Safe Coding: Avoid Problems Caused by CWE 416 – Free Use in Java

The use after free (UAF) vulnerability described in Common Weakness Enumeration CWE-416 occurs when a program continues to use a pointer after it has been freed. This can open security vulnerabilities and lead to undefined behavior, including crashes and even data corruption. The problem arises because the memory referenced by the pointer may be reallocated for other purposes. This in turn can be exploited by attackers.

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Reasons for CWE-416 – Use Free (UAF)

Incorrect memory management: Forgot to invalidate pointers after freeing memory.

Hanging hands: Maintain references to shared storage and access them later.

Dual release: Memory is released more than once, resulting in multiple operations at the same location.

Possible results of CWE-416

Security Vulnerabilities: Attackers can exploit UAF to execute arbitrary code, escalate privileges, or cause a denial of service (DoS).

data corruption: UAF may result in unexpected data changes resulting in program instability or irregular behavior.

Program Crash: Accessing shared memory may cause segmentation or other runtime errors.

Meaning of CWE-416 in Java

Through automatic garbage collection, Java naturally avoids many types of memory management problems, including use-after-free vulnerabilities. However, problems like UAF in Java can still occur if resources other than memory (such as file handles or network connections) are mismanaged. Below are some scenarios in Java that are similar to UAF and strategies to mitigate them.

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Scenario 1: Mismanagement of external resources

Although memory in Java is managed by the garbage collector (GC), other resources such as file handles or network sockets can be mismanaged, causing post-closure usage problems.

Code example:

import java.io.*;

public class UseAfterFreeExample {
    public static void main(String() args) {
        FileInputStream fis = null;
        try {
            fis = new FileInputStream("example.txt");
            // Use the file input stream...
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

        // Attempt to use the file input stream after it has been closed
        try {
            int data = fis.read();  // This will throw an IOException
        } catch (IOException e) {
            System.out.println("Attempted to read from a closed stream.");
        }
    }
}

In this example, an attempt to read from the resource after closing fails FileInputStream to one IOException,

Automated resource management with try-with-resources:

Use the try-with-resource statement, introduced in Java 7, which ensures that each resource is closed at the end of the statement.

import java.io.*;

   public class UseAfterFreeFixed {
       public static void main(String() args) {
           try (FileInputStream fis = new FileInputStream("example.txt")) {
               // Use the file input stream...
           } catch (IOException e) {
               e.printStackTrace();
           }

           // fis is automatically closed here
           // Attempting to use fis here will cause a compilation error
       }
   }

Clear declaration of emptiness:

Set references to zero after closing to avoid accidental reuse.

   import java.io.*;

   public class UseAfterFreeWithNull {
       public static void main(String() args) {
           FileInputStream fis = null;
           try {
               fis = new FileInputStream("example.txt");
               // Use the file input stream...
           } catch (IOException e) {
               e.printStackTrace();
           } finally {
               if (fis != null) {
                   try {
                       fis.close();
                   } catch (IOException e) {
                       e.printStackTrace();
                   } finally {
                       fis = null;  // Prevent reuse
                   }
               }
           }

           // fis is now null, so the following attempt 
           // to use it will fail at compile-time
           if (fis != null) {
               try {
                   int data = fis.read();  // This will not compile since fis is null
               } catch (IOException e) {
                   System.out.println("Attempted to read from a closed stream.");
               }
           }
       }
   }

Scenario 2: Improper use of weak references

Although this is not directly a UAF issue, misuse of weak references can have negative consequences when objects are accessed after they have been reclaimed by the garbage collector.

import java.lang.ref.WeakReference;

public class WeakReferenceExample {
    public static void main(String() args) {
        Object obj = new Object();
        WeakReference

In this example, after invalidating the strong reference and proposing garbage collection, accesses to the weak reference may return zero, indicating that the object has already been collected by the GC. Avoid using weak references to important objects that are still needed.

Always check if any weak references have been removed before using:

   if (weakRef.get() != null) {
       // Safe to use the object
       Object obj = weakRef.get();
       // Use obj
   } else {
       // Handle the case where the object has been collected
       System.out.println("Object has been garbage collected.");
   }

While Java’s automatic memory management reduces the risk of traditional UAF vulnerabilities, developers still need to be careful with resource management and weak references to avoid similar problems. Features like modern Java Try-with-Ressourcen And respecting object references can help maintain robust and error-free code.

Which CVEs are based on CWE-416 in Java

Here are some CVEs to consider related to the CWE-416 vulnerabilities in Java:

CVE-2022-45146

This vulnerability is found in the Bouncy Castle BC-FJA FIPS Java API before version 1.0.2.4. Changes to the JVM garbage collector in Java 13 and later versions can trigger this problem and cause temporary keys used by modules to be set to null while they are still in use. This leads to errors or possible loss of information. FIPS compliant users are not affected as this applies to Java 7, 8, and 11.

CVE-2023-38703

This CVE affects the PJSIP multimedia communications library, which is written in multiple languages, including Java. The problem occurs when synchronization is not maintained between higher level SRTP media and lower level transports such as UDP – resulting in a UAF vulnerability. This may result in unexpected application termination or memory corruption.

The examples described illustrate the importance of proper memory and resource management in Java applications, even though the language manages memory through garbage collection. More information about these vulnerabilities can be found in the National Vulnerability Database (NVD) or in specific notices on platforms such as GitHub and mvnrepository.