for that purpose. The master password must be memorized and never saved. This requires the password manager to be installed on a system to easily access the password (which could be seen as an inconvenience or a security feature, depending on the situation). Some password managers also have smartphone apps which can be used to display passwords for manual entry on systems without that password manager installed (if that is a common use case, you could still use easily typeable but secure passwords for each service instead of completely random ones, see below). Note that a password manager introduces a single point of failure if you ever forget the master password. Some password managers compute the contained passwords based on the master password and the service name where you want to log in instead of encrypting them, making it possible to use it on a new system without syncing any data.
It can be effective to use a memorable long series of unrelated words as a password. The theory is that if a sufficiently long phrase is used, the gained entropy from the password's length can counter the lost entropy from the use of dictionary words. This xkcd comic demonstrates the entropy tradeoff of this method, taking into account the limited set of possible words for each word in the passphrase. If the set of words you choose from is large (multiple thousand words) and you choose 5-7 or even more random words from it, this method provides great entropy, even assuming the attacker knows the set of possible words chosen from and the number of words chosen. The number of possible passphrases after settling on a set of words and number of words is: (number of words in the set of words to select from) to the power of (the number of words chosen for the passphrase). See e.g. Diceware for more.
Once you pick a strong password, be sure to keep it safe. Watch out for keyloggers (software and hardware), screen loggers, social engineering, 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). Note that password managers that are implemented as browser extensions may be vulnerable to side channel attacks. These can be mitigated by using password managers that run as separate applications.
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 , 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 ends up on an encrypted partition or/and uses a strong key derivation function (i.e. yescrypt/bcrypt/argon2 or sha512 with PBKDF2, but not md5 or low iterations in PBKDF2) for the stored password hash (see SHA password hashes for more information).
If you are backing up your password database, make sure that each copy is not stored behind any other passphrase which in turn is stored in it, e.g. an encrypted drive or an authenticated remote storage service, or you will not be able to access it in case of need; a useful trick is to protect the drives or accounts where the database is backed up using a simple cryptographic hash of the master password. Maintain a list of all the backup locations: if one day you fear that the master passphrase has been compromised you will have to change it immediately on all the database backups and the locations protected with keys derived from the master password.
Version-controlling the database in a secure way can be very complicated: if you choose to do it, you must have a way to update the master password of all the database versions. It may not always be immediately clear when the master password is leaked: to reduce the risk of somebody else discovering your password before you realize that it leaked, you may choose to change it on a periodical basis. If you fear that you have lost control over a copy of the database, you will need to change all the passwords contained in it within the time that it may take to brute-force the master password, according to its entropy.
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.
Enforcing strong passwords with pam_pwquality
pam_pwquality provides protection against Dictionary attacks and helps configure a password policy that can be enforced throughout the system. It is based on pam_cracklib, so it is backwards compatible with its options.
Install the package.
- 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.
- Current security guidelines around passwords, e.g. from NIST, but also from others, do not recommend enforcing special characters, since they often only lead to predictable alterations.
If for example you want to enforce this policy:
- prompt 2 times for password in case of an error (retry option)
- 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 lowercase (lcredit option)
- at least 1 other character (ocredit option)
- cannot contain the words "myservice" and "mydomain"
- enforce the policy for root
/etc/pam.d/passwd file to read as:
#%PAM-1.0 password required pam_pwquality.so retry=2 minlen=10 difok=6 dcredit=-1 ucredit=-1 ocredit=-1 lcredit=-1 [badwords=myservice mydomain] enforce_for_root password required pam_unix.so use_authtok sha512 shadow
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_pwquality.
You can refer to theand man pages for more information.
See microcode for information on how to install important security updates for your CPU's microcode.
Some CPUs contain hardware vulnerabilities. See the kernel documentation on hardware vulnerabilities for a list of these vulnerabilities, as well as mitigation selection guides to help customize the kernel to mitigate these vulnerabilities for specific usage scenarios.
To check if you are affected by a known vulnerability, run the following:
$ grep -r . /sys/devices/system/cpu/vulnerabilities/
In most cases, updating the kernel and microcode will mitigate vulnerabilities.
Simultaneous multithreading (hyper-threading)
Simultaneous multithreading (SMT), also called hyper-threading on Intel CPUs, is a hardware feature that may be a source of L1 Terminal Fault and Microarchitectural Data Sampling vulnerabilities. The Linux kernel and microcode updates contain mitigations for known vulnerabilities, but disabling SMT may still be required on certain CPUs if untrusted virtualization guests are present.
SMT can often be disabled in your system's firmware. Consult your motherboard or system documentation for more information. You can also disable SMT in the kernel by adding the following kernel parameter:
hardened_malloc ( AUR, AUR) is a hardened replacement for glibc's malloc(). The project was originally developed for integration into Android's Bionic and musl by Daniel Micay, of GrapheneOS, but he has also built in support for standard Linux distributions on the x86_64 architecture.
While hardened_malloc is not yet integrated into glibc (assistance and pull requests welcome) it can be used easily with LD_PRELOAD. In testing so far, it only causes issues with a handful of applications if enabled globally in
/etc/ld.so.preload. Since hardened_malloc has a performance cost, you may want to decide which implementation to use on a case-by-case basis based on attack surface and performance needs.
To try it out in a standalone manner, use the hardened-malloc-preload wrapper script, or manually start an application with the proper preload value:
Proper usage with Firejail can be found on its wiki page, and some configurable build options for hardened_malloc can be found on the github repo.
Data-at-rest encryption, preferably full-disk encryption with a strong passphrase, is the only way to guard data against physical recovery. This provides data confidentiality 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 to be secure.
While the block-device or filesystem-based encryption types compared in the data-at-rest encryption article are useful at protecting data on physical media, most can not be used to protect data on a remote system that you can not control (such as cloud storage). In some cases, individual file encryption will be useful.
These are some methods to encrypt files:
- Some archiving and compressing tools also provide basic encryption. Some examples are (
-eflag). The encryption should only be relied on particular care, because the tools may use custom algorithms for cross-platform compatibility.
- GnuPG can be used to encrypt files.
- is a simple and easy to use file encryption tool. It also supports multiple recipients and encryption using SSH keys, which is useful for secure file sharing.
The kernel now prevents security issues related to hardlinks and symlinks if the
fs.protected_symlinks sysctl switches are enabled, so there is no longer a major security benefit from separating out world-writable directories.
File systems containing world-writable directories can still be kept separate as a coarse way of limiting the damage from disk space exhaustion. However, filling
/tmp is enough to take down services. More flexible mechanisms for dealing with this concern exist (like quotas), and some file systems include related features themselves (Btrfs has quotas on subvolumes).
Following the principle of least privilege, file systems 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 file system.
/homedisallows executable scripts and breaks Wine, Steam, PyCharm, .NET, etc.
- Wine does not need the
execflag for opening Windows binaries. It is only needed when Wine itself is installed in
- To keep Steam working you can mount
execin fstab by adding the following:
/home/user/.local/share/Steam /home/user/.local/share/Steam none defaults,bind,user,exec,nofail 0 0
- Wine does not need the
- Some packages (building
/var. for example) may require
File systems used for data should always be mounted with
Potential file system mounts to consider:
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
nobody users. You can use chmod to take away all permissions from the group and others:
# chmod go-r path_to_hide
gfrom the command (or re-add the permission with
chmod g+r pathif already ran) if the group is relied on.
Some paths to consider are:
/boot: The boot directory, which includes the vmlinuz and initramfs images.
/etc/nftables.conf: The nftables configuration, applicable to and .
/etc/iptables: The legacy iptables configuration, applicable to .
The default umask
0022 can be changed to improve security for newly created files. The NSA RHEL5 Security Guide suggests a umask of
0077 for maximum security, which makes new files not readable by users other than the owner. To change this, see Umask#Set the mask value. If you use sudo, consider configuring it to use the default root umask.
SUID and SGID files
It is important to be aware of any files with the Setuid or Setgid bit. Examples of relevant files in
/usr/bin with the SUID bit set and owned by root:
Files with the SUID bit set and not owned by root, or files with the SGID bit set typically have less potential impact but can theoretically still do decent damage if vulnerable. It is usually possible to avoid using Setuid or Setgid by assigning Capabilities instead.
/usr/bin for files with either the SUID or SGID bit:
$ find /usr/bin -perm "/u=s,g=s"
Regularly create backups of important data. Regularly test the integrity of the backups. Regularly test that the backups can be restored.
Make sure that at least one copy of the data is stored offline, i.e. not connected to the system under threat in any way. Ransomware and other destructive attacks may also attack any connected backup systems.
SATA SSD frozen mode
Do not use the root account for daily use
Following the principle of least privilege, do not use the root user for daily use. Create a non-privileged user account for each person using the system. Use sudo as necessary for temporary privileged access.
Enforce a delay after a failed login attempt
Add the following line to
/etc/pam.d/system-login to add a delay of at least 4 seconds between failed login attempts:
auth optional pam_faildelay.so delay=4000000
4000000 is the time in microseconds to delay.
Lock out user after three failed login attempts
pam_faillock.so is enabled by default to lock out users for 10 minutes after 3 failed login attempts in a 15 minute period (see FS#67644). The lockout only applies to password authentication (e.g. login and sudo), public key authentication over SSH is still accepted. To prevent complete denial-of-service, this lockout is disabled for the root user by default.
To unlock a user, do:
$ faillock --user username --reset
By default, the lock mechanism is a file per-user located at
/run/faillock/. Deleting or emptying the file unlocks that user—the directory is owned by root, but the file is owned by the user, so the
faillock command only empties the file, therefore does not require root.
pam_faillock.so can be configured with the file
/etc/security/faillock.conf. The lockout parameters:
unlock_time— the lockout time (in seconds, default 10 minutes).
fail_interval— the time in which failed logins can cause a lockout (in seconds, default 15 minutes).
deny— the number of failed logins before lockout (default 3).
deny = 0will disable the lockout mechanism entirely.
By default, all user locks are lost after reboot. If your attacker can reboot the machine, it is more secure if locks persist. To make locks persist, change the
dir parameter in
No restart is required for changes to take effect. Seefor further configuration options, such as enabling lockout for the root account, disabling for centralized login (e.g. LDAP), etc.
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
The current number of threads for each user can be found with
ps --no-headers -Leo user | sort | uniq --count. This may help with determining appropriate values for the limits.
If you must run Xorg, it is recommended to avoid running it as root. Within Wayland, the Xwayland compatibility layer will automatically use rootless Xorg.
The root user is, by definition, the most powerful user on a system. It is also difficult to audit the root user account. It is therefore important to restrict usage of the root user account as much as possible. There are a number of ways to keep the power of the root user while limiting its ability to cause harm.
Use sudo instead of su
- 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.
sudoprevents 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:
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:
This allows all users who are members of the group users to run the commands
/sbin/umount.cifs from any machine (ALL).
EDITOR=nano visudo is regarded as a severe security risk since everything can be used as an
Editing files using sudo
See Sudo#Editing files. Alternatively, you can use an editor like
rnano which has restricted capabilities in order to be safe to run as root.
Restricting root login
Allow only certain users
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.
Specify acceptable login combinations with access.conf
When someone attempts to log in with PAM,
/etc/security/access.conf is checked for the first combination that matches their login properties. Their attempt then fails or succeeds based on the rule for that combination.
Rules can be set for specific groups and users. In this example, the user archie is allowed to login locally, as are all users in the wheel and adm groups. All other logins are rejected:
+:archie:LOCAL +:(wheel):LOCAL +:(adm):LOCAL -:ALL:ALL
Read more at
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-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 around 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.
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 an 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.
Kernel self-protection / exploit mitigation
The basic kernel hardening patch set and more security-focused compile-time configuration options than the package. A custom build can be made to choose a different compromise between security and performance than the security-leaning defaults.package uses a
However, it should be noted that several packages will not work when using this kernel. For example:
Userspace ASLR comparison
Thepackage provides an improved implementation of Address Space Layout Randomization for userspace processes. The command can be used to obtain an estimate of the provided entropy:
Anonymous mapping randomization test : 32 quality bits (guessed) Heap randomization test (ET_EXEC) : 40 quality bits (guessed) Heap randomization test (PIE) : 40 quality bits (guessed) Main executable randomization (ET_EXEC) : 32 quality bits (guessed) Main executable randomization (PIE) : 32 quality bits (guessed) Shared library randomization test : 32 quality bits (guessed) VDSO randomization test : 32 quality bits (guessed) Stack randomization test (SEGMEXEC) : 40 quality bits (guessed) Stack randomization test (PAGEEXEC) : 40 quality bits (guessed) Arg/env randomization test (SEGMEXEC) : 44 quality bits (guessed) Arg/env randomization test (PAGEEXEC) : 44 quality bits (guessed) Offset to library randomisation (ET_EXEC): 34 quality bits (guessed) Offset to library randomisation (ET_DYN) : 34 quality bits (guessed) Randomization under memory exhaustion @~0: 32 bits (guessed) Randomization under memory exhaustion @0 : 32 bits (guessed)
Anonymous mapping randomization test : 28 quality bits (guessed) Heap randomization test (ET_EXEC) : 28 quality bits (guessed) Heap randomization test (PIE) : 28 quality bits (guessed) Main executable randomization (ET_EXEC) : 28 quality bits (guessed) Main executable randomization (PIE) : 28 quality bits (guessed) Shared library randomization test : 28 quality bits (guessed) VDSO randomization test : 20 quality bits (guessed) Stack randomization test (SEGMEXEC) : 30 quality bits (guessed) Stack randomization test (PAGEEXEC) : 30 quality bits (guessed) Arg/env randomization test (SEGMEXEC) : 22 quality bits (guessed) Arg/env randomization test (PAGEEXEC) : 22 quality bits (guessed) Offset to library randomisation (ET_EXEC): 28 quality bits (guessed) Offset to library randomisation (ET_DYN) : 28 quality bits (guessed) Randomization under memory exhaustion @~0: 29 bits (guessed) Randomization under memory exhaustion @0 : 29 bits (guessed)
Anonymous mapping randomization test : 28 quality bits (guessed) Heap randomization test (ET_EXEC) : 28 quality bits (guessed) Heap randomization test (PIE) : 28 quality bits (guessed) Main executable randomization (ET_EXEC) : 28 quality bits (guessed) Main executable randomization (PIE) : 28 quality bits (guessed) Shared library randomization test : 28 quality bits (guessed) VDSO randomization test : 19 quality bits (guessed) Stack randomization test (SEGMEXEC) : 30 quality bits (guessed) Stack randomization test (PAGEEXEC) : 30 quality bits (guessed) Arg/env randomization test (SEGMEXEC) : 22 quality bits (guessed) Arg/env randomization test (PAGEEXEC) : 22 quality bits (guessed) Offset to library randomisation (ET_EXEC): 28 quality bits (guessed) Offset to library randomisation (ET_DYN) : 28 quality bits (guessed) Randomization under memory exhaustion @~0: 28 bits (guessed) Randomization under memory exhaustion @0 : 28 bits (guessed)
32-bit processes (on an x86_64 kernel)
Anonymous mapping randomization test : 16 quality bits (guessed) Heap randomization test (ET_EXEC) : 22 quality bits (guessed) Heap randomization test (PIE) : 27 quality bits (guessed) Main executable randomization (ET_EXEC) : No randomization Main executable randomization (PIE) : 18 quality bits (guessed) Shared library randomization test : 16 quality bits (guessed) VDSO randomization test : 16 quality bits (guessed) Stack randomization test (SEGMEXEC) : 24 quality bits (guessed) Stack randomization test (PAGEEXEC) : 24 quality bits (guessed) Arg/env randomization test (SEGMEXEC) : 28 quality bits (guessed) Arg/env randomization test (PAGEEXEC) : 28 quality bits (guessed) Offset to library randomisation (ET_EXEC): 18 quality bits (guessed) Offset to library randomisation (ET_DYN) : 16 quality bits (guessed) Randomization under memory exhaustion @~0: 18 bits (guessed) Randomization under memory exhaustion @0 : 18 bits (guessed)
Anonymous mapping randomization test : 8 quality bits (guessed) Heap randomization test (ET_EXEC) : 13 quality bits (guessed) Heap randomization test (PIE) : 13 quality bits (guessed) Main executable randomization (ET_EXEC) : No randomization Main executable randomization (PIE) : 8 quality bits (guessed) Shared library randomization test : 8 quality bits (guessed) VDSO randomization test : 8 quality bits (guessed) Stack randomization test (SEGMEXEC) : 19 quality bits (guessed) Stack randomization test (PAGEEXEC) : 19 quality bits (guessed) Arg/env randomization test (SEGMEXEC) : 11 quality bits (guessed) Arg/env randomization test (PAGEEXEC) : 11 quality bits (guessed) Offset to library randomisation (ET_EXEC): 8 quality bits (guessed) Offset to library randomisation (ET_DYN) : 13 quality bits (guessed) Randomization under memory exhaustion @~0: No randomization Randomization under memory exhaustion @0 : No randomization
Restricting access to kernel pointers in the proc filesystem
kernel.kptr_restrict to 1 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 are compiling your own kernel, this can help mitigating local root exploits. This will break some commands when used by non-root users (but many features require root access anyway). See FS#34323 for more information.
kernel.kptr_restrict to 2 will hide kernel symbol addresses in
/proc/kallsyms regardless of privileges.
kernel.kptr_restrict = 1
kptr_restrict=2by default rather than
BPF is a system used to load and execute bytecode within the kernel dynamically during runtime. It is used in a number of Linux kernel subsystems such as networking (e.g. XDP, tc), tracing (e.g. kprobes, uprobes, tracepoints) and security (e.g. seccomp). It is also useful for advanced network security, performance profiling and dynamic tracing.
BPF was originally an acronym of Berkeley Packet Filter since the original classic BPF was used for packet capture tools for BSD. This eventually evolved into Extended BPF (eBPF), which was shortly afterwards renamed to just BPF (not an acronym). BPF should not be confused with packet filtering tools like iptables or netfilter, although BPF can be used to implement packet filtering tools.
BPF code may be either interpreted or compiled using a Just-In-Time (JIT) compiler. The Arch kernel is built with
CONFIG_BPF_JIT_ALWAYS_ON which disables the BPF interpreter and forces all BPF to use JIT compilation. This makes it harder for an attacker to use BPF to escalate attacks that exploit SPECTRE-style vulnerabilities. See the kernel patch which introduced CONFIG_BPF_JIT_ALWAYS_ON for more details.
The kernel includes a hardening feature for JIT-compiled BPF which can mitigate some types of JIT spraying attacks at the cost of performance and the ability to trace and debug many BPF programs. It may be enabled by setting
1 (to enable hardening of unprivileged code) or
2 (to enable hardening of all code).
net.core.bpf_* settings in the kernel documentation for more details.
net.core.bpf_jit_harden=2by default rather than
- By default, BPF programs can be run even by unprivileged users. To change that behaviour set
ptrace is commonly used by debugging tools including gdb, strace, perf, reptyr and other debuggers. However, it also provides a means by which a malicious process can read data from and take control of other processes.
Arch enables the Yama LSM by default, which provides a
kernel.yama.ptrace_scope kernel parameter. This parameter is set to
1 (restricted) by default which prevents tracers from performing a
ptrace call on traces outside of a restricted scope unless the tracer is privileged or has the
CAP_SYS_PTRACE capability. This is a significant improvement in security compared to the classic permissions. Without this module, there is no separation between processes running as the same user (in the absence of additional security layers such as ).
ptraceby running them as privileged processes, e.g. using sudo.
If you do not need to use debugging tools, consider setting
2 (admin-only) or
ptrace possible) to harden the system.
The kernel has the ability to hide other users' processes, normally accessible via
/proc, from unprivileged users by mounting the
proc filesystem with the
gid= options documented in https://docs.kernel.org/filesystems/proc.html.
This greatly complicates an intruder's task of gathering information about running processes, whether some daemon runs with elevated privileges, whether other user runs some sensitive program, whether other users run any program at all, makes it impossible to learn whether any user runs a specific program (given the program does not reveal itself by its behaviour), and, as an additional bonus, poorly written programs passing sensitive information via program arguments are now protected against local eavesdroppers.
proc group, provided by the package, acts as a whitelist of users authorized to learn other users' process information. If users or services need access to
/proc/<pid> directories beyond their own, add them to the group.
For example, to hide process information from other users except those in the
proc /proc proc nosuid,nodev,noexec,hidepid=2,gid=proc 0 0
For user sessions to work correctly, an exception needs to be added for systemd-logind:
Restricting module loading
The default Arch kernel has
CONFIG_MODULE_SIG_ALL enabled, which signs all kernel modules built as part of the package. This allows the kernel to only load modules signed with a valid key, i.e. out-of-tree modules compiled locally or provided by packages such as cannot be loaded.
Kexec allows replacing the current running kernel.
kernel.kexec_load_disabled = 1
Kernel lockdown mode
Since Linux 5.4 the kernel has gained an optional lockdown feature, intended to strengthen the boundary between UID 0 (root) and the kernel. When enabled some applications may cease to work which rely on low-level access to either hardware or the kernel.
To use lockdown, its LSM must be initialized and a lockdown mode must be set.
All officially supported kernels initialize the LSM, but none of them enforce any lockdown mode.
Lockdown has two modes of operation:
integrity: kernel features that allow userland to modify the running kernel are disabled (kexec, bpf).
confidentiality: kernel features that allow userland to extract confidential information from the kernel are also disabled.
It is recommended to use
integrity, unless your specific threat model dictates otherwise.
To enable kernel lockdown at runtime, run:
# echo mode > /sys/kernel/security/lockdown
To enable kernel lockdown on boot, use the kernel parameter
- Kernel lockdown cannot be disabled at runtime.
- Kernel lockdown disables hibernation.
Linux Kernel Runtime Guard (LKRG)
LKRG ( AUR) is a kernel module which performs integrity checking of the kernel and detection of exploit attempts.
Disable emergency shell
The emergency shell is used to interactively troubleshoot the machine during the boot process. However, it is also a gadget that an attacker can use to access secure resources such as the TPM. See this article for a practical example. The difficulty of attacks can be increased by disabling the emergency shell, at the tradeoff of removing a tool to troubleshoot early boot failures.
To disable the emergency shell, See Systemd#Disable emergency mode on remote machine.
See also Wikipedia:Sandbox (computer security).
CONFIG_USER_NSis currently enabled in , , and . Lack of it may prevent certain sandboxing features from being made available to applications.
CONFIG_USER_NS_UNPRIVILEGED) is enabled by default in , and , which greatly increases the attack surface for local privilege escalation (see FS#36969).
To mitigate this, either :
- use the kernel which has the safe default, or
- set the
Firejail is an easy to use tool for sandboxing applications and servers alike. It was originally created for browsers and internet facing applications, but supports a large number of applications by now. To establish a sandboxed environment with a variety of features, it is installed as a suid binary and builds a sandboxed runtime environment for the target application based on black and white lists.
bubblewrap is a sandbox application developed for unprivileged container tools like Flatpak with a significantly smaller resource footprint and complexity than Firejail. While it lacks certain features such as file path whitelisting, bubblewrap does offer bind mounts as well as the creation of user/IPC/PID/network/cgroup namespaces and can support both simple and complex sandboxes.
Manual chroot jails can also be constructed to build sandboxed process environments. It is much more limited than other sandboxing technologies; the extent of its sandboxing is file path isolation.
Linux Containers are another good option when you need more separation than the other options (short of full system virtualization) provide. LXC is run on top of the existing kernel in a pseudo-chroot with their own virtual hardware.
Full virtualization options
Using full virtualization options such as VirtualBox, KVM, Xen or Qubes OS (based on Xen) can also improve isolation and security in the event you plan on running risky applications or browsing dangerous websites.
Network and firewalls
While the stock Arch kernel is capable of using Netfilter's iptables and nftables, they are 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 and nftables for general information.
- See Simple stateful firewall for a guide on setting up an iptables firewall.
- See Category:Firewalls for other ways of setting up netfilter.
- See Ipset for blocking lists of ip addresses, such as those from Bluetack.
- is a configurable inbound and outbound firewall with support for configurable rules by application, port, host, etc.
Some services listen for inbound traffic on open network ports. It is important to only bind these services to the addresses and interfaces that are strictly necessary. It may be possible for a remote attacker to exploit flawed network protocols to access exposed services. This can even happen with processes bound to localhost.
In general, if a service only needs to be accessible to the local system, bind to a Unix domain socket (
localhost instead of a non-loopback address like
If a service needs to be accessible to other systems via the network, control the access with strict firewall rules and configure authentication, authorization and encryption whenever possible.
You can list all current open ports with
ss -l. To show all listening processes and their numeric tcp and udp port numbers:
# ss -lpntu
Seefor more options.
To mitigate brute-force attacks it is recommended to enforce key-based authentication. For OpenSSH, see OpenSSH#Force public key authentication. Alternatively Fail2ban or Sshguard offer lesser forms of protection by monitoring logs and writing firewall rules but open up the potential for a denial of service, since an attacker can spoof packets as if they came from the administrator after identifying their address. Spoofing IP has lines of defense, such as by reverse path filtering and disabling ICMP redirects.
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 also a good practice, both for tracing intrusions and adding an additional layer of security before root access. For OpenSSH, see OpenSSH#Deny.
Mozilla publishes an OpenSSH configuration guide which configures more verbose audit logging and restricts ciphers.
The default domain name resolution (DNS) configuration is highly compatible but has security weaknesses. See DNS privacy and security for more information.
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.
Managing TLS certificates
See TLS#Trust management.
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 by default. For Thunderbolt, you can restrict the direct memory access completely or to known devices, see Thunderbolt#User device authorization. For Firewire and PCI Express, 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.
#Data-at-rest 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.
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, which only leaves some parts of the bootloader code unencrypted. GRUB's configuration, kernel and initramfs are encrypted.
systemd-boot disables editing of kernel parameters when #Secure Boot is enabled. Alternatively, see systemd-boot#Kernel parameters editor with password protection for a more traditional password-based option.
Secure Boot is a feature of UEFI that allows authentication of the files your computer boots. This helps preventing some evil maid attacks such as replacing files inside the boot partition. Normally computers come with keys that are enrolled by vendors (OEM). However these can be removed and allow the computer to enter Setup Mode which allows the user to enroll and manage their own keys.
The secure boot page guides you through how to set secure boot up by using your own keys.
Trusted Platform Module (TPM)
TPMs are hardware microprocessors which have cryptographic keys embedded. This forms the fundamental root of trust of most modern computers and allows end-to-end verification of the boot chain. They can be used as internal smartcards, attest the firmware running on the computer and allow users to insert secrets into a tamper-proof and brute-force resistant store.
Boot partition on removable flash drive
One popular idea is to place the boot partition on a flash drive in order to render the system unbootable without it. Proponents of this idea often use full-disk encryption alongside, and some also use detached encryption headers placed on the boot partition.
This method can also be merged with encrypting /boot.
For example, the following will automatically log out from virtual consoles (but not terminal emulators in X11):
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.
Protect against rogue USB devices
Install USBGuard, which is a software framework that helps to protect your computer against rogue USB devices (a.k.a. BadUSB, PoisonTap or LanTurtle) by implementing basic whitelisting and blacklisting capabilities based on device attributes.
Volatile data collection
A computer that is powered on may be vulnerable to volatile data collection. It is a best practice to turn a computer completely off at times it is not necessary for it to be on, or if the computer's physical security is temporarily compromised (e.g. when passing through a security checkpoint).
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.
It is important to regularly upgrade the system.
Follow vulnerability alerts
Subscribe to the Common Vulnerabilities and Exposure (CVE) Security Alert updates, made available by National Vulnerability Database, and found on the NVD Download webpage. The Arch Linux Security Tracker serves as a particularly useful resource in that it combines Arch Linux Security Advisory (ASA), Arch Linux Vulnerability Group (AVG) and CVE data sets in tabular format. The tool can be used to check for vulnerabilities affecting the running system. A graphical system tray, , can also be used. See also Arch Security Team.
You should also consider subscribing to the release notifications for software you use, especially if you install software through means other than the main repositories or AUR. Some software have mailing lists you can subscribe to for security notifications. Source code hosting sites often offer RSS feeds for new releases.
Packages can be rebuilt and stripped of undesired functions and features as a means to reduce attack surface. For example,
bzip2recover in an attempt to circumvent CVE-2016-3189. Custom hardening flags can also be applied either manually or via a wrapper.
|-D_FORTIFY_SOURCE=2||Run-time buffer overflow detection|
|-D_GLIBCXX_ASSERTIONS||Run-time bounds checking for C++ strings and containers|
|-fasynchronous-unwind-tables||Increased reliability of backtraces|
|-fexceptions||Enable table-based thread cancellation|
|-fpie -Wl,-pie||Full ASLR for executables|
|-fpic -shared||No text relocations for shared libraries|
|-fplugin=annobin||Generate data for hardening quality control|
|-fstack-clash-protection||Increased reliability of stack overflow detection|
|-fstack-protector or -fstack-protector-all||Stack smashing protector|
|-g||Generate debugging information|
|-grecord-gcc-switches||Store compiler flags in debugging information|
|-mcet -fcf-protection||Control flow integrity protection|
|-pipe||Avoid temporary files, speeding up builds|
|-Wall||Recommended compiler warnings|
|-Werror=format-security||Reject potentially unsafe format string arguments|
|-Werror=implicit-function-declaration||Reject missing function prototypes|
|-Wl,-z,defs||Detect and reject underlinking|
|-Wl,-z,now||Disable lazy binding|
|-Wl,-z,relro||Read-only segments after relocation|