File permissions security questions

Posted on 2004-08-03
Medium Priority
Last Modified: 2010-05-18
I am planning to set up a spider trap that will automatically add all bad robots IPs to my .htaccess file. The problem is that in order to achieve that, I will need to make my .htaccess file word-writable (together with another directory which will usually stay empty). Naturally, I don't feel comfortable with that. The .htaccess file is made non-readable for web browsers via httpd.conf (deny from all), but I assume it would still be an unsafe arrangement. Any ideas how I can make it safer without recompiling apache?
Question by:yosmc
LVL 23

Accepted Solution

Mysidia earned 1000 total points
ID: 11712367
Instead of having a web script edit your .htaccess file, have the script add their host to a special "data file"
that only contains a list of hostnames that were detected and possibly timestamps

And you setup a cron job to periodically verify the integrity of the "bad hosts file", and update the .htaccess file to ban the new hosts if its timestamp is newer than the .htaccess file.

And there are other ways like Unix sockets and IPC.
LVL 23

Expert Comment

ID: 11712382
Another alternative is to run scripts setuid... or use apache SuExec to run CGI scripts and the 'User' directive in the
server's httpd.conf so the scripts for certain hosts run as a different user.
See http://httpd.apache.org/docs/suexec.html
LVL 40

Expert Comment

ID: 11712385
Have the "spider trap" drop the bad IP's into a file and run a job from cron, say every 5 minutes, that retrieves any found IP's and adds them to the .htaccess file. The cron job would run as the user that owns the .htaccess file and thus it wouldn't need to be world writable.

Another approach would be to have the "spider trap" write the IP's to a FIFO implemented in say Perl that modifies the .htaccess file.
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LVL 51

Expert Comment

ID: 11713862
chmod 400 .htaccess

then write a script which does (pseudo code follows):
  chmod 600 .htaccess && echo "deny from IP" >> .htaccess; chmod 400 .htaccess

run this scipt as the same user as apache runs, probaly with cron as jlevie suggested
LVL 16

Expert Comment

ID: 11758546
why not make your script store the IP somewere and have a crontab
add the IPs to the file :) that way it will be more secure
LVL 23

Expert Comment

ID: 11760319
Few/none of these methods ensures that it's impossible for a third party to inject ip addresses into your ban list, if that's one of the attack models you need to secure against, then you need to use SuExec or a script setuid to a user others can't access or something similar in order to create a privilege isolation between your web scripts and any CGI scripts run/created by other users.

(Or configure apache so only trusted users can deploy CGI scripts)

note that things like 'chmod 600 file && write && chmod 400 file' create a race condition... not that mode 400 as oposed to 600 provides any assurance security, it provides none against any attack model where the user is able to run arbitrary code as the Apache user: like anyone who can deploy their own CGI scripts on the system can.

As for cron script reading a list of ips collected by the web script, here's a possibility...


my %badips;

# New badips list, to be writable by the web script
my $new_badip_file = '/home/blah/new_badips.dat';

# Stored database
my $old_badip_file = '/home/blah/banned_ips.dat';



sub write_htaccess
  my $file = shift;
  my $temp = $file . ".tmp$$";

  open HTACCESS, ">$temp" || die "Unable to open $temp";
  print HTACCESS q/
        order deny, allow

  for(keys %badips) {
      print HTACCESS "deny from " . $_ . "\n";

  print HTACCESS q%

  close HTACCESS;
  rename $temp, $file;

sub read_badips
  my $file = shift;
  open IPS, $file || die "Unable to open file $file";

  while ($line = <IPS>)
     if ($line =~ /^([0-9]+\.){3,}[0-9]+$/) {
         $badips{$line} = 1;
  close IPS;

sub write_badips
    my $out = shift;
    my $temp = "$out" . ".tmp$$";
    open(IPS, ">$temp") || die "Unable to open file $temp";

    for(keys %badips) {
        print IPS $_ . "\n";
    close IPS;
    rename $temp, $out;


Author Comment

ID: 11916591
Just an update that I am still working on the problem (and thanks for the suggestions so far). I was a little reluctant to enable .htaccess files (I'm the only user and I normally put everything into httpd.conf - makes things faster), so I was looking into perl modules (Apache::BlockAgent, Apache::BlockIP), but unfortunately couldn't get them to work.

Now I'm still looking into my options (maybe there are other more current modules; I also saw an approach that uses mySQL - but should I waste database resources to keep out the bots?) which is why I can't say yet which method for adding IPs will work best.

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