The digital world relies heavily on authentication protocols, and one of the cornerstones in enterprise environments is Kerberos. A January 2024 security update from Microsoft caught a lot of eyes: CVE-2024-20674, a Security Feature Bypass vulnerability in Windows Kerberos, left millions of systems potentially open to attack. In this post, we’ll break down what this flaw is, how it can be exploited, and provide a basic proof-of-concept to help demonstrate the risk involved.

What is CVE-2024-20674?

CVE-2024-20674 is a vulnerability in Windows Kerberos protocol that allows an attacker to potentially bypass the client authentication mechanism. This means an attacker can trick a system into thinking they are someone else, even if they can’t actually provide the right password, certificate, or token.

Severity: High (CVSS 8.1)

> Reference: Microsoft Security Guide: CVE-2024-20674

How Did This Happen?

Kerberos is designed to securely authenticate users in a Windows domain. When you log in, your system talks to a Key Distribution Center (KDC), which then issues tickets for authentication.

The flaw in CVE-2024-20674 arose from improper verification of the public key used in a Kerberos authentication session. An attacker could manipulate this exchange, tricking the server into accepting a key that wasn’t properly validated. This essentially bypasses the authentication, giving the attacker the same access as a legitimate, authenticated user.

Technical Details

The heart of the issue is in the Kerberos KDC's failure to properly validate the public key (especially with PKINIT, Kerberos’ Public Key Cryptography for Initial Authentication extension). If the validation step is skipped or done incorrectly, a malicious actor could supply their own crafted key and impersonate a legitimate user.

Proof-of-Concept (PoC) - Demonstrating the Risk

The following Python snippet uses the popular Impacket library, which can help attackers interact with Kerberos. This is for educational testing on labs only!

Let's simulate sending crafted data to trick the KDC.

from impacket.krb5.asn1 import AS_REQ
from pyasn1.codec.der import encoder
from impacket.krb5 import constants

def craft_malicious_as_req(user, domain, crafted_key):
    # Build an AS_REQ with a bogus/crafted public key for PKINIT
    as_req = AS_REQ()
    # Set normal fields...
    as_req['pvno'] = 5
    as_req['msg-type'] = 10  # AS_REQ
    # Add other fields (omitted for brevity)
    # # Here, instead of valid public key, insert crafted_key
    as_req['req-body']['additional-tickets'] = crafted_key

    encoded = encoder.encode(as_req)
    return encoded

if __name__ == "__main__":
    fake_key = b"malicious_public_key"  # This is just a dummy example
    pkt = craft_malicious_as_req("victimuser", "domain.internal", fake_key)
    print(pkt.hex())
    # Send this packet to KDC and observe if it gets accepted (in unpatched systems)

Disclaimer: This code does not exploit the vulnerability directly but shows how one might craft data as part of a proof-of-concept. Exploiting this on any production network is illegal.

Real-World Exploitation

According to Microsoft, exploitation is *"more likely"* if an attacker can already access your network. They could:

Potentially escalate privileges

That’s serious—especially in organizations that still rely heavily on older Windows Server domains.

Verify your domain controllers have applied January 2024 security updates.

3. Where possible, enable additional Kerberos protections (like enforcement of strong authentication, and monitoring for unusual ticket activity).

Patch details:
Microsoft Guidance for CVE-2024-20674

Final Thoughts

CVE-2024-20674 shows us that authentication mechanisms are only as strong as their weakest code path. If you manage Windows domains or Active Directory environments, patch without delay. Even a small error in authentication can bring huge risks to your organization!

Further Reading

- Official CVE record (NIST)
- Kerberos protocol basics - Microsoft Docs

Timeline

Published on: 01/09/2024 18:15:50 UTC
Last modified on: 01/14/2024 22:37:10 UTC