CWE-492: Use of Inner Class Containing Sensitive Data

What is Use of Inner Class Containing Sensitive Data?

• Overview: Use of Inner Class Containing Sensitive Data refers to a vulnerability where inner classes in Java, which appear private, can be accessed at package scope level due to how they are compiled to bytecode, potentially exposing sensitive data.

• Exploitation Methods:

• Attackers can exploit this by accessing the compiled peer class that holds the inner class, gaining access to sensitive fields.
• Common attack patterns include reverse engineering bytecode to identify and access inner classes that handle private data.

• Security Impact:

• Direct consequences include unauthorized access to private fields and methods intended to be secure.
• Potential cascading effects could lead to further exposure of sensitive information and compromise of application integrity.
• Business impact might involve data breaches, loss of customer trust, and potential legal liabilities.

• Prevention Guidelines:

• Specific code-level fixes include avoiding the use of inner classes for handling sensitive data or ensuring proper encapsulation.
• Security best practices involve reviewing the design to limit access exposure and using access modifiers effectively.
• Recommended tools and frameworks include using static analysis tools to detect potential vulnerabilities and adhering to secure coding practices outlined in Java security guidelines.

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Technical Details

Likelihood of Exploit: Medium

Affected Languages: Java

Affected Technologies: Not specified

Vulnerable Code Example

// This is a vulnerable example of using an inner class to store sensitive data
public class SensitiveDataManager {
    // Sensitive data that should be kept private
    private String secretKey = "SuperSecretKey123";

    // Vulnerable inner class that exposes sensitive data
    class InnerSensitiveData {
        // Directly accessing the outer class's sensitive data
        public String getSecretKey() {
            return secretKey; // Exposes the sensitive data directly
        }
    }
}

Explanation:

• Security Issue: The InnerSensitiveData inner class has direct access to the secretKey of its enclosing class. This access can lead to accidental exposure of sensitive information if the inner class is accessed from outside the SensitiveDataManager class.
• Impact: Unauthorized access to sensitive data (secretKey) can occur, compromising security.

How to fix Use of Inner Class Containing Sensitive Data?

To address this vulnerability, sensitive data should be encapsulated properly, and access should be controlled. Static nested classes should be used instead of inner classes to prevent implicit access to the outer class's instance.

Specific Fixes:

1. Encapsulation: Maintain the privacy of sensitive data and control access through well-defined methods.
2. Use Static Nested Classes: If nested functionality is required, use static nested classes to eliminate implicit references to the outer class's instance.

Fixed Code Example

// Fixed code with properly encapsulated sensitive data
public class SecureDataManager {
    // Sensitive data is kept private and access is strictly controlled
    private String secretKey = "SuperSecretKey123";

    // Static nested class, which does not have an implicit reference to the outer class's instance
    static class InnerSecureData {
        // No direct access to the outer class's sensitive data
        // Operations on sensitive data should be done through controlled methods
    }

    // Controlled method to access sensitive data
    public String getEncryptedSecretKey() {
        // Encrypt or obfuscate the key before returning
        return encrypt(secretKey);
    }

    // Example encryption method (for demonstration purposes)
    private String encrypt(String data) {
        // Implement a proper encryption algorithm here
        return "Encrypted" + data; // Placeholder for encryption logic
    }
}

Explanation:

• Static Nested Class: By using a static nested class (InnerSecureData), implicit access to the outer class's instance and its data is avoided.
• Controlled Access: The getEncryptedSecretKey method provides a controlled mechanism to access the sensitive data, ensuring it is encrypted or obfuscated before being exposed.
• Encapsulation: Sensitive data is encapsulated within SecureDataManager, and access is only available through methods that enforce security measures.

This approach mitigates the risk of exposing sensitive data inadvertently, adhering to secure coding practices.