The World Wide Web Security FAQ

6. CGI (Server) Scripts

Q30: What's the problem with CGI scripts?

The problem with CGI scripts is that each one presents yet another opportunity for exploitable bugs. CGI scripts should be written with the same care and attention given to Internet servers themselves, because, in fact, they are miniature servers. Unfortunately, for many Web authors, CGI scripts are their first encounter with network programming.

CGI scripts can present security holes in two ways:

  1. They may intentionally or unintentionally leak information about the host system that will help hackers break in.
  2. Scripts that process remote user input, such as the contents of a form or a "searchable index" command, may be vulnerable to attacks in which the remote user tricks them into executing commands.

CGI scripts are potential security holes even though you run your server as "nobody". A subverted CGI script running as "nobody" still has enough privileges to mail out the system password file, examine the network information maps, or launch a log-in session on a high numbered port (it just needs to execute a few commands in Perl to accomplish this). Even if your server runs in a chroot directory, a buggy CGI script can leak sufficient system information to compromise the host.

Q31: Is it better to store scripts in the cgi-bin directory, or to store them anywhere in the document tree and identify them to the server using the .cgi extension?

Although there's nothing intrinsically dangerous about scattering CGI scripts around the document tree, it's better to store them in the cgi-bin directory. Because CGI scripts are such potentially large security holes, it's much easier to keep track of what scripts are installed on your system if they're kept in a central location rather than being scattered around among multiple directories. This is particularly true in an environment with multiple Web authors. It's just too easy for an author to inadverently create a buggy CGI script and install it somewhere in the document tree. By restricting CGI scripts to the cgi-bin directory and by setting up permissions so that only the Web administrator can install these scripts, you avoid this chaotic situation.

There's also a risk of a hacker managing to create a .cgi file somewhere in your document tree and then executing it remotely by requesting its URL. A cgi-bin directory with tightly-controls lessens the possibility of this happening.

Q32: Are compiled languages such as C safer than interpreted languages like Perl and shell scripts?

The answer is "yes", but with many qualifications and explanations.

First of all is the issue of the remote user's access to the script's source code. The more the hacker knows about how a script works, the more likely he is to find bugs to exploit. With a script written in a compiled language like C, you can compile it to binary form, place it in cgi-bin/, and not worry about intruders gaining access to the source code. However, with an interpreted script, the source code is always potentially available. Even though a properly-configured server will not return the source code to an executable script, there are many scenarios in which this can be bypassed.

Consider the following scenario. For convenience's sake, you've decided to identify CGI scripts to the server using the .cgi extension. Later on, you need to make a small change to an interpreted CGI script. You open it up with the Emacs text editor and modify the script. Unfortunately the edit leaves a backup copy of the script source code lying around in the document tree. Although the remote user can't obtain the source code by fetching the script itself, he can now obtain the backup copy by blindly requesting the URL:


(This is another good reason to limit CGI scripts to cgi-bin and to make sure that cgi-bin is separate from the document root.)

Of course in many cases the source code to a CGI script written in C is freely available on the Web, and the ability of hackers to steal the source code isn't an issue.

Another reason that compiled code may be safer than interpreted code is the size and complexity issue. Big software programs, such as shell and Perl interpreters, are likely to contain bugs. Some of these bugs may be security holes. They're there, but we just don't know about them.

A third consideration is that the scripting languages make it extremely easy to send data to system commands and capture their output. As explained below, the invocation of system commands from within scripts is one of the major potential security holes. In C, it's more effort to invoke a system command, so it's less likely that the programmer will do it. In particular, it's very difficult to write a shell script of any complexity that completely avoids dangerous constructions. Shell scriptig languages are poor choices for anything more than trivial CGI programs.

All this being said, please understand that I am not guaranteeing that a compiled program will be safe. C programs can contain many exploitable bugs, as the net's experiences with NCSA httpd 1.3 and sendmail shows. Counterbalancing the problems with interpreted scripts is that they tend to be shorter and are therefore more easily understood by other people than the author. Furthermore, Perl contains a number of built-in features that were designed to catch potential security holes. For example, the taint checks (see below) catch many of the common pitfalls in CGI scripting, and may make a Perl scripts safer in some respects than the equivalent C program.

Q33: I found a great CGI script on the Web and I want to install it. How can I tell if it's safe?

You can never be sure that a script is safe. The best you can do is to examine it carefully and understand what it's doing and how it's doing it. If you don't understand the language the script's written in, show it to someone who does.

Things to think about when you examine a script:

  1. How complex is it? The longer it is, the more likely it is to have problems.
  2. Does it read or write files on the host system? Programs that read files may inadvertently violate access restrictions you've set up, or pass sensitive system information to hackers. Programs that write files have the potential to modify or damage documents, or, in the worst case, introduce trojan horses to your system.
  3. Does it interact with other programs on your system? For example, many CGI scripts send e-mail in response to a form input by opening up a connection with the sendmail program. Is it doing this in a safe way?
  4. Does it run with suid (set-user-id) privileges? In general this is a very dangerous thing and scripts need to have excellent reasons for doing this.
  5. Does the author validate user input from forms? Checking form input is a sign that the author is thinking about security issues.
  6. Does the author use explicit path names when invoking external programs? Relying on the PATH environment variable to resolve partial path names is a dangerous practice.

Q34: What CGI scripts are known to contain security holes?

Quite a number of widely distributed CGI scripts contain known security holes. All the ones that are identified here have since been caught and fixed, but if you are running an older version of the script you may still be vulnerable. Get rid of it and obtain the latest version.
"phf" phone book script, distributed with NCSA httpd and Apache

The holes in the first two of these scripts were discovered by Paul Phillips (, who also wrote the CGI security FAQ. The hole in the PHF (phone book) script was discovered by Jennifer Myers (, and is representative of a potential security hole in all CGI script that use NCSA's util.c library. Here's a patch to fix the problem in util.c.

Reports of other buggy scripts will be posted here on an intermittent basis.

In addition, one of the scripts given as an example of "good CGI scripting" in the published book "Build a Web Site" by net.Genesis and Devra Hall contains the classic error of passing an unchecked user variable to the shell. The script in question is in Section 11.4, "Basic Search Script Using Grep", page 443. Other scripts in this book may contain similar security holes.

This list is far from complete. No centralized authority is monitoring all the CGI scripts that are released to the public. Ultimately it's up to you to examine each script and make sure that it's not doing anything unsafe.

Q35: I'm developing custom CGI scripts. What unsafe practices should I avoid?

  1. Avoid giving out too much information about your site and server host.

    Although they can be used to create neat effects, scripts that leak system information are to be avoided. For example, the "finger" command often prints out the physical path to the fingered user's home directory and scripts that invoke finger leak this information (you really should disable the finger daemon entirely, preferably by removing it). The w command gives information about what programs local users are using. The ps command, in all its shapes and forms, gives would-be intruders valuable information on what daemons are running on your system.

  2. If you're coding in a compiled language like C, avoid making assumptions about the size of user input.

    A MAJOR source of security holes has been coding practices that allowed character buffers to overflow when reading in user input. Here's a simple example of the problem:

       #include <stdlib.h>
    #include <stdio.h> static char query_string[1024]; char* read_POST() {
    int query_size; query_size=atoi(getenv("CONTENT_LENGTH")); fread(query_string,query_size,1,stdin); return query_string; }
    The problem here is that the author has made the assumption that user input provided by a POST request will never exceed the size of the static input buffer, 1024 bytes in this example. This is not good. A wily hacker can break this type of program by providing input many times that size. The buffer overflows and crashes the program; in some circumstances the crash can be exploited by the hacker to execute commands remotely.

    Here's a simple version of the read_POST() function that avoids this problem by allocating the buffer dynamically. If there isn't enough memory to hold the input, it returns NULL:

       char* read_POST() {
    int query_size=atoi(getenv("CONTENT_LENGTH")); char* query_string = (char*) malloc(query_size); if (query_string != NULL) fread(query_string,query_size,1,stdin); return query_string; }
    Of course, once you've read in the data, you should continue to make sure your buffers don't overflow. Watch out for strcpy(), strcat() and other string functions that blindly copy strings until they reach the end. Use the strncpy() and strncat() calls instead.
       #define MAXSTRINGLENGTH 256
       char myString[MAXSTRINGLENGTH];
       char* query = read_POST();
       myString[MAXSTRINGLENGTH-1]='\0';      /* ensure null byte */
       strncpy(myString,query,MAXSTRINGLENGTH-1); /* don't overwrite null byte */
    (Note that the semantics of strncpy are nasty when the input string is exactly MAXSTRINGLENGTH bytes long, leading to some necessary fiddling with the terminating NULL.)
  3. Never, never, never pass unchecked remote user input to a shell command.

    In C this includes the popen(), and system() commands, all of which invoke a /bin/sh subshell to process the command. In Perl this includes system(), exec(), and piped open() functions as well as the eval() function for invoking the Perl interpreter itself. In the various shells, this includes the exec and eval commands.

    Backtick quotes, available in shell interpreters and Perl for capturing the output of programs as text strings, are also dangerous.

    The reason for this bit of paranoia is illustrated by the following bit of innocent-looking Perl code that tries to send mail to an address indicated in a fill-out form.

       $mail_to = &get_name_from_input; # read the address from form
       open (MAIL,"| /usr/lib/sendmail $mail_to");
       print MAIL "To: $mailto\nFrom: me\n\nHi there!\n";
       close MAIL;
    The problem is in the piped open() call. The author has assumed that the contents of the $mail_to variable will always be an innocent e-mail address. But what if the wiley hacker passes an e-mail address that looks like this?;mail</etc/passwd;
    Now the open() statement will evaluate the following command:
    /usr/lib/sendmail; mail</etc/passwd
    Unintentionally, open() has mailed the contents of the system password file to the remote user, opening the host to password cracking attack.

Q36: But if I avoid eval(), exec(), popen() and system(), how can I create an interface to my database/search engine/graphics package?

You don't have to avoid these calls completely. You just have to understand what you're doing before you call them. In some cases you can avoid passing user-inputted variables through the shell by calling external programs differently. For example, sendmail supports a -t option, which tells it to ignore the address given on the command line and take its To: address from the e-mail header. The example above can be rewritten in order to take advantage of this feature as shown below (it also uses the -oi flag to prevent sendmail from ending the message prematurely if it encounters a period at the start of a line):
   $mailto = &get_name_from_input; # read the address from form
   open (MAIL,"| /usr/lib/sendmail -t -oi");
   print MAIL <<END;
   To: $mailto
   From: me (me\
   Subject: nothing much

   Hi there!
   close MAIL;
C programmers can use the exec family of commands to pass arguments directly to programs rather than going through the shell. This can also be accomplished in Perl using the technique described below.

You should try to find ways not to open a shell. In the rare cases when you have no choice, you should always scan the arguments for shell metacharacters and remove them. The list of shell metacharacters is extensive:

Notice that it contains the carriage return and newline characters, something that someone at NCSA forgot when he or she wrote the widely-distributed util.c library as an example of CGI scripting in C.

It's a better policy to make sure that all user input arguments are exactly what you expect rather than blindly remove shell metacharacters and hope there aren't any unexpected side-effects. Even if you avoid the shell and pass user variables directly to a program, you can never be sure that they don't contain constructions that reveal holes in the programs you're calling.

For example, here's a way to make sure that the $mail_to address created by the user really does look like a valid address:

  $mail_to = &get_name_from_input; # read the address from form
  unless ($mail_to =~ /^[\w-.]+\@[\w-.]+$/) {
     die 'Address not in form';
(This particular pattern match may be too restrictive for some sites. It doesn't allow UUCP-style addresses or any of the many alternative addressing schemes).

Q37: Is it safe to rely on the PATH environment variable to locate external programs?

Not really. One favorite hacker's trick is to alter the PATH environment variable so that it points to the program he wants your script to execute rather than the program you're expecting. In addition to avoiding passing unchecked user variabes to external programs, you should also invoke the programs using their full absolute pathnames rather than relying on the PATH environment variable. That is, instead of this fragment of C code:
   system("ls -l /local/web/foo");
use this:
   system("/bin/ls -l /local/web/foo");
If you must rely on the PATH, set it yourself at the beginning of your CGI script:

In general it's not a good idea to put the current directory (".") into the path.

Q38: I hear there's a package called cgiwrap that makes CGI scripts safe?

This is not quite true. cgiwrap (by Nathan Neulinger <>, was designed for multi-user sites like university campuses where local users are allowed to create their own scripts. Since CGI scripts run under the server's user ID (e.g. "nobody"), it is difficult under these circumstances for administrators to determine whose script is generating bounced mail, errors in the server log, or annoying messages on other user's screens. There are also security implications when all users' scripts run with the same permissions: one user's script can unintentionally (or intentionally) trash the database maintained by another user's script.

cgiwrap allows you to put a wrapper around CGI scripts so that a user's scripts now run under his own user ID. This policy can be enforced so that users must use cgiwrap in order to execute CGI scripts. Although this simplifies administration and prevents users from interfering with each other, it does put the individual user at tremendous risk. Because his scripts now run with his own permissions, a subverted CGI script can trash his home directory by executing the command

    rm -r ~

Worse, since the subverted CGI script has write access to the user's home directory, it could place a trojan horse in the user's directory that will subvert the security of the entire system. The "nobody" user, at least, usually doesn't have write permission anywhere.

Q39: People can only use scripts if they're accessed from a form that lives on my local system, right?

Not right. Although you can restrict access to a script to certain IP addresses or to user name/password combinations, you can't control how the script is invoked. A script can be invoked from any form, anywhere in the world. Or its form interface can be bypassed entirely and the script invoked by directly requesting its URL. Don't assume that a script will always be invoked from the form you wrote to go with it. Anticipate that some parameters will be missing or won't have the expected values.

When restricting access to a script, remember to put the restrictions on the _script_ as well as any HTML forms that access it. It's easiest to remember this when the script is of the kind that generates its own form on the fly.

Q40: Can people see or change the values in "hidden" form variables?

They sure can! The hidden variable is visible in the raw HTML that the server sends to the browser. To see the hidden variables, a user just has to select "view source" from the browser menu. In the same vein, there's nothing preventing a user from setting hidden variables to whatever he likes and sending it back to your script. Don't rely on hidden variables for security.

Q41: Is using the "POST" method for submitting forms more private than "GET"?

If you are concerned about your queries showing up in server logs, or those of Web proxies along the way, this is true. Queries submitted with POST usually don't appear in logs, while GET queries do. In other respects, however, there's no substantial difference in security between the two methods. It is just as easy to intercept unencrypted GET queries as POST queries. Furthermore, unlike some early implementations of HTTP encryption, the current generation of data encrypting server/browser combinations do just as good a job encrypting GET requests as they do for POST requests.

Q42: Where can I learn more about safe CGI scripting?

The CGI security FAQ, maintained by Paul Phillips (, can be found at:

CGI security is also covered by documentation maintained at NCSA:

An excellent all-round introduction to Perl and CGI Scripting can be found in the Perl CGI FAQ,
written by Tom Christiansen ( and Shishir Gundavaram (

Lincoln D. Stein,
Whitehead Institute/MIT Center for Genome Research
Last modified: Fri Nov 8 10:07:21 EST 1996