sbox is a CGI wrapper script that allows Web site hosting services to safely grant CGI authoring privileges to untrusted clients. In addition to changing the process privileges of client scripts to match their owners, it goes beyond other wrappers by placing configurable ceilings on script resource usage, avoiding unintentional (as well as intentional) denial of service attacks. It also optionally allows the Webmaster to place client's CGI scripts in a chroot'ed shell restricted to the author's home directories.
sbox is compatible with all Web servers running under BSD-derived flavors of Unix. You can use and redistribute it freely.
The current release is 1.07. Download it from the Web at http://stein.cshl.org/WWW/software/sbox/.
Older versions are also available.
Poorly-written CGI scripts are the single major source of server security holes on the World Wide Web. Every CGI script should be scrutinized and extensively tested before installing it on a server, and subject to periodic review thereafter.
For Web hosting services, however, this advice is impractical. Hosting services must sponsor multiple Web authors of different levels of competence and reliability. Web authors do not trust each other, and the Web hosting service does not trust the authors. In such a situation, CGI scripts are even more problematic than usual. Because all CGI scripts run under the Web server's user ID, one author's scripts can interfere with another's. For example a malicious author could create a script that deletes files created by another author's script, or even cause another author's script to crash by sending it a kill signal. A poorly written script that contains a security hole can compromise the entire site's security by, for example, transmitting the contents of the system password file to a malicious remote user. The same problems are faced by large academic sites which provide Web pages for students.
For most Web hosting services it would be impossible to subject each and every author's CGI scripts to code review. Nor is it practical to cut off CGI scripting privileges entirely. In the competitive world of ISP's, customers will just move elsewhere.
The most popular solution to this problem is the use of "wrapper" scripts. In this system, untrusted author's CGI scripts are never invoked directly. Instead a small wrapper script is called on to execute the author's script, the target. The wrapper is SUID to root. When the wrapper runs, it subjects the target to certain safety checks (for example, checking that the script is not world-writable). The wrapper then changes its process ID to match the owner of the target and executes it. The result is that the author's script is executed with his own identity and privileges, preventing it from interfering with other author's scripts. The system also leads to increased accountability. Any files that an misbehaving script creates or modifies will bear the fingerprints of its creator. Without a wrapper, it can be impossible to determine which author's script is causing problems.
The limitations of wrapper scripts are three-fold:
sbox is highly configurable. It can be configured to chroot without changing its process ID, to change its process ID without performing the chroot, to change its group ID without changing its user ID, to establish resource ceilings without doing anything else, or any other combination that suits you.
sbox is designed to run with any Unix-based Web server. The package should compile correctly on any standard Unix system; however the resource limits use the BSD-specific setrlimit() and setpriority() calls. If you do not know whether your system supports these calls, check for the existence of the file /usr/include/system/resource.h. If this file does not exist, then chances are slim that you can use the resource limits. You can run sbox without the limits by setting the preprocessor define SET_LIMITS to FALSE (see below).
After unpacking the package, you should have the following files:
Makefile README.html (this file) README.txt (this file as text) sbox.h sbox.c env.cYou will first examine and edit the Makefile, then change sbox.h to suit your site configuration and preferences. It is suggested that you keep copies of the unaltered files for future reference.
Using your favorite text editor, examine and change the value of the INSTALL_DIRECTORY variable. This is the location in which sbox will be installed, and should correspond to your site-wide CGI directory.
You may also need to fiddle with the options for the install program. The default is to make sbox owned by user "root" and group "bin", and installed with permissions -rws--x--x. This configuration is SUID to root, necessary in order for the chroot and process ID changing functions to work.
If you wish to adjust the C compiler and its flags, change the CC and CFLAGS variables as needed.
This is the fun part. sbox.h contains several dozen flags that affect the script's features. These flags are implemented as compile-time defines rather than as run-time configuration variables for security reasons. There is less chance that the behavior of sbox can be maliciously altered if it has no dependences on external configuration files.
You should review sbox.h with a text editor and change the settings as needed. A typical entry looks like this:
/* * ECHO_FAILURES -- If set to TRUE, will echo fatal error messages * to the browser. Set to FALSE to inhibit error messages. */ #ifndef ECHO_FAILURES #define ECHO_FAILURES TRUE #endif
This section sets a feature called ECHO_FAILURES to TRUE. To change the value to FALSE, simply edit the line that begins with "#define" to read like this:
#define ECHO_FAILURES FALSE
These variables correspond to general sbox settings such as logging and environment consistency checking.
sbox can be set to log all its actions, including both failures and successful launches of author's scripts. Log entries are time stamped and labeled with the numeric IDs of the user and group that the target script was launched under.
These variables controls sbox's chroot functionality. The path names are relative to the document root. In the case of virtual hosts, this will be whatever is specified by the DocumentRoot directive in the server's configuration file. In the case of user-supported directories, it will be the user's public_html directory.
If you use LIMIT_FSIZE_HARD or _SOFT and are logging to stderr, be careful! If your web server error log is larger than the limit, no logging will occur.
Compile the sbox binary by typing make. If it compiles successfully, become root and type make install to install it in your site's cgi-bin directory (at the location specified in the Makefile.)
You can also install sbox manually by copying it into your cgi-bin directory and settings its permissions to ---s--x--x. This can be done with the following commands while logged in as the root user:
# chown root sbox # chgrp bin sbox # chmod 4111 sbox
In order for sbox to be effective, CGI scripts should be turned off in all user-supported directories and document directories. All CGI scripts should be placed in the main cgi-bin directory. No one but authorized site administrators should have write or listing privileges for this directory. If you are using the Apache server, a typical entry for a virtual host will look like this:
<VirtualHost www.fred.com> ServerAdmin fred@fred.com ServerName www.fred.com DocumentRoot /home/fred/pub/html TransferLog /home/fred/logs/access_log ErrorLog /home/fred/logs/error_log <Location /> Options Indexes SymbolicLinks order allow,deny allow from all </Location> </VirtualHost>
sbox enforces a directory-based CGI scripting scheme. Web authors' scripts must be located in a single directory tree whose position relative to their document tree is hard-coded in the CGI_BIN define. To avoid the possibility that an author's scripts can be downloaded by a remote user, I suggest that the scripts directory be placed outside the author's document root, for example in "../cgi-bin".
With the virtual host definition given above, the author's HTML documents will now reside in /home/fred/pub/html, while his scripts will reside in /home/fred/pub/cgi-bin, entirely outside his virtual site's document root.
When sbox runs, it will chroot() to the directory specified by the ROOT define, cutting the target script off from most system resources. Dynamically linked programs (including interpreters and the like) will not be happy unless they can find the shared libraries they rely on. Therefore, this directory should be set up like a miniature root directory, containing whatever is necessary for programs to run. This list is different from system to system. See tips for some advice on setting it up.
Below is the structure of an author's directory, assuming that the virtual host uses ~fred/pub/html as its document root.
% ls -l ~fred/pub total 10 drwxr-xr-x 2 fred users 1024 Oct 23 06:27 bin/ system binaries drwxr-xr-x 3 fred users 1024 Oct 19 20:44 cgi-bin/ CGI scripts drwxr-xr-x 2 fred users 1024 Oct 12 16:59 dev/ device special files drwxr-xr-x 2 fred users 1024 Oct 19 17:57 etc/ configuration files drwxr-xr-x 2 fred users 1024 Oct 22 19:14 html/ HTML document root drwxr-xr-x 3 fred users 1024 Oct 19 20:35 lib/ shared libraries drwxr-xr-x 2 fred users 1024 Oct 23 05:48 tmp/ temporary files
The same type of directory structure should be used for user-supported directories. Generally you will want to set it up in the directory that contains public_html. In this case, when sbox runs, its root directory will be the user's home directory.
For improved security, you can restrict sbox even further by putting making the ROOT some subdirectory of the user's home directory. This configuration ensures that the only way a script can get access to a file is if the user places it in the restricted directory deliberately. One simple scheme goes as follows:
Relevant directories relative to $HOME:
public_html/ # document root public_html/sbox/ -> ../sbox/html/ # symbolic link sbox/bin/ sbox/cgi-bin/ sbox/html/ sbox/lib/
Defines for sbox.h:
#define ROOT "../sbox" #define CGI_BIN "../sbox/cgi-bin"
Under this scheme, the sbox-accessible scripts are located at:
http://yoursite.com/cgi-bin/sbox/~fred/env
and sbox-accessible pages are available via HTTP at:
http://yoursite.com/~fred/sbox/mypage.html
You do not have to do any special directory configuration if you do not take advantage of sbox's chroot feature.
To use sbox create URLs like this one:
http://www.virtual.host.com/cgi-bin/sbox/script_name
^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^
virtual host name author's script
The first part of the URL is the path to the sbox script. The second part is the path to the author's script, relative to the cgi-bin directory in his home directory. If the author's script needs access to additional path information, you can append it in the natural way:
http://www.virtual.host.com/cgi-bin/sbox/script_name/additional/path/info
For user-supported directories, use this format:
http://www.virtual.host.com/cgi-bin/sbox/~fred/script_name
Authors are free to organize their script directories into a hierarchy. They need only modify script URLs to reflect the correct path:
http://www.virtual.host.com/cgi-bin/sbox/foo/bar/script_name
Here are a few pieces of advice and tips on making best use of sbox.
Many CGI scripts will not run correctly in a chroot environment unless they can find the resources they need. Compiled C programs often need access to shared libraries and/or device special files. Interpreted scripts need access to their interpreters, for example Perl. Feature-rich programs like sendmail depend on their configuration files being present in /etc.
As described above, you will need to turn the chroot directory into a miniature root file system, complete with /etc, /lib, /bin, /tmp and /dev directories. I recommend that you create and test a chroot directory for one virtual host, then use it as a master copy for creating new virtual hosts every time you add a new author account. Both the cpio and the tar commands can be used to copy shared libraries and device special files safely.
Programs that check file ownerships may need access to password and/or group files in order for them to translate from numeric uid's and gid's to text names. In order to support CGI scripts that perform this type of action, you should place dummy copies of /etc/passwd and /etc/group in the author's /etc directory. These files should not contain real passwords, and should only contain standard system user accounts (e.g. "bin" and "mail"), plus any needed by the script. You probably don't want to make the complete list of user account names available to authors' CGI scripts!
If CGI scripts require access to the DNS system in order to resolve host names and IP addresses, you should place a copy of /etc/resolv.conf into the chroot directory. You may need to copy other configuration files to use certain feature-rich programs. For example, if scripts send e-mail using the sendmail program, you will need to install its configuration program, sendmail.cf.
Many programs redirect their output to the device special file /dev/null. Other programs need access to /dev/zero or other special files. You can copy these files from the real /dev directory using either cpio or tar. Alternately you can create the files from scratch using mknod, but only if you know what you're doing. You'll need to have superuser privileges to accomplish either of these tasks.
The Unix time system expects to find information about the local timezone in a compiled file named /usr/lib/zoneinfo/localtime. You may need to copy this into your chroot directory in order for the timezone to be correctly displayed. You can confirm that the correct timezone is being found by examining the output of the "env" executable.
There are two ways to finesse the problem of shared libraries. For compiled C scripts, one option is to link the program statically (by providing the -static flag to the linker). A less laborious solution is to place copies of the required shared libraries in the new root's /lib directory (or /slib, for systems that use that directory for shared libraries). Many systems have a utility that lists the shared libraries required by a binary. Use this program to determine which shared libraries are required, and copy them over into each author's /lib directory. In addition to the shared libraries, you may need to copy the dynamic linker itself into the /lib directory. On my linux system, this file is "ld-linux.so".
If a executable cannot find its shared libraries at run time, it will usually fail with a specific error message that will lead you to the problem -- look in the server error log. If you get silent failures, it's probably the dynamic linker itself that can't be found.
Linux, and possibly some other systems, uses a cache file named /etc/ld.so.cache to resolve the location of library files. If this file isn't found at run time, the system will generate a warning but find the correct shared libraries nevertheless. The quick and dirty way to get rid of this warning is to copy the current cache file from the real /etc directory to the chroot one. However, this may have bad side effects (I haven't actually encountered any, but I worry about it.) It's better to make this cache file from scratch in the chroot environment itself. To do this, run the ldconfig program with the command-line version of chroot. You'll need to be root to do this:
# cd /sbin # chroot ~fred/pub ./ldconfig
Perl scripts, in addition to requiring the Perl interpreter, will often need access to the Perl lib directory in order to get at useful modules (such as CGI.pm). It's easiest to copy the whole Perl library tree to the correct location in the chroot directory, being careful to get the path right. For example, if the real Perl library is located in /usr/local/lib/perl5, you'll need to create a parallel /usr hierarchy in the chroot directory. On my system, I recompiled Perl to use /lib/perl5 and dumped the modules into that directory. If things get bolluxed up, you can always tell Perl where to look for its libraries by appending something like this to the top of CGI scripts:
#!/bin/perl
BEGIN { push(@INC,'/lib/perl5','/lib/perl5/i586-linux/5.004'); }
Some CGI scripts act as filters on static HTML documents. Examples include PHP and various guestbook scripts. Such scripts often include the path to the static document appended to the end of the script's URL as "additional path information." For example:
http://your.site/~fred/guestbook.cgi/~fred/guestbook/data.txt
The script will be passed two environment variables, PATH_INFO, containing the additional path information, and PATH_TRANSLATED, containing the path information translated into an absolute filename. In the example above, the values of these variables might be:
| PATH_INFO | /~fred/guestbook/data.txt |
|---|---|
| PATH_TRANSLATED | /home/fred/public_html/guestbook/data.txt |
When sbox is running it interprets the additional path information as relative to the user's document root. This means that a document located in Fred's public_html directory can be referred to this way:
http://your.site/cgi-bin/sbox/~fred/guestbook.cgi/guestbook/data.txt
After performing the chroot(), sbox attempts to adjust PATH_TRANSLATED so that it continues to point to a valid file. If the user's document root is located within the chroot directory, then PATH_TRANSLATED is trimmed so that it is relative to the new root directory:
| PATH_INFO | /guestbook/data.txt |
|---|---|
| PATH_TRANSLATED | /public_html/guestbook/data.txt |
However, if the document root is entirely outside the new root directory, then sbox will simply use the same value for PATH_INFO and PATH_TRANSLATED:
| PATH_INFO | /guestbook/data.txt |
|---|---|
| PATH_TRANSLATED | /guestbook/data.txt |
The default resource limits are reasonable. Most authors won't have problems with them unless they need to do number crunching or manipulate many files simultaneously. If need be, authors can raise the soft resource limits up to the levels imposed by the hard limit ceilings, which are very liberal. C programmers can do this directly by making calls to setrlimit(). Perl scripters should download and install Jarkko Hietaniemi's BSD::Resource module from CPAN.
Because of design conflicts, the "#exec" style server-side include do not work correctly with sbox. However, the "#include virtual" command, which does almost exactly the same thing, does work correctly. To include the output of sbox-wrapped CGI scripts in server-side-include files, just write something like this:
<!--#include virtual="/cgi-bin/sbox/~fred/guestbook"-->
If you are running Apache 1.2 or higher, you can take advantage of the rewrite rule module to make sbox transparent. For virtual hosts, you can add something like the following to the <VirtualHost> section:
This replaces all URLs that start with "/cgi" with "/cgi-bin/sbox". This lets authors refer to their scripts with:RewriteEngine on RewriteRule ^/cgi/(.*) /cgi-bin/sbox/$1 [PT]
http://www.virtual.host.com/cgi/script_nameand to main Web server scripts with:
http://www.virtual.host.com/cgi-bin/guestbookFor user-supported directories, this rewrite rule will allow users to refer to their scripts using http://www.host.com/~username/cgi/script_name:
RewriteEngine on RewriteRule ^/~(.+)/cgi/(.+) /cgi-bin/sbox/~$1/$2 [PT,NS]
Additional path information, such as the pointer to a guestbook file, can be passed as additional path information. It will be interpreted as relative to the user's document root directory:
http://www.host.com/~user/cgi/guestbook/guestbook.txt
This distribution comes with a small statically linked binary called "env" that you can call as a CGI script. It prints out some information about the current environment, including the user and group ID's, the current working directory, and the environment variables, to help you determine whether sbox is configured correctly and working as expected.
This utility is ©1997-2003 Lincoln D. Stein. It can be used freely and redistributed in source code and binary form. I request that this documentation, including the copyright statement, remain attached to the utility if you redistribute it. You are free to make modifications, but please attach a note stating the changes you made.