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Re^2: Debugging cgi-bin script

by walkingthecow (Friar)
on Jan 06, 2013 at 01:29 UTC ( #1011820=note: print w/ replies, xml ) Need Help??


in reply to Re: Debugging cgi-bin script
in thread Debugging cgi-bin script

Here's the full code:

#!/usr/local/perl5/bin/perl use strict; use diagnostics; $|++; $ENV{PATH} = "/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/ +sbin"; use CGI qw(:all delete_all escapeHTML); if (my $session = param('session')) { # returning to pick up session d +ata my $cache = get_cache_handle(); my $data = $cache->get($session); unless ($data and ref $data eq "ARRAY") { # something is wrong show_form(); exit 0; } print header; print start_html(-title => "Traceroute Results", ($data->[0] ? () : (-head => ["<meta http-equiv=refresh content=5>"])) +); print h1("Traceroute Results"); print pre(escapeHTML($data->[1])); print p(i("... working ...")) unless $data->[0]; print end_html; } elsif (my $host = param('host')) { # returning to select host if ($host =~ /^([a-zA-Z0-9.\-]{1,100})\z/) { # create a session $host = $1; # untainted now print h1("Yes"); my $session = get_session_id(); my $cache = get_cache_handle(); $cache->set($session, [0, ""]); # no data yet if (my $pid = fork) { # parent does delete_all(); # clear parameters param('session', $session); print redirect(self_url()); } elsif (defined $pid) { # child does close STDOUT; # so parent can go on unless (open F, "-|") { open STDERR, ">&=1"; exec "/usr/sbin/traceroute", $host; die "Cannot execute traceroute: $!"; } my $buf = ""; while (<F>) { $buf .= $_; $cache->set($session, [0, $buf]); } $cache->set($session, [1, $buf]); exit 0; } else { die "Cannot fork: $!"; } } else { show_form(); } } else { # display form show_form(); } exit 0; sub show_form { print header, start_html("Traceroute"), h1("Traceroute"); print start_form; print submit('traceroute to this host:'), " ", textfield('host'); print end_form, end_html; } sub get_cache_handle { require Cache::FileCache; Cache::FileCache->new ({ namespace => 'traceroute', username => 'nobody', default_expires_in => '30 minutes', auto_purge_interval => '4 hours', }); } sub get_session_id { require Digest::MD5; Digest::MD5::md5_hex(Digest::MD5::md5_hex(time().{}.rand().$$)); }


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Re^3: Debugging cgi-bin script
by quester (Vicar) on Jan 06, 2013 at 08:21 UTC

    This may be a bit off topic, but...

    Designing your own random number generator in a high-level language is a terrible, terrible idea. There just isn't any way for a normal process to get access to as much entropy as the operating system can gather from timing I/O completions.

    How many bits of entropy are actually in the return from get_session_id? Let's add it up: essentially nothing from the call to time() (because the attacker knows what time it is), about 13 bits from the memory address from {} (estimated on perl 5.14.3 on Linux 3.6), 32 bits from the call to rand() (because an strace shows that perl seeded it by reading four bytes from /dev/urandom), and 15 bits at most from $$ (unless you change /proc/sys/kernel/pid_max and start a lot of processes on your system).

    That's at most 60 bits of randomness that get_session_id tries to magically inflate into 128 bits by calling Digest::MD5::md5_hex a second time. Whatever the second call was intended to do, it's not going to be able to do it.

    Both Solaris and RHEL have had /dev/urandom for a long time; it became standard in Solaris 9 in 2002 and was available as a patch since 2.6 in 1997. It's been standard in every release of RHEL, and was in the old pre-RHEL Red Hat since 4.0 in 1996.

    To make sure your /dev/urandom is working, try

    od -x /dev/urandom | head

    To use it in your script, try this:

    sub get_session_id { require Digest::MD5; open my $ur, "<", "/dev/urandom" or die "Cannot open /dev/urandom, $!"; my $buflen = sysread( $ur, my $buf, 16 ); defined $buflen or die "Failed to read /dev/urandom, $!"; $buflen == 16 or die "Tried to read 16 bytes from /dev/urandom but got $bufl +en"; Digest::MD5::md5_hex( $buf ); }

    Incidentally, never use /dev/random instead of /dev/urandom in this application. For a web server, it exposes the server to a denial-of-service attack where the attacker removes entropy from the system-wide pool, by starting new sessions, faster than entropy is added to the pool, by (mostly) I/O completions.

      Designing your own random number generator in a high-level language is a terrible, terrible idea.

      :) FWIW, merlyn didn't design it, he copied from the fallback Apache::Session::Generate::MD5

      I don't know from entrophy and randomness, but this isn't encryption we're dealing with, no authentication or authorization, no financial transactions -- if the attacker has access to the application, guessing doesn't get him anything he didn't already have access to

      You might like Re^3: Randomness encountered with CGI Session where afoken talks bits

      FYI/FMI Session::Token - Portable, secure, efficient, simple random session token generation that satisfies those OWASP recommendations

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