7.4. Minimize Privileges

As noted earlier, it is an important general principle that programs have the minimal amount of privileges necessary to do its job (this is termed ``least privilege''). That way, if the program is broken, its damage is limited. The most extreme example is to simply not write a secure program at all - if this can be done, it usually should be. For example, don't make your program setuid or setgid if you can; just make it an ordinary program, and require the administrator to log in as such before running it.

In Linux and Unix, the primary determiner of a process' privileges is the set of id's associated with it: each process has a real, effective and saved id for both the user and group (a few very old Unixes don't have a ``saved'' id). Linux also has, as a special extension, a separate filesystem UID and GID for each process. Manipulating these values is critical to keeping privileges minimized, and there are several ways to minimize them (discussed below). You can also use chroot(2) to minimize the files visible to a program, though using chroot() can be difficult to use correctly. There are a few other values determining privilege in Linux and Unix, for example, POSIX capabilities (supported by Linux 2.2 and greater, and by some other Unix-like systems).

7.4.1. Minimize the Privileges Granted

Perhaps the most effective technique is to simply minimize the highest privilege granted. In particular, avoid granting a program root privilege if possible. Don't make a program setuid root if it only needs access to a small set of files; consider creating separate user or group accounts for different function.

A common technique is to create a special group, change a file's group ownership to that group, and then make the program setgid to that group. It's better to make a program setgid instead of setuid where you can, since group membership grants fewer rights (in particular, it does not grant the right to change file permissions).

This is commonly done for game high scores. Games are usually setgid games, the score files are owned by the group games, and the programs themselves and their configuration files are owned by someone else (say root). Thus, breaking into a game allows the perpetrator to change high scores but doesn't grant the privilege to change the game's executable or configuration file. The latter is important; if an attacker could change a game's executable or its configuration files (which might control what the executable runs), then they might be able to gain control of a user who ran the game.

If creating a new group isn't sufficient, consider creating a new pseudouser (really, a special role) to manage a set of resources - often a new pseudogroup (again, a special role) is also created just to run a program. Web servers typically do this; often web servers are set up with a special user (``nobody'') so that they can be isolated from other users. Indeed, web servers are instructive here: web servers typically need root privileges to start up (so they can attach to port 80), but once started they usually shed all their privileges and run as the user ``nobody''. However, don't use the ``nobody'' account (unless you're writing a webserver); instead, create your own pseudouser or new group. The purpose of this approach is to isolate different programs, processes, and data from each other, by exploiting the operating system's ability to keep users and groups separate. If different programs shared the same account, then breaking into one program would also grant privileges to the other. Usually the pseudouser should not own the programs it runs; that way, an attack who breaks into the account cannot change the program it runs. By isolating different parts of the system into running separate users and groups, breaking one part will not necessarily break the whole system's security.

If you're using a database system (say, by calling its query interface), limit the rights of the database user that the application uses. For example, don't give that user access to all of the system stored procedures if that user only needs access to a handful of user-defined ones. Do everything you can inside stored procedures. That way, even if someone does manage to force arbitrary strings into the query, the damage that can be done is limited. If you must directly pass a regular SQL query with client supplied data (and you usually shouldn't), wrap it in something that limits its activities (e.g., sp_sqlexec). (My thanks to SPI Labs for these database system suggestions).

If you must give a program privileges usually reserved for root, consider using POSIX capabilities as soon as your program can minimize the privileges available to your program. POSIX capabilities are available in Linux 2.2 and in many other Unix-like systems. By calling cap_set_proc(3) or the Linux-specific capsetp(3) routines immediately after starting, you can permanently reduce the abilities of your program to just those abilities it actually needs. For example the network time daemon (ntpd) traditionally has run as root, because it needs to modify the current time. However, patches have been developed so ntpd only needs a single capability, CAP_SYS_TIME, so even if an attacker gains control over ntpd it's somewhat more difficult to exploit the program.

I say ``somewhat limited'' because, unless other steps are taken, retaining a privilege using POSIX capabilities requires that the process continue to have the root user id. Because many important files (configuration files, binaries, and so on) are owned by root, an attacker controlling a program with such limited capabilities can still modify key system files and gain full root-level privilege. A Linux kernel extension (available in versions 2.4.X and 2.2.19+) provides a better way to limit the available privileges: a program can start as root (with all POSIX capabilities), prune its capabilities down to just what it needs, call prctl(PR_SET_KEEPCAPS,1), and then use setuid() to change to a non-root process. The PR_SET_KEEPCAPS setting marks a process so that when a process does a setuid to a nonzero value, the capabilities aren't cleared (normally they are cleared). This process setting is cleared on exec(). However, note that PR_SET_KEEPCAPS is a Linux-unique extension for newer versions of the linux kernel.

One tool you can use to simplify minimizing granted privileges is the ``compartment'' tool developed by SuSE. This tool, which only works on Linux, sets the filesystem root, uid, gid, and/or the capability set, then runs the given program. This is particularly handy for running some other program without modifying it. Here's the syntax of version 0.5:

Syntax: compartment [options] /full/path/to/program

  --chroot path   chroot to path
  --user user     change UID to this user
  --group group   change GID to this group
  --init program  execute this program before doing anything
  --cap capset    set capset name. You can specify several
  --verbose       be verbose
  --quiet         do no logging (to syslog)

Thus, you could start a more secure anonymous ftp server using:

  compartment --chroot /home/ftp --cap CAP_NET_BIND_SERVICE anon-ftpd

At the time of this writing, the tool is immature and not available on typical Linux distributions, but this may quickly change. You can download the program via http://www.suse.de/~marc. A similar tool is dreamland; you can that at http://www.7ka.mipt.ru/~szh/dreamland.

Note that not all Unix-like systems, implement POSIX capabilities, and PR_SET_KEEPCAPS is currently a Linux-only extension. Thus, these approaches limit portability. However, if you use it merely as an optional safeguard only where it's available, using this approach will not really limit portability. Also, while the Linux kernel version 2.2 and greater includes the low-level calls, the C-level libraries to make their use easy are not installed on some Linux distributions, slightly complicating their use in applications. For more information on Linux's implementation of POSIX capabilities, see http://linux.kernel.org/pub/linux/libs/security/linux-privs.

FreeBSD has the jail() function for limiting privileges; see the jail documentation for more information. There are a number of specialized tools and extensions for limiting privileges; see Section 3.10.

7.4.2. Minimize the Time the Privilege Can Be Used

As soon as possible, permanently give up privileges. Some Unix-like systems, including Linux, implement ``saved'' IDs which store the ``previous'' value. The simplest approach is to reset any supplemental groups if appropriate (e.g., using setgroups(2)), and then set the other id's twice to an untrusted id. In setuid/setgid programs, you should usually set the effective gid and uid to the real ones, in particular right after a fork(2), unless there's a good reason not to. Note that you have to change the gid first when dropping from root to another privilege or it won't work - once you drop root privileges, you won't be able to change much else. Note that in some systems, just setting the group isn't enough, if the process belongs to supplemental groups with privileges. For example, the ``rsync'' program didn't remove the supplementary groups when it changed its uid and gid, which created a potential exploit.

It's worth noting that there's a well-known related bug that uses POSIX capabilities to interfere with this minimization. This bug affects Linux kernel 2.2.0 through 2.2.15, and possibly a number of other Unix-like systems with POSIX capabilities. See Bugtraq id 1322 on http://www.securityfocus.com for more information. Here is their summary:

POSIX "Capabilities" have recently been implemented in the Linux kernel. These "Capabilities" are an additional form of privilege control to enable more specific control over what privileged processes can do. Capabilities are implemented as three (fairly large) bitfields, which each bit representing a specific action a privileged process can perform. By setting specific bits, the actions of privileged processes can be controlled -- access can be granted for various functions only to the specific parts of a program that require them. It is a security measure. The problem is that capabilities are copied with fork() execs, meaning that if capabilities are modified by a parent process, they can be carried over. The way that this can be exploited is by setting all of the capabilities to zero (meaning, all of the bits are off) in each of the three bitfields and then executing a setuid program that attempts to drop privileges before executing code that could be dangerous if run as root, such as what sendmail does. When sendmail attempts to drop privileges using setuid(getuid()), it fails not having the capabilities required to do so in its bitfields and with no checks on its return value . It continues executing with superuser privileges, and can run a users .forward file as root leading to a complete compromise.

One approach, used by sendmail, is to attempt to do setuid(0) after a setuid(getuid()); normally this should fail. If it succeeds, the program should stop. For more information, see http://sendmail.net/?feed=000607linuxbug. In the short term this might be a good idea in other programs, though clearly the better long-term approach is to upgrade the underlying system.

7.4.3. Minimize the Time the Privilege is Active

Use setuid(2), seteuid(2), setgroups(2), and related functions to ensure that the program only has these privileges active when necessary, and then temporarily deactivate the privilege when it's not in use. As noted above, you might want to ensure that these privileges are disabled while parsing user input, but more generally, only turn on privileges when they're actually needed.

Note that some buffer overflow attacks, if successful, can force a program to run arbitrary code, and that code could re-enable privileges that were temporarily dropped. Thus, there are many attacks that temporarily deactivating a privilege won't counter - it's always much better to completely drop privileges as soon as possible. There are many papers that describe how to do this, such as "Designing Shellcode Demystified". Some people even claim that ``seteuid() [is] considered harmful'' because of the many attacks it doesn't counter. Still, temporarily deactivating these permissions prevents a whole class of attacks, such as techniques to convince a program to write into a file that perhaps it didn't intend to write into. Since this technique prevents many attacks, it's worth doing if permanently dropping the privilege can't be done at that point in the program.

7.4.4. Minimize the Modules Granted the Privilege

If only a few modules are granted the privilege, then it's much easier to determine if they're secure. One way to do so is to have a single module use the privilege and then drop it, so that other modules called later cannot misuse the privilege. Another approach is to have separate commands in separate executables; one command might be a complex tool that can do a vast number of tasks for a privileged user (e.g., root), while the other tool is setuid but is a small, simple tool that only permits a small command subset (and does not trust its invoker). The small, simple tool checks to see if the input meets various criteria for acceptability, and then if it determines the input is acceptable, it passes the data on to the complex tool. Note that the small, simple tool must do a thorough job checking its inputs and limiting what it will pass along to the complex tool, or this can be a vulnerability. The communication could be via shell invocation, or any IPC mechanism. These approaches can even be layered several ways, for example, a complex user tool could call a simple setuid ``wrapping'' program (that checks its inputs for secure values) that then passes on information to another complex trusted tool.

This approach is the normal approach for developing GUI-based applications which requre privilege, but must be run by unprivileged users. The GUI portion is run as a normal unprivileged user process; that process then passes security-relevant requests on to another process that has the special privileges (and does not trust the first process, but instead limits the requests to whatever the user is allowed to do). Never develop a program that is privileged (e.g., using setuid) and also directly invokes a graphical toolkit: Graphical toolkits aren't designed to be used this way, and it would be extremely difficult to audit graphical toolkits in a way to make this possible. Fundamentally, graphical toolkits must be large, and it's extremely unwise to place so much faith in the perfection of that much code, so there is no point in trying to make them do what should never be done. Feel free to create a small setuid program that invokes two separate programs: one without privileges (but with the graphical interface), and one with privileges (and without an external interface). Or, create a small setuid program that can be invoked by the unprivileged GUI application. But never combine the two into a single process. For more about this, see the statement by Owen Taylor about GTK and setuid, discussing why GTK_MODULES is not a security hole.

Some applications can be best developed by dividing the problem into smaller, mutually untrusting programs. A simple way is divide up the problem into separate programs that do one thing (securely), using the filesystem and locking to prevent problems between them. If more complex interactions are needed, one approach is to fork into multiple processes, each of which has different privilege. Communications channels can be set up in a variety of ways; one way is to have a "master" process create communication channels (say unnamed pipes or unnamed sockets), then fork into different processes and have each process drop as many privileges as possible. If you're doing this, be sure to watch for deadlocks. Then use a simple protocol to allow the less trusted processes to request actions from the more trusted process(es), and ensure that the more trusted processes only support a limited set of requests. Setting user and group permissions so that no one else can even start up the sub-programs makes it harder to break into.

Some operating systems have the concept of multiple layers of trust in a single process, e.g., Multics' rings. Standard Unix and Linux don't have a way of separating multiple levels of trust by function inside a single process like this; a call to the kernel increases privileges, but otherwise a given process has a single level of trust. This is one area where technologies like Java 2, C# (which copies Java's approach), and Fluke (the basis of security-enhanced Linux) have an advantage. For example, Java 2 can specify fine-grained permissions such as the permission to only open a specific file. However, general-purpose operating systems do not typically have such abilities at this time; this may change in the near future. For more about Java, see Section 10.6.

7.4.5. Consider Using FSUID To Limit Privileges

Each Linux process has two Linux-unique state values called filesystem user id (FSUID) and filesystem group id (FSGID). These values are used when checking against the filesystem permissions. If you're building a program that operates as a file server for arbitrary users (like an NFS server), you might consider using these Linux extensions. To use them, while holding root privileges change just FSUID and FSGID before accessing files on behalf of a normal user. This extension is fairly useful, and provides a mechanism for limiting filesystem access rights without removing other (possibly necessary) rights. By only setting the FSUID (and not the EUID), a local user cannot send a signal to the process. Also, avoiding race conditions is much easier in this situation. However, a disadvantage of this approach is that these calls are not portable to other Unix-like systems.

7.4.6. Consider Using Chroot to Minimize Available Files

You can use chroot(2) to limit the files visible to your program. This requires carefully setting up a directory (called the ``chroot jail'') and correctly entering it. This can be a fairly effective technique for improving a program's security - it's hard to interfere with files you can't see. However, it depends on a whole bunch of assumptions, in particular, the program must lack root privileges, it must not have any way to get root privileges, and the chroot jail must be properly set up (e.g., be careful what you put inside the chroot jail, and make sure that users can never control its contents before calling chroot). I recommend using chroot(2) where it makes sense to do so, but don't depend on it alone; instead, make it part of a layered set of defenses. Here are a few notes about the use of chroot(2):

7.4.7. Consider Minimizing the Accessible Data

Consider minimizing the amount of data that can be accessed by the user. For example, in CGI scripts, place all data used by the CGI script outside of the document tree unless there is a reason the user needs to see the data directly. Some people have the false notion that, by not publicly providing a link, no one can access the data, but this is simply not true.

7.4.8. Consider Minimizing the Resources Available

Consider minimizing the computer resources available to a given process so that, even if it ``goes haywire,'' its damage can be limited. This is a fundamental technique for preventing a denial of service. For network servers, a common approach is to set up a separate process for each session, and for each process limit the amount of CPU time (et cetera) that session can use. That way, if an attacker makes a request that chews up memory or uses 100% of the CPU, the limits will kick in and prevent that single session from interfering with other tasks. Of course, an attacker can establish many sessions, but this at least raises the bar for an attack. See Section 3.6 for more information on how to set these limits (e.g., ulimit(1)).