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CERT* Advisory CA-97.25.CGI_metachar
Original issue date: Nov. 10, 1997
Last revised: --
Topic: Sanitizing User-Supplied Data in CGI Scripts
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The CERT Coordination Center has received reports and seen mailing list
discussions of a problem with some CGI scripts, which allow an attacker to
execute arbitrary commands on a WWW server under the effective user-id of the
server process. The problem lies in how the scripts are written, NOT in the
scripting languages themselves.
The CERT/CC team urges you to check all CGI scripts that are available via the
World Wide Web services at your site and ensure that they sanitize
user-supplied data. We have written a tech tip on how to do this (see Section
III).
We will update the tech tip (rather than this advisory) if we receive
additional information.
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I. Description
Some CGI scripts have a problem that allows an attacker to execute
arbitrary commands on a WWW server under the effective user-id of the
server process. The cause of the problem is not the CGI scripting
language (such as Perl and C). Rather, the problem lies in how an
individual writes his or her script. In many cases, the author of the
script has not sufficiently sanitized user-supplied input.
II. Impact
If user-supplied data is not sufficiently sanitized, local and remote
users may be able to execute arbitrary commands on the HTTP server with
the privileges of the httpd daemon. They may then be able to compromise
the HTTP server and under certain configurations gain privileged access.
III. Solution
We strongly encourage you to review all CGI scripts that are available
via WWW services at your site. You should ensure that these scripts
sufficiently sanitize user-supplied data.
We recommend carrying out this review on a regular basis and whenever new
scripts are made available.
For advice about what to look for and how to address the problem,
see our tech tip on meta-characters in CGI scripts, available from
ftp://ftp.cert.org/pub/tech_tips/cgi_metacharacters
We have placed the November 5, 1997, version of this tech tip in an
appendix of this advisory for your convenience. We may update this tech
tip in the future, so please check the electronic version for the most
current information.
If you believe that a script does not sufficiently sanitize
user-supplied data then we encourage you to disable the script and
consult the script author for a patch.
If the script author is unable to supply a patched version, sites with
sufficient expertise may wish to patch the script themselves, adapting
the material in our tech tip to meet whatever specification is required
(such as the appropriate RFC).
(NOTE: We cannot offer any further assistance on source code patching
than that given in the tech tip mentioned above.)
.............................................................................
Appendix - Tech Tip on CGI Metacharacters (version 1.1, Nov. 5, 1997)
The tech tip below may be updated in the future. For the most current version,
see ftp://ftp.cert.org/pub/tech_tips/cgi_metacharacters
How To Remove Meta-characters From User-Supplied Data In CGI Scripts
1. Definition of the Problem
We have noticed several reports to us and to public mailing lists about CGI
scripts that allow an attacker to execute arbitrary commands on a WWW
server under the effective user-id of the server process.
In many of these cases, the author of the script has not sufficiently
sanitized user-supplied input.
2. Definition of "Sanitize"
Consider an example where a CGI script accepts user-supplied data. In
practice, this data may come from any number of sources of user-supplied
data; but for this example, we will say that the data is taken from an
environment variable $QUERY_STRING. The manner in which data was inserted
into the variable is not important - the important point here is that the
programmer needs to gain control over the contents of the data in
$QUERY_STRING before further processing can occur. The act of gaining this
control is called "sanitizing" the data.
3. A Common But Inadvisable Approach
A script writer who is aware of the need to sanitize data may decide to
remove a number of well-known meta-characters from the script and replace
them with underscores. A common but inadvisable way to do this is by
removing particular characters.
For instance, in Perl:
#!/usr/local/bin/perl
$user_data = $ENV{'QUERY_STRING'}; # Get the data
print "$user_data\n";
$user_data =~ s/[\/ ;\[\]\<\>&\t]/_/g; # Remove bad characters. WRONG!
print "$user_data\n";
exit(0);
In C:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int
main(int argc, char *argv[], char **envp)
{
static char bad_chars[] = "/ ;[]<>&\t";
char * user_data; /* our pointer to the environment string */
char * cp; /* cursor into example string */
/* Get the data */
user_data = getenv("QUERY_STRING");
printf("%s\n", user_data);
/* Remove bad characters. WRONG! */
for (cp = user_data; *(cp += strcspn(cp, bad_chars)); /* */)
*cp = '_';
printf("%s\n", user_data);
exit(0);
}
In this method, the programmer determines which characters should NOT be
present in the user-supplied data and removes them. The problem with this
approach is that it requires the programmer to predict all possible inputs.
If the user uses input not predicted by the programmer, then there is the
possibility that the script may be used in a manner not intended by the
programmer.
4. A Recommended Approach
A better approach is to define a list of acceptable characters and replace any
character that is NOT acceptable with an underscore. The list of valid input
values is typically a predictable, well-defined set of manageable size. For
example, consider the tcp_wrappers package written by Wietse Venema. In the
percent_x.c module, Wietse has defined the following:
char *percent_x(...)
{
{...}
static char ok_chars[] = "1234567890!@%-_=+:,./\
abcdefghijklmnopqrstuvwxyz\
ABCDEFGHIJKLMNOPQRSTUVWXYZ";
{...}
for (cp = expansion; *(cp += strspn(cp, ok_chars)); /* */ )
*cp = '_';
{...}
The benefit of this approach is that the programmer is certain that
whatever string is returned, it contains only characters now under his or her
control.
This approach contrasts with the approach we discussed earlier. In the earlier
approach, which we do not recommend, the programmer must ensure that he or she
traps all characters that are unacceptable, leaving no margin for error. In
the recommended approach, the programmer errs on the side of caution and only
needs to ensure that acceptable characters are identified; thus the programmer
can be less concerned about what characters an attacker may try in an attempt
to bypass security checks.
Building on this philosophy, the Perl program we presented above could be
thus sanitized to contain ONLY those characters allowed. For example:
#!/usr/cert/bin/perl
$_ = $user_data = $ENV{'QUERY_STRING'}; # Get the data
print "$user_data\n";
$OK_CHARS='a-zA-Z0-9_\-\.@'; # A restrictive list, which
# should be modified to match
# an appropriate RFC, for example.
eval "tr/[$OK_CHARS]/_/c";
$user_data = $_;
print "$user_data\n";
exit(0);
Likewise, the same updated example in C:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int
main(int argc, char *argv[], char **envp)
{
static char ok_chars[] = "abcdefghijklmnopqrstuvwxyz\
ABCDEFGHIJKLMNOPQRSTUVWXYZ\
1234567890_-.@";
char * user_data; /* our pointer to the environment string */
char * cp; /* cursor into example string */
user_data = getenv("QUERY_STRING");
printf("%s\n", user_data);
for (cp = user_data; *(cp += strspn(cp, ok_chars)); /* */)
*cp = '_';
printf("%s\n", user_data);
exit(0);
}
5. Recommendation
We strongly encourage you to review all CGI scripts available via your web
server to ensure that any user-supplied data is sanitized using the approach
described in Section 4, adapting the example to meet whatever specification
you are using (such as the appropriate RFC).
6. Additional Tips
The following comments appeared in CERT Advisory CA-97.12 "Vulnerability in
webdist.cgi" and AUSCERT Advisory AA-97.14, "SGI IRIX webdist.cgi
Vulnerability."
We strongly encourage all sites should consider taking this opportunity
to examine their entire httpd configuration. In particular, all CGI
programs that are not required should be removed, and all those
remaining should be examined for possible security vulnerabilities.
It is also important to ensure that all child processes of httpd are
running as a non-privileged user. This is often a configurable option.
See the documentation for your httpd distribution for more details.
Numerous resources relating to WWW security are available. The
following pages may provide a useful starting point. They include
links describing general WWW security, secure httpd setup, and secure
CGI programming.
The World Wide Web Security FAQ:
http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
The following book contains useful information including sections on
secure programming techniques.
_Practical Unix & Internet Security_, Simson Garfinkel and
Gene Spafford, 2nd edition, O'Reilly and Associates, 1996.
Please note that the CERT/CC and AUSCERT do not endorse the URL that
appears above. If you have any problem with the sites, please contact
the site administrator.
Another resource that sites can consider is the CGI.pm module. Details
about this module are available from:
http://www.genome.wi.mit.edu/ftp/pub/software/WWW/cgi_docs.html
This module provides mechanisms for creating forms and other web-based
applications. Be aware, however, that it does not absolve the programmer from
the safe-coding responsibilities discussed above.
Copyright 1997 Carnegie Mellon University. Conditions for use, disclaimers,
and sponsorship information can be found in
http://www.cert.org/legal_stuff.html and ftp://info.cert.org/pub/legal_stuff .
If you do not have FTP or web access, send mail to This email address is being protected from spambots. You need JavaScript enabled to view it. with
"copyright" in the subject line.
CERT is registered in the U.S. Patent and Trademark Office.
This file: ftp://ftp.cert.org/pub/tech_tips/cgi_metacharacters
Last revised November 5, 1997
Version 1.1
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The CERT Coordination Center thanks Wietse Venema for some of the material
used in the cgi_metacharacters tech tip.
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If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (see http://www.first.org/team-info/).
CERT/CC Contact Information
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Email This email address is being protected from spambots. You need JavaScript enabled to view it.
Phone +1 412-268-7090 (24-hour hotline)
CERT personnel answer 8:30-5:00 p.m. EST(GMT-5) / EDT(GMT-4)
and are on call for emergencies during other hours.
Fax +1 412-268-6989
Postal address
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
USA
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Getting security information
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Copyright 1997 Carnegie Mellon University. Conditions for use, disclaimers,
and sponsorship information can be found in
http://www.cert.org/legal_stuff.html and ftp://ftp.cert.org/pub/legal_stuff .
If you do not have FTP or web access, send mail to This email address is being protected from spambots. You need JavaScript enabled to view it. with
"copyright" in the subject line.
*CERT is registered in the U.S. Patent and Trademark Office.
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This file: ftp://ftp.cert.org/pub/cert_advisories/CA-97.25.CGI_metachar
http://www.cert.org
click on "CERT Advisories"
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Revision history
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