PSF-2005-001 - SimpleXMLRPCServer.py allows unrestricted traversal

Python Security Advisory


Advisory ID:  PSF-2005-001
Issue Date:   February 3, 2005
Product:      Python
Versions:     2.2 all versions, 2.3 prior to 2.3.5, 2.4
CVE Names:    CAN-2005-0089

Python is an interpreted, interactive, object-oriented programming language. It is often compared to Tcl, Perl, Scheme or Java.

The Python development team has discovered a flaw in the SimpleXMLRPCServer library module which can give remote attackers access to internals of the registered object or its module or possibly other modules. The flaw only affects Python XML-RPC servers that use the register_instance() method to register an object without a _dispatch() method. Servers using only register_function() are not affected.

On vulnerable XML-RPC servers, a remote attacker may be able to view or modify globals of the module(s) containing the registered instance's class(es), potentially leading to data loss or arbitrary code execution. If the registered object is a module, the danger is particularly serious. For example, if the registered module imports the os module, an attacker could invoke the os.system() function.

But the attack is not limited to registered object modules; for example, the code in the Python cookbook recipe at http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/165375 is vulnerable to an attack using im_func.func_globals.update which allows reading or modifying the global variable accessList.

The Common Vulnerabilities and Exposures project (cve.mitre.org) has assigned the name CAN-2005-0089 to this issue.

Python 2.3.5 will be released from www.python.org within a few days containing a fix for this issue. Python 2.4.1 will be released later this month containing the same fix. Patches for Python 2.2, 2.3 and 2.4 are also immediately available:

http://python.org/security/PSF-2005-001/patch-2.2.txt (Python 2.2)
http://python.org/security/PSF-2005-001/patch.txt (Python 2.3, 2.4)

Note that these patches disable recursive traversal, potentially resulting in reduced functionality of XML-RPC applications depending on this feature.

Acknowledgement: thanks to Graham Dumpleton for reporting this issue.

Postscripts

The fix for this problem has been incorporated into Python 2.3.5, which was released on Feb 8th, 2005, and into Python 2.4.1, which was released March 30th, 2005.