CWE-586: Explicit Call to Finalize()

What is Explicit Call to Finalize()?

• Overview: This vulnerability occurs when Java code explicitly calls an object's finalize() method from outside the finalizer, which is typically not intended. This can lead to multiple invocations of finalize(), causing unpredictable behavior.

• Exploitation Methods:

• Attackers can exploit this by triggering resource cleanup prematurely or multiple times, potentially leading to inconsistent object states.
• Common attack patterns include manipulating object lifecycle and resource management to cause unexpected behavior.

• Security Impact:

• Direct consequences include resource leaks or improper resource deallocation.
• Potential cascading effects include application crashes or data corruption due to inconsistent states.
• Business impact may involve compromised application reliability, leading to user dissatisfaction and increased maintenance costs.

• Prevention Guidelines:

• Avoid calling finalize() explicitly in your code; rely on the garbage collector to handle object finalization.
• Follow security best practices by using try-with-resources or finally blocks for resource management.
• Recommended tools and frameworks include static analysis tools that can detect improper finalizer calls in codebases.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Java

Affected Technologies: Not specified

Vulnerable Code Example

Java Example

public class ResourceHandler {
    // Simulated resource
    private boolean isResourceReleased = false;
    
    // Vulnerable method calling finalize explicitly
    public void releaseResource() {
        // Explicitly calling finalize, which should not be done
        finalize();  // This is a security risk
    }
    
    // Finalizer to release the resource
    @Override
    protected void finalize() {
        if (!isResourceReleased) {
            System.out.println("Releasing resource in finalize.");
            isResourceReleased = true;
        }
    }
}

Explanation:

• Line 10: The code explicitly calls the finalize() method, which is a security risk. The finalize() method is intended to be called by the garbage collector to allow an object to clean up resources before being reclaimed. Calling it explicitly can lead to unpredictable behavior, resource leaks, or security issues if the object is not properly set up for finalization. Additionally, relying on finalize() for resource management can lead to performance issues and is deprecated in recent Java versions.

How to fix Explicit Call to Finalize()?

To fix this vulnerability:

1. Remove explicit calls to finalize(): The finalize() method should only be called by the Java garbage collector. Manually invoking it can bypass the normal garbage collection process, leading to potential resource leaks or inconsistent program states.
2. Implement a proper resource cleanup method: Design and implement a method such as close() or dispose(), which should be explicitly called by the developer when the resources need to be released.
3. Use try-with-resources or finally blocks: Where applicable, use the try-with-resources statement or finally blocks to ensure resources are automatically cleaned up.

Fixed Code Example

public class ResourceHandler implements AutoCloseable {
    // Simulated resource
    private boolean isResourceReleased = false;
    
    // Proper method to release resources
    public void close() {
        if (!isResourceReleased) {
            System.out.println("Releasing resource in close.");
            isResourceReleased = true;
        }
    }
    
    // Implement AutoCloseable to support try-with-resources
    @Override
    public void close() {
        close();
    }
}

Explanation:

• Lines 1-12: Introduced a close() method as part of the AutoCloseable interface, which is explicitly called to release resources. This pattern ensures that resource management is explicit and predictable.
• Line 11: Implemented the AutoCloseable interface to leverage try-with-resources for automatic resource management.
• Removed the finalize() method entirely since it's no longer necessary and inappropriate for explicit resource cleanup. This adheres to modern Java best practices, ensuring that resources are managed deterministically and efficiently.