Incomplete Identification of Uploaded File Variables (PHP)

Description

These global variables could be overwritten by POST requests, cookies, or other methods of populating or overwriting these variables. This could be used to read or process arbitrary files by providing values such as "/etc/passwd".

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

As of 2006, the "four globals" method is probably in sharp decline, but older PHP applications could have this issue.

In the "four globals" method, PHP sets the following 4 global variables (where "varname" is application-dependent):

$varname = name of the temporary file on local machine
$varname_size = size of file
$varname_name = original name of file provided by client
$varname_type = MIME type of the file

Example Two

"The global $_FILES exists as of PHP 4.1.0 (Use $HTTP_POST_FILES instead if using an earlier version). These arrays will contain all the uploaded file information."

$_FILES['userfile']['name'] - original filename from client
$_FILES['userfile']['tmp_name'] - the temp filename of the file on the server

** note: 'userfile' is the field name from the web form; this can vary.

See Also