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Restricting UNIX Users

Created: 30 Apr 2002 • Updated: 02 Nov 2010
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by Anton Chuvakin

Restricting UNIX Users
by Anton Chuvakin, Ph.D.
last updated May 1, 2002

Stories of cruel system administrators oppressing poor users have been around since the rise of UNIX in the 1970s. Users are inherently limited in what they can do on a UNIX system due to file permissions, passwords and other standard UNIX controls. However, it is often necessary to further restrict system users in other ways, both to protect them from themselves and to protect the system from the malicious or overly "playful" users. This article will discuss ways in which security administrators can limit what users are able to do on a UNIX system, with a particular focus on Linux. Both local and remote users will be considered. However, restricting root users from doing things on the system (while possible) is a somewhat different story and will not be addressed in detail here.

Limiting what users can do is also a good method preventing "insider attacks", which account for majority of losses from infosec-related incidents. (According to the annual CPI/FBI survey, 59% of companies surveyed said they have had one or more attacks reported internally. Almost 8% of those companies reported 60 or more internal incidents. The survey is available here. Great perimeter defenses and network IDS will not stop the console user from wreaking havoc upon a system. Furthermore, organizational security policies might call for increased restrictions for end-users in order to reduce the IT management costs.

Objectives of Restricting Users

It is important to keep in mind the *purpose* of imposing a particular restriction on system users. Different environments imply different goals, which will require different tools to be used. For example, "saving" casual users from "complexities" of the UNIX shell is a different problem from keeping a determined expert attacker from exploiting a buggy SUID binary. In addition, a different degree of restriction might be needed for different environments, such as only allowing certain applications or only denying certain applications. Curiously, the latter is often futile since there are simply too many ways to break loose, just like a "default-allow" policy for the firewall usually provide only little security.

File Permissions

Let’s start from the weakest limitation one can impose: use of UNIX permissions. In fact, it is probably sufficient for most environments. While seemingly inflexible, permissions allow control over what binaries users can execute. If all binaries except the ones that need to be executed by the untrusted users are not world executable, and all user-writable partitions such as /tmp and /home are mounted with noexec flag (see man mount for the details on that), the security of the set-up is adequate. Noexec flag is needed to prevent users from downloading or building their own versions of binaries. If there is a pool of users that need the higher privileges, those can be gathered in a special group and binaries are made group-executable.

Linux file attributes, such as immutable, append-only, and others belong to the same groups of controls; they are, however, more useful to restrict root somewhat.

Permissions through PAM

Another simple (and weak) restriction of a different kind can be implemented via Linux system resource limits. It will prevent users from hogging system resources (disk space, RAM, CPU cycles), both in the case of abuse (fork bomb, mail bomb) or accidents (program fills the disk, spawns too many child processes, etc). Linux Pluggable Authentication Modules (PAM) can be used to impose resource limits. There are many guides available for doing this, such as chapter 5.15 of Securing and Optimizing Linux: RedHat Edition - A Hands-on Guide and Using Pam by Dave Wreski. For overview of PAM see the SecurityFocus articles Pluggable Authentication Manuals, Part One and Part Two. The summary is shown below:

Limits for all users (apparently, except "root"):

 File: /etc/security/limits.conf  *       hard    core    0 *       hard    rss     10000 *       hard    nproc   100 

Shown above are limits for the size of core files, resident memory for the process and number of owned processes. "Hard" limits cannot be changed by the user, while "soft" can.

and for the specific group "lusers":

 @lusers     hard  core    0 @lusers     hard  rss     2000 @lusers     hard  nproc   200 @lusers	    hard  fsize	  100000 @lusers     hard  nofile  100 @lusers	    hard  cpu	  10 @lusers     hard  priority 5              # same as  "nice -n 5" on all processes 

(in addition, maximum file size, number of open files, maximum CPU time and process priority are set)

Also, adding a line to the file /etc/pam.d/login is needed to make PAM check the above config file:

 session         required        /lib/security/pam_limits.so 

It should be noted, that PAM allows even more granular restrictions to be created. The RedHat whitepaper Enhanced Console Access outlines using PAM to limit (or, rather, to selectively enable) user's access to devices, X Windows applications, shutdown functionality and other privileges for console and remote users.

Restricted Shell

The next degree of limitation is to be a restricted shell. In this case, a version of a normal bash shell will prevent users from changing the directory and environment variables, redirecting output, running commands with absolute pathnames, using exec command and some other actions. Restrictions are not enforced for shell scripts. See man bash for more details. Combining rbash with a restrictive configuration of UNIX permissions can help achieve further security.

Rbash is a viable choice if you are trying to somewhat contain trusted users. Its restrictions can be easily overcome (see below).

To test rbash restricted shell functionality:

 # adduser luser # ln -s /bin/bash /bin/rbash # echo "/bin/rbash" >> /etc/shells # chsh -s /bin/rbash luser # cd ~luser # su luser $ 

and then:

 $ cd / rbash: cd: restricted 

Menu-Based Shells

Another solution is a custom application that only provides access to selected system resources. UNIX menu-based shells fall into this category. Let’s look at several such solutions. Pdmenu provides a curses-based color interface for launching applications under Linux. It can safely be used as the user's log-in shell and only allows access to specified console applications such as mail or Web browser. It can also launch an application and then present the output to the user, such as /bin/ls for browsing directories. Apparently, users should not be given rights to add or modify the menu. Ideally, the menu shell should not require, or even allow, the user to type anything, but to only allow them choose from the list of presented options.

Flash is another menu-based Linux, using older and somewhat less-refined interface. It provides similar functionality with menus, commands and hot-keys.

These shells work on remote users (connected via SSH or telnet) and local users not using an X Window system.

Leaving the convenience features of those applications aside, let’s concentrate on the extra security they might provide. If used as log-in shells, they limit user to only running applications that can be chosen from the menu. Provided that the normal shell (such as /bin/bash, or a /bin/chsh command to change a default shell) is not one of the applications and the user cannot modify the command line, there is seemingly no way to start anything else. Apparently, menu shell has no control over what happens after application is launched. And herein lies one of the pitfalls of the menu shells, which will be discussed later in this article, in the section on breaking out.

Similar to rbash above, this feature is more of a protection of non-technical users from themselves, rather than a true security mechanism.

Chroot

Another way to restrict users is to confine them to a certain directory via chroot. It does provide a degree of security in case that some conditions are met. For more details, see my analysis of chroot security Using Chroot Securely. Briefly, if there is no way to get root privileges within chroot-ed directory, chroot does provide effective security. To prevent users from getting root within the chrooted directory, the set of application present within their jail should be carefully selected.

Combining restricted shell with a chroot protection increases the security of the whole set-up by providing defense in-depth. If the user manages to break out of the confines of the restricted shell, they would still end up in a directory without a method to get extra privileges. If chroot directory is well designed, the only thing gained after the breaking out is an ability to modify a command line for the allowed applications and not much else.

Moving outside of Linux realm, FreeBSD's jail command (described in detail at Chapter 12 of the FreeBSD Developers' Handbook, The Jail Subsystem) is a significant enhancement of simple chrooting. Jail is a great way to impose restrictions on remote users. Using both user-level and kernel-space code, jail creates a better confinement than simple chroot call by also restricting inter-process communication, network connectivity and some other system calls.

Kernel-Based Capabilities

Now let’s turn to more serious security measures to further restrict users - kernel based capabilities. As outlined in the SecurityFocus article Linux Kernel Hardening, several Linux security kernel patches implement mandatory access controls and role-based security. Using this functionality, one can take away almost any user access rights in various combinations. For example, LIDS kernel patch can prevent a user from binding network sockets, using or even seeing certain files, devices and processes, changing ownership of files and using other UNIX functions.

GRSecurity kernel patch (described in detail in the SecurityFocus article Grsecurity) also has features to restrict users. CONFIG_GRKERNSEC_TPE option allows one to create a group for "untrusted" users who can only execute binaries from specified root-owned directories.

Admittedly, kernel-level protection is not fun to configure, unless one is into tweaking multiple ACL lists and tracing applications for allowed system calls. It is also not something that will work out of the box, since there will always be some heavy customization required. Kernel-level protection is the most difficult or even impossible to bypass, provided it is configured properly.

Overall, below the method usability summary is provided.

Restricted shell mostly "convenience" feature for limiting the non-technical users
Menu shell provides some security if implemented correctly and applications are audited
Chroot provides good security provided there is no root inside
Chroot+menu combines security and convenience of the above two approaches
Kernel high security, but also hard to configure, can be used to restrict expert users

Breaking Out of Various Restrictions

Now it is time to touch upon the breaking out of various protective mechanisms.

First, let’s look at the restricted shell. There are simply too many ways things can go wrong: modifying environment variables (such as EDITOR, VISUAL, and others unlocked by rbash), starting applications with shell escapes, changing user's shell, simply running the downloaded shell binary and numerous other methods can let the bird fly out of the cage.

A well-written menu shell is a significant improvement. The "breaking focus" is now on the applications that are being launched from the menu shell. The problem lies in the fact that many UNIX console applications have a shell escape. Examples include pine (Ctrl-Z), vi (:!'bash'), ftp (!), telnet (!), emacs (M-x shell), gdb (shell) and many others. (figures in brackets are the commands that invoke a shell.) It is interesting to note that some of the applications will spawn a new copy of restricted shell and some will use regular /bin/sh. It is worth noting that vi (vim) has a special restricted mode with no shell escape feature. In can be invoked by calling rvim in a manner similar to bash and rbash. To make applications resistant to such an attack, one has to manually remove the shell escapes from the source code, which is probably too time-consuming for most uses.

Even if the launched application does not provide for shell escape, one can crash the application using the overflow shellcode (/bin/sh) typically used for overflowing SUID binaries and gaining root. If shellcode is used for crashing the application or the menu-based shell itself, the clean copy of /bin/sh will be launched (provided that one exists, which might not be the case in chroot environment).

For methods to break out of chroot, see Using Chroot Securely. However, breaking out of well-written menu-shell, which only launches source-audited application all running in well-designed chroot environment, appears to be impossible.

Kernel-level protection is the closest to being unbreakable, if not for the bugs in the kernel security patch code (LIDS had a serious bug earlier this year) or configuration errors. The "good news" for the attackers, is that configuration errors are likely, due to complexity of the typical rule setup.

In addition, many scenarios in which the need to restrict shell users appears can be solved using a different technology. For instance, shell e-mail users can be moved to Web mail application and Web site editors can be given a CGI application to update their pages. Some virual hosting solutions operate this way.

Conclusion

To conclude, many tools are available to restrict what users can do on a Linux system. Depending upon business requirements and other needs, one is able to choose the right combination of security and functionality. As usual, there is a trade-off between security and ease of set-up and use.

Anton Chuvakin, Ph.D. is a Senior Security Analyst with netForensics, a security information management company that provides real-time network security monitoring solutions.

This article originally appeared on SecurityFocus.com -- reproduction in whole or in part is not allowed without expressed written consent.