CWE-616: Incomplete Identification of Uploaded File Variables (PHP)

Description

The PHP application uses an old method for processing uploaded files by referencing the four global variables that are set for each file (e.g. $varname, $varname_size, $varname_name, $varname_type). These variables could be overwritten by attackers, causing the application to process unauthorized files.

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".

Potential Impact

Confidentiality, Integrity

Read Files or Directories, Modify Files or Directories

Demonstrative Examples

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):

Bad

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

"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."

Bad

$_FILES['userfile']['name'] - original filename from client$_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.

Mitigations & Prevention

Architecture and Design

Use PHP 4 or later.

Architecture and Design

If you must support older PHP versions, write your own version of is_uploaded_file() and run it against $HTTP_POST_FILES['userfile']))

Implementation

For later PHP versions, reference uploaded files using the $HTTP_POST_FILES or $_FILES variables, and use is_uploaded_file() or move_uploaded_file() to ensure that you are dealing with an uploaded file.

Real-World CVE Examples

CVE ID	Description
CVE-2002-1460	Forum does not properly verify whether a file was uploaded or if the associated variables were set by POST, allowing remote attackers to read arbitrary files.
CVE-2002-1759	Product doesn't check if the variables for an upload were set by uploading the file, or other methods such as $_POST.
CVE-2002-1710	Product does not distinguish uploaded file from other files.

Taxonomy Mappings

PLOVER: — Incomplete Identification of Uploaded File Variables (PHP)
Software Fault Patterns: SFP25 — Tainted input to variable

Frequently Asked Questions

What is CWE-616?

CWE-616 (Incomplete Identification of Uploaded File Variables (PHP)) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Variant-level weakness. The PHP application uses an old method for processing uploaded files by referencing the four global variables that are set for each file (e.g. $varname, $varname_size, $varname_name, $varname_type). T...

How can CWE-616 be exploited?

Attackers can exploit CWE-616 (Incomplete Identification of Uploaded File Variables (PHP)) to read files or directories, modify files or directories. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-616?

Key mitigations include: Use PHP 4 or later.

What is the severity of CWE-616?

CWE-616 is classified as a Variant-level weakness (Low-Medium abstraction). It has been observed in 3 real-world CVEs.