Variable Extraction Error

Description

This vulnerability occurs when an application uses unvalidated external input to dynamically select which variables to populate with data. Without proper checks, this can allow an attacker to overwrite critical internal variables, leading to unexpected behavior or security breaches.

Extended Description

In languages like PHP, functions such as `extract()` or `import_request_variables()` can mimic the dangerous behavior of the deprecated `register_globals` feature if called incorrectly. An attacker can manipulate input to overwrite global variables, including superglobals like `$_SESSION` or `$_GET`, potentially bypassing security controls or altering application logic. This pattern is not exclusive to PHP. Many interpreted languages and custom frameworks offer similar dynamic variable assignment features. Developers must always validate or whitelist which variable names are allowed to be set externally, treating user input as data, not as code that defines the program's structure.

Common Consequences 1

Scope: Integrity

Impact: Modify Application Data

An attacker could modify sensitive data or program variables.

Potential Mitigations 3

Phase: Implementation

Strategy: Input Validation

Use allowlists of variable names that can be extracted.

Phase: Implementation

Consider refactoring your code to avoid extraction routines altogether.

Phase: Implementation

In PHP, call extract() with options such as EXTR_SKIP and EXTR_PREFIX_ALL; call import_request_variables() with a prefix argument. Note that these capabilities are not present in all PHP versions.

Demonstrative Examples 1

ID : DX-107

This code uses the credentials sent in a POST request to login a user.

Code Example:

Bad

PHP

//Log user in, and set $isAdmin to true if user is an administrator*

php

The call to extract() will overwrite the existing values of any variables defined previously, in this case $isAdmin. An attacker can send a POST request with an unexpected third value "isAdmin" equal to "true", thus gaining Admin privileges.

Observed Examples 5

CVE-2006-7135extract issue enables file inclusion

CVE-2006-7079Chain: PHP app uses extract for register_globals compatibility layer (Variable Extraction Error), enabling path traversal (Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal'))

CVE-2007-0649extract() buried in include files makes post-disclosure analysis confusing; original report had seemed incorrect.

CVE-2006-6661extract() enables static code injection

CVE-2006-2828import_request_variables() buried in include files makes post-disclosure analysis confusing

Applicable Platforms

Languages:

PHP : Undetermined

Modes of Introduction

Implementation

Alternate Terms

Variable overwrite

Related Weaknesses

ChildOf:

Improper Control of Dynamically-Identified Variables (CWE-914)

CanPrecede:

Modification of Assumed-Immutable Data (MAID) (CWE-471)

Taxonomy Mapping

Software Fault Patterns

Notes

Research GapProbably under-reported for PHP. Seems under-studied for other interpreted languages.