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CAPEC-13: Subverting Environment Variable Values

Subverting Environment Variable Values
Definition in a New Window Definition in a New Window
Attack Pattern ID: 13
Abstraction: Detailed
Status: Draft
Completeness: Complete
Presentation Filter:
+ Summary

The attacker directly or indirectly modifies environment variables used by or controlling the target software. The attacker's goal is to cause the target software to deviate from its expected operation in a manner that benefits the attacker.

+ Attack Steps
  1. The attacker probes the application for information. Which version of the application is running? Are there known environment variables? etc.

  1. The attacker gains control of an environment variable and ties to find out what process(es) the environment variable controls.

  1. The attacker modifies the environment variable to abuse the normal flow of processes or to gain access to privileged resources.

+ Attack Prerequisites
  • An environment variable is accessible to the user.

  • An environment variable used by the application can be tainted with user supplied data.

  • Input data used in an environment variable is not validated properly.

  • The variables encapsulation is not done properly. For instance setting a variable as public in a class makes it visible and an attacker may attempt to manipulate that variable.

+ Typical Severity

Very High

+ Typical Likelihood of Exploit

Likelihood: Very High

+ Methods of Attack
  • Injection
  • Modification of Resources
  • Protocol Manipulation
+ Examples-Instances


Environment variables

Changing the LD_LIBRARY_PATH environment variable in TELNET will cause TELNET to use an alternate (possibly Trojan) version of a function library. The Trojan library must be accessible using the target file system and should include Trojan code that will allow the user to log in with a bad password. This requires that the attacker upload the Trojan library to a specific location on the target.

As an alternative to uploading a Trojan file, some file systems support file paths that include remote addresses, such as \\\shared_files\trojan_dll.dll.

Related Vulnerabilities

Path Manipulation (CVE-1999-0073)

+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Low

In a web based scenario, the client controls the data that it submitted to the server. So anybody can try to send malicious data and try to bypass the authentication mechanism.

Skill or Knowledge Level: Medium

Skill or Knowledge Level: High

Some more advanced attacks may require knowledge about protocols and probing technique which help controlling a variable. The malicious user may try to understand the authentication mechanism in order to defeat it.

+ Probing Techniques

An attacker can intentionally modify the client side parameter and monitor how the server behaves in response to that modification. For instance an attacker will look at the cookie data, the URL parameters, the hidden variables in forms, variables used in system calls, etc.

If the client uses a program in binary format to connect to the server, disassembler can be used to identify parameter within the binary code, and then the attacker would try to simulate the client application and change some of the parameters sent to the server. For instance the attacker may find that a secret key or a path is hard coded in the binary client application.

Environment variables are frequently stored in cleartext configuration files. If the attacker can modify those configuration files, he can control the environment variables. Even a read access can potentially be dangerous since this may give sensitive information to perform this type of attack. Indeed knowing which environment variables the application uses is a prerequisite to this type of attack.

+ Obfuscation Techniques

The attacker may try to obfuscate its attempts to subvert the target process (such as authentication) by using valid values for the variable she controls. By using valid values the user tries to understand the authentication mechanism. This would be in preparation to a more serious attack.

+ Solutions and Mitigations

Protect environment variables against unauthorized read and write access.

Protect the configuration files which contain environment variables against illegitimate read and write access.

Assume all input is malicious. Create a white list that defines all valid input to the software system based on the requirements specifications. Input that does not match against the white list should not be permitted to enter into the system.

Apply the least privilege principles. If a process has no legitimate reason to read an environment variable do not give that privilege.

+ Attack Motivation-Consequences
ScopeTechnical ImpactNote
Execute unauthorized code or commands
Run Arbitrary Code
Bypass protection mechanism
Unexpected State
Read application data
+ Injection Vector

The client controlled parameter

+ Payload

The new value of the client controlled parameter.

+ Activation Zone

The activation zone is the server side function where the client controlled parameter is consumed.

+ Payload Activation Impact

Consuming an attacker-controlled parameter can defeat the normal process of the application.

+ Purposes
  • Penetration
+ CIA Impact
Confidentiality Impact: MediumIntegrity Impact: HighAvailability Impact: Low
+ Technical Context
Architectural Paradigms
+ References
[R.13.1] [REF-2] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.
[R.13.2] [REF-3] "Common Weakness Enumeration (CWE)". CWE-20 - Input Validation. Draft. The MITRE Corporation. 2007. <http://cwe.mitre.org/data/definitions/20.html>.
+ Content History
CAPEC Content TeamThe MITRE Corporation2014-06-23Internal_CAPEC_Team
CAPEC Content TeamThe MITRE Corporation2017-01-09Updated Related_Attack_PatternsInternal

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Page Last Updated or Reviewed: August 04, 2017