When it comes to Windows Server security, DNS has always been a prime target. In 2022, a particularly dangerous flaw was found in Microsoft’s DNS server software – CVE-2022-26812. This post will walk you through what this vulnerability is all about, how it works, code snippets for proof-of-concept, and how you can keep your systems safe.

What is CVE-2022-26812?

CVE-2022-26812 is a remote code execution (RCE) vulnerability in the way that Windows DNS Server handles certain requests. If exploited, an attacker can run code of their choice, with the same permissions as the DNS Server process – that’s usually SYSTEM, the highest level you can get.

And some earlier supported versions

This vulnerability is distinct from a bunch of others disclosed around the same time, including CVE-2022-26811, CVE-2022-26813, and others in the CVE-2022-24536 to CVE-2022-26829 range.

Official References

- Microsoft Security Update Guide for CVE-2022-26812
- NIST NVD entry for CVE-2022-26812
- Microsoft Patch Tuesday April 2022

Vulnerability Details

A bug in the way Windows DNS Server processes received packets (specially crafted DNS requests) allows an unauthenticated attacker to send a network packet that causes the server to execute code they control.

The vulnerability is considered wormable—it could potentially be used to spread from one vulnerable DNS Server to another, automatically, if left unpatched.

How Does the Exploit Work?

The specific weakness is a buffer handling error when DNS packets are parsed. An attacker can send a malformed packet, such as a crafted DNS query, that overflows some crucial buffer, overwriting server memory.

They craft a DNS packet with malformed contents targeting the vulnerable parsing code.

- DNS server blindly processes the packet and, because of poor bounds checking, it overwrites memory on the stack or heap.

Proof-of-Concept (PoC) Code Snippet

Below is a basic outline (not a weaponized exploit!) for sending a custom DNS query to a target DNS server. This doesn’t perform exploitation outright, but illustrates how an attacker connects and sends crafted DNS packets.

import socket

def send_custom_dns_query(target_ip, crafted_packet):
    # DNS uses UDP by default, port 53
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    try:
        sock.sendto(crafted_packet, (target_ip, 53))
        data, _ = sock.recvfrom(512)
        print("Received:", data)
    except Exception as e:
        print("Failed:", e)
    finally:
        sock.close()

# Example: a (non-malicious) simple DNS query
# Real attack would carefully set bytes to trigger vulnerability
basic_query = b'\x12\x34\x01\x00\x00\x01\x00\x00\x00\x00\x00\x00' \
              b'\x03foo\x03bar\x00\x00\x01\x00\x01'

send_custom_dns_query("TARGET_DNS_IP", basic_query)

Note: Actual exploit code would craft the packet body (data after the header) to trigger the memory error specifically.

A real exploit would

- Reproduce the vulnerable code path (typically in the way labels and lengths are decoded inside the DNS query).

Once run, give the attacker SYSTEM privileges on the DNS server.

Public exploit code for this vulnerability has not been widely released, but similar attacks often use C or Python to craft the packet and send it in bulk, then watch for remotely-exploitable effects such as a crash or shell.

How to Stay Safe

Patch! Microsoft released patches in April 2022.

Apply the latest Windows Server updates on every DNS server.

Microsoft’s Update Guide for CVE-2022-26812

Don’t expose DNS servers directly to the internet unless absolutely necessary.

3. Use a firewall to restrict access to UDP/TCP port 53.

Summary Table

| Affected Product      | Patched? | Action           |
|--------------------------|--------------|----------------------|
| Windows Server 2019      | Yes          | Patch immediately    |
| Windows Server 2016      | Yes          | Patch immediately    |
| Windows Server 2012 R2   | Yes          | Patch immediately    |

Final Thoughts

CVE-2022-26812 is a perfect example of how a simple bug in network protocol handling can let attackers get total control. If you run Windows DNS, double-check your patching.

Don’t confuse this CVE with others in the same announcement batch! Each one has unique triggers and impacts—even similar-looking CVEs like CVE-2022-24536 or CVE-2022-26814.

- Microsoft Security Update Guide
- NIST NVD Record

Timeline

Published on: 04/15/2022 19:15:00 UTC
Last modified on: 04/18/2022 20:23:00 UTC