Plex Media Server is a popular tool for streaming movies, TV shows, and music to all kinds of devices.   But did you know that older versions of Plex, specifically up to version 1.21, have a serious security issue? This problem, tracked as CVE-2021-33959, allows attackers to use your Plex server as a weapon in a DDoS reflection attack—even if you never notice!

Let’s break down what this vulnerability means, how it works, and most importantly—how you can fix it.

What is a DDoS Reflection Attack?

A Distributed Denial-of-Service (DDoS) reflection attack is a sneaky way for bad guys to amplify their bandwidth by tricking many innocent servers into bombarding a target with unwanted traffic. Instead of attacking directly, they use features of other servers—like Plex—so the attack seems to come from everywhere.

How Plex Was Vulnerable

In versions 1.21 and earlier, Plex’s network service could be abused as a DDoS reflector. That happened because:
- Plex listened for unauthenticated requests on network ports (by default, 32414/UDP for GDM).
- Anyone could send a small UDP message to your open port and trigger a much larger reply, even without a Plex account.

Attackers could easily bounce high volumes of traffic at an unsuspecting third party, causing their network to slow down or crash.

The Technical Details & PoC

Plex uses a discovery protocol (GDM—General Discovery Mechanism) to help clients locate servers. But before the fix, anyone could send a forged UDP “discovery” message to port 32414, and your server would faithfully respond—even to spoofed source IPs.

Sample Code to Trigger the Response

Just for education (don’t use for malicious purposes!), here’s a Python code snippet to send a GDM discovery UDP message:

import socket

PMS_IP = 'victim-ip-here'      # Replace with the server to test
GDM_PORT = 32414
GDM_MESSAGE = b'M\\\'

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.sendto(GDM_MESSAGE, (PMS_IP, GDM_PORT))

# Listen for response only if testing on your own network!
data, addr = sock.recvfrom(1024)
print(f"Received: {data} from {addr}")


> Warning: Misusing this code against servers you don’t own is illegal. The above is for research and defensive testing only!

Discovery and References

- Original CVE Record: CVE-2021-33959
- SSD-Disclosure #22
- Rapid7 Advisory

Researchers discovered this was being abused “in the wild.” At the height, over 27,000 Plex servers were being used as amplification points for DDoS attacks!

1. Update Plex!

This issue is fixed in version 1.22..4163 and above. Visit Plex Downloads and grab the latest version for your platform.

3. Check for Leaky Ports

You can check your ports using tools like Shodan.io or canyouseeme.org. Search for open port 32414 from the outside.

4. Monitor Logs for Abuse

If you see lots of unexplained requests or your server is sluggish, it might be because someone is abusing your setup!

Conclusion

CVE-2021-33959 is a prime example of how “just a media server” can become a weapon in larger internet wars. Keeping your software up-to-date, using sensible firewall rules, and knowing your network can help protect not just yourself—but the whole internet.

Remember: Running network services means being responsible for their security.

Stay safe, keep streaming, and always patch early!

*Exclusive write-up by ChatGPT. For questions or feedback, reach out through the comments!*

Timeline

Published on: 01/18/2023 14:15:00 UTC
Last modified on: 01/26/2023 16:15:00 UTC