What is Double Free (CWE-415)?

TL;DR: Double Free (CWE-415) is a memory management vulnerability in C/C++ where the same pointer is freed twice, leading to crashes or arbitrary code execution. Fix it by ensuring each allocation is freed only once.

What is Double Free?

Double Free (CWE-415) is a type of memory management vulnerability that occurs when the same pointer is freed twice. As defined by the MITRE Corporation under CWE-415, and classified by the OWASP Foundation under [mapping]…

Quick Summary

Double Free vulnerabilities occur in C/C++ applications where a program calls free() twice on the same memory address. This can lead to severe consequences such as crashes or arbitrary code execution. Jump to: What is Double Free? · Quick Summary · Overview · How It Works · Business Impact · Attack Scenario · Detection · Fixes

Jump to: Quick Summary · Double Free Overview · How Double Free Works · Business Impact of Double Free · Double Free Attack Scenario · How to Detect Double Free · How to Fix Double Free · Framework-Specific Fixes for Double Free · How to Ask AI to Check Your Code for Double Free · Double Free Best Practices Checklist · Double Free FAQ · Vulnerabilities Related to Double Free · References · Scan Your Own Site

Double Free Overview

What

Double Free (CWE-415) occurs when a program calls free() twice on the same memory address.

Why it matters

This can lead to severe consequences such as crashes or arbitrary code execution, compromising system integrity and availability.

Where it occurs

It primarily affects C/C++ applications that manage memory manually without proper tracking mechanisms.

Who is affected

Developers of C/C++ applications who do not properly manage memory allocations and deallocations are at risk.

Who is NOT affected

Applications using automatic memory management, such as Java or Python, are generally immune to this issue.

How Double Free Works

Root Cause

Double Free occurs when a program calls free() twice on the same memory address without proper tracking mechanisms in place.

Attack Flow

1. The attacker identifies a pointer that has been freed.
2. The attacker triggers another call to free() with the same pointer.
3. Memory management data structures become corrupted, leading to crashes or arbitrary code execution.

Prerequisites to Exploit

• The application must be using manual memory management in C/C++.
• There must be an opportunity for a second free() call on the same address.

Vulnerable Code

#include <stdlib.h>

void example_function(char* ptr) {
    // Free the pointer once
    free(ptr);
    
    // Free the pointer again (double free)
    free(ptr);
}

This code is vulnerable because it calls free() twice on the same memory address.

Secure Code

#include <stdlib.h>

void example_function(char* ptr) {
    if (ptr != NULL) {
        free(ptr); // Free the pointer once
        ptr = NULL; // Set pointer to NULL after freeing
    }
}

This code is secure because it sets the pointer to NULL after freeing, preventing a second call to free().

Business Impact of Double Free

Integrity

Corrupted memory can lead to unexpected behavior and data modification.

Availability

Crashes due to corrupted memory management structures disrupt system availability.

Financial

Potential for significant financial loss from downtime or security breaches.

Compliance

Non-compliance with security standards and regulations, leading to legal penalties.

Double Free Attack Scenario

1. The attacker identifies a pointer that has been freed in the application’s codebase.
2. They trigger another call to free() on the same address through an error condition or crafted input.
3. This leads to memory corruption and potential execution of arbitrary code.

How to Detect Double Free

Manual Testing

• Review code for patterns where the same pointer is freed multiple times.
• Check error handling paths for double free opportunities.
• Verify that pointers are set to NULL after freeing them.

Automated Scanners (SAST / DAST)

Static analysis can identify instances of free() called twice on the same address. Dynamic testing may be needed to confirm actual exploitation scenarios.

PenScan Detection

PenScan’s scanner engines, such as ZAP and Nuclei, detect potential Double Free vulnerabilities in C/C++ applications.

False Positive Guidance

False positives occur when a tool misidentifies safe uses of memory management functions as potential vulnerabilities. Verify that the pointer is indeed freed twice before marking it as an issue.

How to Fix Double Free

• Ensure each allocation is freed only once.
• Set pointers to NULL after freeing them.
• Use smart pointers or RAII techniques for automatic resource management.

Framework-Specific Fixes for Double Free

C++

#include <memory>

void example_function(std::unique_ptr<char> ptr) {
    // Automatically manages memory and prevents double free
}

This code uses std::unique_ptr to automatically manage memory, preventing double free issues.

How to Ask AI to Check Your Code for Double Free

Double Free Best Practices Checklist

• ✅ Ensure each allocation is freed only once.
• ✅ Set pointers to NULL after freeing them.
• ✅ Use smart pointers or RAII techniques for automatic resource management.

Double Free FAQ

How does double freeing memory occur in C++?

In C++, double freeing occurs when the same pointer is freed twice, often due to complex error handling or cleanup routines that fail to track which resources have already been released.

What are the consequences of a double free vulnerability?

A double free can corrupt memory management data structures, leading to crashes, unexpected behavior, and potential execution of arbitrary code.

How does static analysis help detect double frees?

Static analysis tools analyze source code without executing it to identify instances where free() is called twice on the same pointer.

What are best practices for preventing double frees in C++ applications?

Ensure each allocation is freed only once, set pointers to NULL after freeing them, and use smart pointers or automatic memory management techniques.

How can I detect double free vulnerabilities manually?

Manually review code for patterns where the same pointer is freed multiple times, especially in error handling paths.

What are common false positives when detecting double frees with automated tools?

False positives may occur if a tool misidentifies safe uses of memory management functions as potential vulnerabilities.

How can I prevent double free issues using modern C++ features?

Utilize smart pointers and RAII (Resource Acquisition Is Initialization) techniques to manage resource lifetimes automatically.

Vulnerabilities Related to Double Free

| CWE | Name | Relationship | |—|—|—| | CWE-825 | Expired Pointer Dereference | ChildOf | | CWE-1341 | Multiple Releases of Same Resource or Handle | ChildOf | | CWE-672 | Operation on a Resource after Expiration or Release | ChildOf | | CWE-672 | Operation on a Resource after Expiration or Release | ChildOf | | CWE-672 | Operation on a Resource after Expiration or Release | ChildOf | | CWE-666 | Operation on Resource in Wrong Phase of Lifetime | ChildOf | | CWE-416 | Use After Free | PeerOf | | CWE-123 | Write-what-where Condition | PeerOf |

References

• MITRE: CWE-415
• NVD

Scan Your Own Site

Manual code review catches what you know to look for. An automated scan catches what you didn’t. Scan your own website using PenScan to find Double Free and other risks before an attacker does.