Dynamic Variable Evaluation

Opis

In a language where the user can influence the name of a variable at runtime, if the variable names are not controlled, an attacker can read or write to arbitrary variables, or access arbitrary functions.

The resultant vulnerabilities depend on the behavior of the application, both at the crossover point and in any control/data flow that is reachable by the related variables or functions.

Inne nazwy

Dynamic evaluation

Skutki

• Confidentiality, Integrity, Availability

  Modify Application Data; Execute Unauthorized Code or Commands

  An attacker could gain unauthorized access to internal program variables and execute arbitrary code.

Zapobieganie

• Implementation · Refactoring

  Refactor the code to avoid dynamic variable evaluation whenever possible.

• Implementation · Input Validation

  Use only allowlists of acceptable variable or function names.

• Implementation

  For function names, ensure that you are only calling functions that accept the proper number of arguments, to avoid unexpected null arguments.

Przykłady podatności

• CVE-2009-0422 Chain: Dynamic variable evaluation allows resultant remote file inclusion and path traversal.
• CVE-2007-2431 Chain: dynamic variable evaluation in PHP program used to modify critical, unexpected $_SERVER variable for resultant XSS.
• CVE-2006-4904 Chain: dynamic variable evaluation in PHP program used to conduct remote file inclusion.
• CVE-2006-4019 Dynamic variable evaluation in mail program allows reading and modifying attachments and preferences of other users.

Słabość nadrzędna

• CWE-914 Improper Control of Dynamically-Identified Variables

Pokrewna

• CWE-183 Permissive List of Allowed Inputs

Kiedy powstaje

• Implementation

Dotyczy

PHP, Perl

Uwagi

Under-studied, probably under-reported. Few researchers look for this issue; most public reports are for PHP, although other languages are affected. This issue is likely to grow in PHP as developers begin to implement functionality in place of register_globals.

Źródła

1. Dynamic Evaluation Vulnerabilities in PHP applications — Steve Christey, Full-Disclosure, 2006
2. A Study In Scarlet: Exploiting Common Vulnerabilities in PHP Applications — Shaun Clowes