CVE-2023-28216 - Inside the Windows ALPC Elevation of Privilege Vulnerability

In April 2023, Microsoft patched a critical local privilege escalation vulnerability tracked as CVE-2023-28216. This flaw impacted Windows’ Advanced Local Procedure Call (ALPC) mechanism—a core part of Windows that allows processes to communicate with each other efficiently and securely.

If you’re a security enthusiast or a blue teamer, understanding these bugs is key to defending Windows endpoints. In this post, I’ll break down the vulnerability in simple terms, show a code snippet to illustrate exploitation, link to the original references, and explain how attackers can leverage this bug for EoP (Elevation of Privilege).

What is ALPC?

ALPC (Advanced Local Procedure Call) is a Windows feature used by processes (programs) to talk with one another on the same machine. It's basically a messaging channel; think of it like passing sticky notes between different apps, but much more sophisticated.

Windows uses ALPC everywhere—your system’s processes exchange information rapidly and quietly via this channel. That makes flaws here extremely dangerous.

A Simple Look at the Vulnerability

CVE-2023-28216 revolves around how ALPC improperly handles privileges when managing requests. An attacker with basic access (like a regular user) could trick ALPC into granting SYSTEM privileges. This could allow them to run code as SYSTEM—a holy grail for attackers.

This was classified as a local privilege escalation: the attacker must already have some access to the target machine (even as a low-privileged user).

Exploit Breakdown

According to Microsoft’s advisory, the flaw arises due to improper input validation. While full exploit code hasn’t been published by Microsoft’s policy, some security researchers have demonstrated the technique post-patch.

Malicious ALPC request crafted to a system service.

3. The ALPC system service incorrectly validates the request and runs a command under SYSTEM's badge.
4. Attacker gains SYSTEM privileges and can do anything on the machine—install malware, dump secrets, etc.

Realistic Code Snippet

Suppose a C-based proof-of-concept (POC) exploit for a similar ALPC EoP. Here’s a simplified version to get the idea (for educational purposes only!):

// Pseudocode to demonstrate ALPC abuse
#include <Windows.h>
#include <stdio.h>

int main() {
    HANDLE hAlpcPort;
    UNICODE_STRING PortName;
    OBJECT_ATTRIBUTES objAttr;
    ALPC_PORT_ATTRIBUTES PortAttrib;
    
    // Set up the ALPC port (details omitted)
    RtlInitUnicodeString(&PortName, L"\\RPC Control\\VulnerableService");
    InitializeObjectAttributes(&objAttr, &PortName, , NULL, NULL);

    // Try to connect to vulnerable ALPC port
    NTSTATUS status = NtAlpcConnectPort(
        &hAlpcPort,
        &PortName,
        &objAttr,
        &PortAttrib, , , NULL, NULL, NULL, NULL, NULL
    );

    if (NT_SUCCESS(status)) {
        // Craft malicious message for privilege escalation
        // (details omitted for safety)
        printf("Successfully connected to ALPC port. Attempting privilege escalation...\n");
        // Send the message and receive a privileged token...
    } else {
        printf("ALPC port connection failed.\n");
    }
    return ;
}

Note: This code is for educational illustration; the real exploit would utilize specific message formats and abuse the vulnerability directly.

Manipulate system files

- Disable AV/EDR tools

Move laterally inside a network

All without any admin interaction, provided they had an initial foothold.

Mitigation and Patching

Microsoft fixed this on April 11, 2023—if you patch your Windows (including servers!), you’re safe. You can find the official fix in April’s Patch Tuesday bundle:

- Microsoft Security Response Center (MSRC): CVE-2023-28216
- Patch download page (look up the KB for your version)

References and Further Reading

- Microsoft Security Advisory
- Windows ALPC Internals by Alex Ionescu
- Cybersecurity Advisory: Windows ALPC EoP (CERT/CC)
- Project Zero - Exploiting ALPC

Conclusion

CVE-2023-28216 is a sobering reminder: core Windows features like ALPC, if implemented incorrectly, can expose even well-patched systems to local attackers. If you’re responsible for Windows security, patch aggressively, monitor for unusual privilege escalations, and follow vendors like Microsoft and top researchers for advisory alerts.

Stay safe, patch smart, and keep hacking fun—but responsible!

Timeline

Published on: 04/11/2023 21:15:00 UTC
Last modified on: 04/13/2023 01:15:00 UTC