Common Attack Pattern Enumeration and Classification
A Community of Knowledge Resource for Building Secure Software
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 Execution Flow
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 \\172.16.2.100\shared_files\trojan_dll.dll.
Path Manipulation (CVE-1999-0073)
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.
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.
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.
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.
The activation zone is the server side function where the client controlled parameter is consumed.
Consuming an attacker-controlled parameter can defeat the normal process of the application.
[R.13.1] [REF-2] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.