Common Weakness Enumeration

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CWE-627: Dynamic Variable Evaluation

Weakness ID: 627
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ 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.
+ 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
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
+ Relevant to the view "Development Concepts" (CWE-699)
+ 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).
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.


PHP (Undetermined Prevalence)

Perl (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.


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
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.
+ Weakness Ordinalities
(where the weakness exists independent of other weaknesses)
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
MemberOfViewView884CWE Cross-section
MemberOfCategoryCategory990SFP Secondary Cluster: Tainted Input to Command
+ Notes

Research Gap

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
[REF-517] Steve Christey. "Dynamic Evaluation Vulnerabilities in PHP applications". Full-Disclosure. 2006-05-03. <>.
[REF-518] Shaun Clowes. "A Study In Scarlet: Exploiting Common Vulnerabilities in PHP Applications". <>.
+ Content History
Modification DateModifierOrganization
2008-07-01Eric DalciCigital
updated Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Applicable_Platforms, Relationships
2008-10-14CWE Content TeamMITRE
updated Background_Details, Description
2011-03-29CWE Content TeamMITRE
updated Description
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Common_Consequences, Relationships
2012-10-30CWE Content TeamMITRE
updated Potential_Mitigations
2013-02-21CWE Content TeamMITRE
updated Common_Consequences, Observed_Examples, Potential_Mitigations, Relationships, Weakness_Ordinalities
2014-07-30CWE Content TeamMITRE
updated Relationships
2017-11-08CWE Content TeamMITRE
updated References

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Page Last Updated: January 18, 2018