CWE - CWE-627: Dynamic Variable Evaluation (4.4)

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)

Nature	Type	ID	Name
ChildOf	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	914	Improper Control of Dynamically-Identified Variables
PeerOf	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	183	Permissive List of Allowed Inputs

Relevant to the view "Software Development" (CWE-699)

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1006	Bad Coding Practices

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 life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

Phase	Note
Implementation

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.

Languages

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.

Scope	Impact	Likelihood
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

Reference	Description
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.

Potential Mitigations

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.

Weakness Ordinalities

Ordinality	Description
Primary	(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.

Nature	Type	ID	Name
MemberOf	View - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).	884	CWE Cross-section
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	990	SFP 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. <http://seclists.org/fulldisclosure/2006/May/0035.html>.

[REF-518] Shaun Clowes. "A Study In Scarlet: Exploiting Common Vulnerabilities in PHP Applications". <http://www.securereality.com.au/studyinscarlet.txt>.

Content History

Submissions
Submission Date	Submitter	Organization
2007-05-07	CWE Content Team	MITRE
2007-05-07
Modifications
Modification Date	Modifier	Organization
2008-07-01	Eric Dalci	Cigital
2008-07-01	updated Time_of_Introduction
2008-09-08	CWE Content Team	MITRE
2008-09-08	updated Applicable_Platforms, Relationships
2008-10-14	CWE Content Team	MITRE
2008-10-14	updated Background_Details, Description
2011-03-29	CWE Content Team	MITRE
2011-03-29	updated Description
2011-06-01	CWE Content Team	MITRE
2011-06-01	updated Common_Consequences
2012-05-11	CWE Content Team	MITRE
2012-05-11	updated Common_Consequences, Relationships
2012-10-30	CWE Content Team	MITRE
2012-10-30	updated Potential_Mitigations
2013-02-21	CWE Content Team	MITRE
2013-02-21	updated Common_Consequences, Observed_Examples, Potential_Mitigations, Relationships, Weakness_Ordinalities
2014-07-30	CWE Content Team	MITRE
2014-07-30	updated Relationships
2017-11-08	CWE Content Team	MITRE
2017-11-08	updated References
2020-02-24	CWE Content Team	MITRE
2020-02-24	updated Relationships
2020-06-25	CWE Content Team	MITRE
2020-06-25	updated Potential_Mitigations


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Common Weakness Enumeration

CWE-627: Dynamic Variable Evaluation