By default, environment variables are inherited from a process' parent. However, when a program executes another program, the calling program can set the environment variables to arbitrary values. This is dangerous to setuid/setgid programs, because their invoker can completely control the environment variables they're given. Since they are usually inherited, this also applies transitively; a secure program might call some other program and, without special measures, would pass potentially dangerous environment variables values on to the program it calls. The following subsections discuss environment variables and what to do with them.
Some environment variables are dangerous because many libraries and programs are controlled by environment variables in ways that are obscure, subtle, or undocumented. For example, the IFS variable is used by the sh and bash shell to determine which characters separate command line arguments. Since the shell is invoked by several low-level calls (like system(3) and popen(3) in C, or the back-tick operator in Perl), setting IFS to unusual values can subvert apparently-safe calls. This behavior is documented in bash and sh, but it's obscure; many long-time users only know about IFS because of its use in breaking security, not because it's actually used very often for its intended purpose. What is worse is that not all environment variables are documented, and even if they are, those other programs may change and add dangerous environment variables. Thus, the only real solution (described below) is to select the ones you need and throw away the rest.
Normally, programs should use the standard access routines to access environment variables. For example, in C, you should get values using getenv(3), set them using the POSIX standard routine putenv(3) or the BSD extension setenv(3) and eliminate environment variables using unsetenv(3). I should note here that setenv(3) is implemented in Linux, too.
However, crackers need not be so nice; crackers can directly control the environment variable data area passed to a program using execve(2). This permits some nasty attacks, which can only be understood by understanding how environment variables really work. In Linux, you can see environ(5) for a summary how about environment variables really work. In short, environment variables are internally stored as a pointer to an array of pointers to characters; this array is stored in order and terminated by a NULL pointer (so you'll know when the array ends). The pointers to characters, in turn, each point to a NIL-terminated string value of the form ``NAME=value''. This has several implications, for example, environment variable names can't include the equal sign, and neither the name nor value can have embedded NIL characters. However, a more dangerous implication of this format is that it allows multiple entries with the same variable name, but with different values (e.g., more than one value for SHELL). While typical command shells prohibit doing this, a locally-executing cracker can create such a situation using execve(2).
The problem with this storage format (and the way it's set) is that a program might check one of these values (to see if it's valid) but actually use a different one. In Linux, the GNU glibc libraries try to shield programs from this; glibc 2.1's implementation of getenv will always get the first matching entry, setenv and putenv will always set the first matching entry, and unsetenv will actually unset all of the matching entries (congratulations to the GNU glibc implementers for implementing unsetenv this way!). However, some programs go directly to the environ variable and iterate across all environment variables; in this case, they might use the last matching entry instead of the first one. As a result, if checks were made against the first matching entry instead, but the actual value used is the last matching entry, a cracker can use this fact to circumvent the protection routines.
For secure setuid/setgid programs, the short list of environment variables needed as input (if any) should be carefully extracted. Then the entire environment should be erased, followed by resetting a small set of necessary environment variables to safe values. There really isn't a better way if you make any calls to subordinate programs; there's no practical method of listing ``all the dangerous values''. Even if you reviewed the source code of every program you call directly or indirectly, someone may add new undocumented environment variables after you write your code, and one of them may be exploitable.
The simple way to erase the environment in C/C++ is by setting the global variable environ to NULL. The global variable environ is defined in <unistd.h>; C/C++ users will want to #include this header file. You will need to manipulate this value before spawning threads, but that's rarely a problem, since you want to do these manipulations very early in the program's execution (usually before threads are spawned).
The global variable environ's definition is defined in various standards; it's not clear that the official standards condone directly changing its value, but I'm unaware of any Unix-like system that has trouble with doing this. I normally just modify the ``environ'' directly; manipulating such low-level components is possibly non-portable, but it assures you that you get a clean (and safe) environment. In the rare case where you need later access to the entire set of variables, you could save the ``environ'' variable's value somewhere, but this is rarely necessary; nearly all programs need only a few values, and the rest can be dropped.
Another way to clear the environment is to use the undocumented clearenv() function. The function clearenv() has an odd history; it was supposed to be defined in POSIX.1, but somehow never made it into that standard. However, clearenv() is defined in POSIX.9 (the Fortran 77 bindings to POSIX), so there is a quasi-official status for it. In Linux, clearenv() is defined in <stdlib.h>, but before using #include to include it you must make sure that __USE_MISC is #defined. A somewhat more ``official'' approach is to cause __USE_MISC to be defined is to first #define either _SVID_SOURCE or _BSD_SOURCE, and then #include <features.h> - these are the official feature test macros.
One environment value you'll almost certainly re-add is PATH, the list of directories to search for programs; PATH should not include the current directory and usually be something simple like ``/bin:/usr/bin''. Typically you'll also set IFS (to its default of `` \t\n'', where space is the first character) and TZ (timezone). Linux won't die if you don't supply either IFS or TZ, but some System V based systems have problems if you don't supply a TZ value, and it's rumored that some shells need the IFS value set. In Linux, see environ(5) for a list of common environment variables that you might want to set.
If you really need user-supplied values, check the values first (to ensure that the values match a pattern for legal values and that they are within some reasonable maximum length). Ideally there would be some standard trusted file in /etc with the information for ``standard safe environment variable values'', but at this time there's no standard file defined for this purpose. For something similar, you might want to examine the PAM module pam_env on those systems which have that module. If you allow users to set an arbitrary environment variable, then you'll let them subvert restricted shells (more on that below).
If you're using a shell as your programming language, you can use the ``/usr/bin/env'' program with the ``-'' option (which erases all environment variables of the program being run). Basically, you call /usr/bin/env, give it the ``-'' option, follow that with the set of variables and their values you wish to set (as name=value), and then follow that with the name of the program to run and its arguments. You usually want to call the program using the full pathname (/usr/bin/env) and not just as ``env'', in case a user has created a dangerous PATH value. Note that GNU's env also accepts the options "-i" and "--ignore-environment" as synonyms (they also erase the environment of the program being started), but these aren't portable to other versions of env.
If you're programming a setuid/setgid program in a language that doesn't allow you to reset the environment directly, one approach is to create a ``wrapper'' program. The wrapper sets the environment program to safe values, and then calls the other program. Beware: make sure the wrapper will actually invoke the intended program; if it's an interpreted program, make sure there's no race condition possible that would allow the interpreter to load a different program than the one that was granted the special setuid/setgid privileges.
If you allow users to set their own environment variables, then users will be able to escape out of restricted accounts (these are accounts that are supposed to only let the users run certain programs and not work as a general-purpose machine). This includes letting users write or modify certain files in their home directory (e.g., like .login), supporting conventions that load in environment variables from files under the user's control (e.g., openssh's .ssh/environment file), or supporting protocols that transfer environment variables (e.g., the Telnet Environment Option; see CERT Advisory CA-1995-14 for more). Restricted accounts should never be allowed to modify or add any file directly contained in their home directory, and instead should be given only a specific subdirectory that they are allowed to modify (if they can modify any).
Given the similarities with certain other security issues, i'm surprised this hasn't been discussed earlier. If it has, people simply haven't paid it enough attention.
This problem is not necessarily ssh-specific, though most telnet daemons that support environment passing should already be configured to remove dangerous variables due to a similar (and more serious) issue back in '95 (ref: ). I will give ssh-based examples here.
Scenario one: Let's say admin bob has a host that he wants to give people ftp access to. Bob doesn't want anyone to have the ability to actually _log into_ his system, so instead of giving users normal shells, or even no shells, bob gives them all (say) /usr/sbin/nologin, a program he wrote himself in C to essentially log the attempt to syslog and exit, effectively ending the user's session. As far as most people are concerned, the user can't do much with this aside from, say, setting up an encrypted tunnel.
The thing is, bob's system uses dynamic libraries (as most do), and /usr/sbin/nologin is dynamically linked (as most such programs are). If a user can set his environment variables (e.g. by uploading a '.ssh/environment' file) and put some arbitrary file on the system (e.g. 'doevilstuff.so'), he can bypass any functionality of /usr/sbin/nologin completely via LD_PRELOAD (or another member of the LD_* environment family).
The user can now gain a shell on the system (with his own privileges, of course, barring any 'UseLogin' issues (ref: )), and administrator bob, if he were aware of what just occurred, would be extremely unhappy.
Granted, there are all kinds of interesting ways to (more or less) do away with this problem. Bob could just grit his teeth and give the ftp users a nonexistent shell, or he could statically compile nologin, assuming his operating system comes with static libraries. Bob could also, humorously, make his nologin program setuid and let the standard C library take care of the situation. Then, of course, there are also the ssh-specific access controls such as AllowGroup and AllowUsers. These may appease the situation in this scenario, but it does not correct the problem.
... Now, what happens if bob, instead of using /usr/sbin/nologin, wants to use (for example) some BBS-type interface that he wrote up or downloaded? It can be a script written in perl or tcl or python, or it could be a compiled program; doesn't matter. Additionally, bob need not be running an ftp server on this host; instead, perhaps bob uses nfs or veritas to mount user home directories from a fileserver on his network; this exact setup is (unfortunately) employed by many bastion hosts, password management hosts and mail servers---to name a few. Perhaps bob runs an ISP, and replaces the user's shell when he doesn't pay. With all of these possible (and common) scenarios, bob's going to have a somewhat more difficult time getting around the problem.
... Exploitation of the problem is simple. The circumvention code would be compiled into a dynamic library and LD_PRELOAD=/path/to/evil.so should be placed into ~user/.ssh/environment (a similar environment option may be appended to public keys in the authohrized_keys file). If no dynamically loadable programs are executed, this will have no effect.
ISPs and universities (along with similarly affected organizations) should compile their rejection (or otherwise restricted) binaries statically (assuming your operating system comes with static libraries)...
Ideally, sshd (and all remote access programs that allow user-definable environments) should strip any environment settings that libc ignores for setuid programs.