Mobile code: non-final public field

Description
This attack aims to manipulate non-final public variables used in mobile code by injecting malicious values on it, mostly in Java and C++ applications.

When a public member variable or class used in mobile code isn’t declared as final, its values can be malicious manipulated by any function that has access to it in order to extend the application code or acquire critical information about the application.

Severity
Medium to High

Likelihood of exploitation
Low

Examples
A Java applet from certain application is acquired and subverted by an attacker. Then, he makes the victim accepts and runs a Trojan or malicious code that was prepared to manipulate non-final objects’ state and behavior. This code is instantiated and executed continuously using default JVM on victim’s machine. When the victim invokes the Java applet from the original application using the same JVM, the malicious process could be mixed with original applet, thus it modifies values of non-final objects and executes under victim’s credentials.

In the following example, the class “any_class” is declared as final and “server_addr” variable is not:

public final class any_class extends class_Applet { public URL server_addr; … }

In this case, the value of “server_addr” variable could be set by any other function that has access to it, thus changing the application behavior. A proper way to declare this variable is:

public class any_class extends class_Applet { public final URL server_addr; … }

When a variable is declared as final its value cannot be modified.

External References
http://cwe.mitre.org/data/definitions/493.html – Mobile Code: non-final public field http://www.fortifysoftware.com/vulncat/ - Unsafe Mobile Code: Access Violation http://www.fortifysoftware.com/vulncat/ - Unsafe Mobile Code: Public finalize Method

Related Threats
Category: Logical Attacks

Related Attacks

 * Mobile code: invoking untrusted mobile code
 * Mobile code: object hijack

Related Vulnerabilities
Category: Unsafe Mobile Code

Related Countermeasures
Category: Access Control