CGI security: Escape newlines.

[jmyers () marigold eecs nwu edu (Jennifer Myers)]

When sending user-supplied data to a shell in a CGI program, it has
become common practice among security-conscious CGI authors to remove
or escape certain shell metacharacters to avoid them being interpreted
by the shell and possibly allowing the user to execute arbritrary
commands at his or her will.

There are a good set of security guidelines at:
http://www.cerf.net/~paulp/cgi-security/safe-cgi.txt:

That document recommends removing or escaping the following characters
in user-supplied data before passing it to a shell:

        ;<>*|`&$!#()[]{}:'"/

There is (at least) one character missing from this list: the new line
character.  I have never seen the new line character included in a list
of metacharaters to filter.

A sampling of widely-available CGI programs turned up many that are
vulnerable.  Just about any CGI program which "un-hexifies" (converts
characters represented by their hex values in a URL by to their actual
character) its input and passes that input to a shell is likely to be
vulnerable.

Here's a toy example:

  #!/usr/local/bin/perl
  # usage: http://your.host/cgi-bin/echo?<string>
  # Echos back the QUERY_STRING to the user.

  $| = 1;
  $in = $ENV{'QUERY_STRING'};
  $in =~ s/%(..)/pack("c",hex($1))/ge;

  # Escape the nasty metacharacters
  # (List courtesy of http://www.cerf.net/~paulp/cgi-security/safe-cgi.txt)
  $in =~ s/([;<>\*\|`&\$!#\(\)\[\]\{\}:'"])/\\$1/g;

  print "Content-type: text/html\n\n";
  system("/bin/echo $in");

Install this program in cgi-bin/echo and
<http://your.host/cgi-bin/echo?hello%20there>, will return a page
containing the text "hello there".

Insert %0A, the newline character, and you can exploit the shell
to run any command you wish.

For example, the URL <http://your.host/cgi-bin/echo?%0Acat%20/etc/passwd>
will get you the password file.

(In Perl, the call to system() should have broken up the arguments:
        system("/bin/echo", $in);
and the problem would disappear.)

While this example uses system() in Perl, the general program will
show up whenever a shell is invoked.

THE FIX:

Very simple.  Add the character \n (the new line character) to the
list of characters to REMOVE from user-supplied data before
suppling it to a shell in a CGI program.

  #!/usr/local/bin/perl
  # usage: http://your.host/cgi-bin/safe-echo?<string>
  # Echos back the QUERY_STRING to the user.

  $| = 1;
  $in = $ENV{'QUERY_STRING'};
  $in =~ s/%(..)/pack("c",hex($1))/ge;

  # Escape the nasty metacharacters
  # (List courtesy of http://www.cerf.net/~paulp/cgi-security/safe-cgi.txt)
  $in =~ s/([;<>\*\|`&\$!#\(\)\[\]\{\}:'"\n])/\\$1/g;

  # SECURITY FIX: REMOVE NEWLINES
  $in =~ tr/\n//d;

  print "Content-type: text/html\n\n";
  system("/bin/echo $in");

Again, this bug exists in MANY CGI programs.  If you maintain CGI
programs on your server, I suggest you check through each of them.
I've only looked through several CGI programs, and found the bug on
some of them (the authors have been contacted). If I have more time in
the near future, I'll post a list of vulnerable programs as well as
alerting he authors.  In the meantime, you should check through the
source of all of your CGI programs for this bug.

--
Jennifer Myers                          http://www.eecs.nwu.edu/~jmyers/