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CWE-627: Dynamic Variable Evaluation

 
Dynamic Variable Evaluation
Weakness ID: 627 (Weakness Base)Status: Incomplete
+ Description

Description Summary

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.

Extended Description

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.

+ Alternate Terms
Dynamic evaluation
+ Time of Introduction
  • Implementation
+ Applicable Platforms

Languages

PHP

Perl

+ Common Consequences
ScopeEffect
Confidentiality
Integrity
Availability

Technical Impact: Modify application data; Execute unauthorized code or commands

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

+ Observed Examples
ReferenceDescription
Chain: Dynamic variable evaluation allows resultant remote file inclusion and path traversal.
Chain: dynamic variable evaluation in PHP program used to modify critical, unexpected $_SERVER variable for resultant XSS.
Chain: dynamic variable evaluation in PHP program used to conduct remote file inclusion.
Dynamic variable evaluation in mail program allows reading and modifying attachments and preferences of other users.
+ Potential Mitigations

Phase: Implementation

Strategy: Refactoring

Refactor the code to avoid dynamic variable evaluation whenever possible.

Phase: Implementation

Strategy: Input Validation

Use only whitelists 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.

+ Background Details

Many interpreted languages support the use of a "$$varname" construct to set a variable whose name is specified by the $varname variable. In PHP, these are referred to as "variable variables." Functions might also be invoked using similar syntax, such as $$funcname(arg1, arg2).

+ Weakness Ordinalities
OrdinalityDescription
Primary
(where the weakness exists independent of other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness BaseWeakness Base914Improper Control of Dynamically-Identified Variables
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory990SFP Secondary Cluster: Tainted Input to Command
Software Fault Pattern (SFP) Clusters (primary)888
PeerOfWeakness BaseWeakness Base183Permissive Whitelist
Research Concepts1000
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
+ Research Gaps

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.

+ References
Steve Christey. "Dynamic Evaluation Vulnerabilities in PHP applications". Full-Disclosure. 2006-05-03. <http://seclists.org/fulldisclosure/2006/May/0035.html>.
Shaun Clowes. "A Study In Scarlet: Exploiting Common Vulnerabilities in PHP Applications". <http://www.securereality.com.au/studyinscarlet.txt>.
+ Content History
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time_of_Introduction
2008-09-08CWE Content TeamMITREInternal
updated Applicable_Platforms, Relationships
2008-10-14CWE Content TeamMITREInternal
updated Background_Details, Description
2011-03-29CWE Content TeamMITREInternal
updated Description
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2012-05-11CWE Content TeamMITREInternal
updated Common_Consequences, Relationships
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations
2013-02-21CWE Content TeamMITREInternal
updated Common_Consequences, Observed_Examples, Potential_Mitigations, Relationships, Weakness_Ordinalities
2014-07-30CWE Content TeamMITREInternal
updated Relationships
Page Last Updated: July 30, 2014