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.

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:

– Implementation

Related Weaknesses

CWE-914
CWE-183

Consequences

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.

Potential Mitigations

Phase: Implementation

Description: 

Refactor the code to avoid dynamic variable evaluation whenever possible.

Phase: Implementation

Description: 

Use only allowlists of acceptable variable or function names.

Phase: Implementation

Description: 

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

CVE References

• 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.