Dynamic Variable Evaluation
Incomplete Variant
Structure: Simple
Description
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.
Common Consequences 1
Scope: ConfidentialityIntegrityAvailability
Impact: Modify Application DataExecute Unauthorized Code or Commands
An attacker could gain unauthorized access to internal program variables and execute arbitrary code.
Potential Mitigations 3
Phase: Implementation
Strategy: Refactoring
Refactor the code to avoid dynamic variable evaluation whenever possible.
Phase: Implementation
Strategy: Input Validation
Use only allowlists of acceptable variable or function names.
Phase: Implementation
For function names, ensure that you are only calling functions that accept the proper number of arguments, to avoid unexpected null arguments.
Observed Examples 4
CVE-2009-0422Chain: Dynamic variable evaluation allows resultant remote file inclusion and path traversal.
CVE-2007-2431Chain: dynamic variable evaluation in PHP program used to modify critical, unexpected $_SERVER variable for resultant XSS.
CVE-2006-4904Chain: dynamic variable evaluation in PHP program used to conduct remote file inclusion.
CVE-2006-4019Dynamic variable evaluation in mail program allows reading and modifying attachments and preferences of other users.
References 2
Dynamic Evaluation Vulnerabilities in PHP applications
Steve Christey
Full-Disclosure
03-05-2006
https://seclists.org/fulldisclosure/2006/May/35(2023-04-07)
ID: REF-517
A Study In Scarlet: Exploiting Common Vulnerabilities in PHP Applications
Shaun Clowes
https://securereality.com.au/study-in-scarlett/(2023-04-07)
ID: REF-518
Applicable Platforms
Languages:
PHP : UndeterminedPerl : Undetermined
Modes of Introduction
Implementation
Alternate Terms
Dynamic evaluation
Related Weaknesses
Notes
Research GapUnder-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.