Race for the Kernel: Anatomy of the Windows Handle Double-Free (CVE-2025-62215)

In the Windows Kernel, handles are the sacred tokens of access. Every file, process, or mutex you interact with is represented by a kernel object, and user-mode applications hold a 'handle' to that object. The kernel acts as the librarian, keeping a strict count (Reference Counting) of how many people are holding a book. When the count hits zero, the book is burned (memory is freed).

But what happens if the librarian gets distracted? CVE-2025-62215 is a classic tale of concurrency gone wrong. It turns out that if two threads scream "I'm returning this book!" at the exact same nanosecond, the kernel might get confused, agree with both of them, and try to burn the book twice.

This isn't just a crash. In the hands of a skilled exploit developer, a Double-Free is the holy grail of memory corruption. It turns the kernel's own memory manager against itself, allowing an attacker to rewrite the laws of physics within the operating system. You start as a nobody (Low Integrity); you end as God (SYSTEM).

Deep inside ntoskrnl.exe, the Object Manager is responsible for lifecycle management. When you call CloseHandle, the kernel decrements the object's reference count. Logic dictates that the cleanup routine should only run when the count hits exactly zero. However, CVE-2025-62215 exposes a Race Condition (CWE-362) in this critical path.

The vulnerability exists in the timing window between checking the handle status and decrementing the object pointer count. If an attacker spawns multiple threads targeting the same object handle, they can desynchronize this logic.

Thread A enters the destruction routine. It passes the check. Before it can mark the object as 'dying', Thread B enters the same routine. Because the state hasn't updated yet, Thread B also passes the check. Both threads proceed to call the internal free routine on the same memory address. The first free works as intended. The second free triggers CWE-415 (Double Free), corrupting the heap metadata that tracks available memory chunks.

While the exact source code of Windows is closed, we can reconstruct the logic failure based on the binary analysis and the patch behavior. The vulnerable pseudo-code likely looked something like this:

// VULNERABLE LOGIC (Conceptual)
void ObpCloseHandleTableEntry(ObjectTable, Entry) {
    // ... validation logic ...
    
    // The Race Window: multiple threads pass here before state updates
    if (Entry->Value & 1) {
        Object = Decode(Entry);
        
        // Thread A is here... Context Switch... Thread B arrives here
        if (InterlockedDecrement(&Object->PointerCount) == 0) {
            // Both threads think they are the last one
            ObpDeleteObject(Object); // DOUBLE FREE
        }
    }
}

The fix involves tightening the synchronization locks around the handle table entry lookup and the reference decrement, ensuring that the operation is atomic. If Thread A is closing the handle, Thread B is blocked or sees an invalid handle immediately, rather than racing down the execution path.

Turning a Double-Free into a stable exploit requires finesse. The goal is Heap Feng Shui: organizing the kernel memory so that the corruption lands exactly where we want it.

1. The Setup: Create a 'target' object. Something simple like a Pipe or an ALPC Port. Create thousands of handles to it.
2. The Race: Spin up 20+ threads. Have them all hammer CloseHandle and OpenProcess in a tight loop. We are fishing for the race condition.
3. The First Free: Thread A wins. The object is freed. The memory slot (chunk) is now marked as 'available' in the Free List.
4. The Spray (The Trap): Immediately spray the heap with fake objects (e.g., IoCompletionReserve or specific WNF states) that are the exact same size as the freed object. The kernel allocator happily places one of our fake objects into the slot vacated in Step 3.
5. The Second Free: Thread B (the slow racer) finally executes its free. It thinks it's freeing the original object, but it actually frees our fake object (the one we just sprayed).

This corrupts the FLINK (Forward Link) and BLINK (Backward Link) pointers of the heap chunk. By carefully crafting the data in our fake object, we can turn this metadata corruption into a Write-What-Where primitive. We use this to overwrite our process's Token pointer with the address of the System Token, effectively promoting ourselves to administrator.

This is not theoretical. CISA added this to the KEV catalog immediately because ransomware operators love this stuff. In a modern attack chain, getting code execution on a box (via Phishing or RCE) is only step one. You usually land as a low-privileged user.

With CVE-2025-62215, that foothold becomes total compromise in seconds. An attacker can:

• Bypass EDR: By loading a malicious kernel driver (BYOVD) after gaining System privileges.
• Dump Credentials: Access lsass.exe memory to steal domain admin hashes.
• Persist: Burry themselves deep in the OS, outside the view of user-mode antivirus.

The fact that this works on the latest Windows 11 builds (24H2) and Server 2025 makes it a universal skeleton key for Windows environments.

Microsoft's November 2025 patch introduces stricter locking mechanisms in the Object Manager (Ob). The remediation is binary: you patch, or you are vulnerable.

> [!NOTE] > Detection Tip: Look for processes generating an absurd number of Handle operations (Event ID 4656) in a short burst, followed by the spawning of a cmd.exe or powershell.exe with System integrity.

If you are a developer, the lesson here is that InterlockedDecrement alone is not a silver bullet for thread safety if the logic surrounding the counter is flawed. Always assume your handle is lying to you.