Incomplete Identification of Uploaded File Variables (PHP)

Description

This vulnerability occurs in PHP applications that rely on the deprecated, auto-registered global variables for file uploads (like $varname, $varname_size). Because these variables are not properly validated, an attacker can overwrite them to trick the application into processing malicious or unauthorized files.

Extended Description

In older PHP configurations, file uploads automatically created a set of global variables for each file. Attackers can exploit this by sending forged POST requests or manipulated cookies that overwrite these variables with their own values. Since the application trusts these globals without verification, it can be deceived into handling files it was never intended to, such as sensitive system files. To prevent this, developers must stop using these legacy global variables entirely. The secure alternative is to use the validated $_FILES superglobal array, which is populated directly by the PHP engine and is immune to this type of user-supplied data overwrite. Modern PHP versions have disabled the automatic creation of these global variables ('register_globals' is off by default), making this a legacy issue for outdated codebases.

Common Consequences 1

Scope: ConfidentialityIntegrity

Impact: Read Files or DirectoriesModify Files or Directories

Potential Mitigations 3

Phase: Architecture and Design

Use PHP 4 or later.

Phase: 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']))

Phase: 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.

Demonstrative Examples 2

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):

Code Example:Bad
PHP
php

"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."

Code Example:Bad
PHP
php

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

Observed Examples 3

CVE-2002-1460Forum 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-2002-1759Product doesn't check if the variables for an upload were set by uploading the file, or other methods such as $_POST.

CVE-2002-1710Product does not distinguish uploaded file from other files.

References 1

A Study in Scarlet - section 5, "File Upload"

Shaun Clowes

ID: REF-502

Applicable Platforms

Languages:

PHP : Undetermined

Modes of Introduction

Implementation

Related Weaknesses

ChildOf:

Insufficient Verification of Data Authenticity (CWE-345)

PeerOf:

PHP External Variable Modification (CWE-473)

Taxonomy Mapping

PLOVER
Software Fault Patterns