The Cisco Secure Web Appliance, powered by AsyncOS Software (formerly known as Cisco Web Security Appliance or WSA), is designed to protect networks from web-based threats. Recently, a vulnerability was identified in the scanning engines of AsyncOS, which could allow an attacker to bypass rule blocks and receive restricted traffic. This blog post will discuss the details of this vulnerability (CVE-2022-20952), the potential risks it poses, and how to mitigate it.

Vulnerability Details

The vulnerability stems from an issue in the scanning engines of AsyncOS where malformed, encoded traffic is not effectively detected. This leads to a loophole where a remote attacker, without any authentication, can exploit this vulnerability by connecting through an affected device to a malicious server, obtaining malformed HTTP responses.

The potential outcome of a successful exploit includes bypassing explicit block rules, allowing the attacker to receive traffic which should have been blocked by the device. The Confidentiality, Integrity, and Availability (CIA) triad could be compromised, leading to severe security concerns for the affected network.

Exploit Details

In order to exploit this vulnerability, the attacker would need to establish a connection through the affected Cisco Secure Web Appliance and create an interaction with a malicious server. The interaction includes malformed HTTP responses that are not detected by the scanning engines of AsyncOS. Below is a basic code snippet that demonstrates this:

GET /malicious HTTP/1.1
Host: malicious-server.com
Connection: Keep-Alive
Content-Type: text/html

HTTP/1.1 200 OK
Content-Encoding: gzip, chunked
Content-Type: text/html

[Malformed HTTP payload]

In this example, the attacker attempts to bypass the block rule by altering the content encoding format and sending a malformed payload. The scanning engine fails to detect this issue, allowing the attacker to connect and receive information that should have been blocked.

1. Cisco Advisory for CVE-2022-20952: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asyncos-rule-bypass-tjzxf5E5
2. CVE Details: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-20952

Mitigations

Cisco has released a software update to address this vulnerability. Users are strongly advised to update to the latest version available, which includes the necessary fixes. To download the update and access information on how to install it, visit Cisco's Software Download Center.

Additionally, network administrators must remain vigilant in monitoring network traffic and the behavior of the devices connected to it. Robust security practices and contingency plans should be in place to minimize the potential impact of an attack.

Conclusion

The CVE-2022-20952 vulnerability in Cisco Secure Web Appliance's AsyncOS scanning engines poses a significant risk to network security. This vulnerability allows an unauthenticated, remote attacker to bypass the configured rule blocks and gain access to restricted traffic. To protect your network, ensure your Cisco Secure Web Appliance is updated with the latest software release and maintain strong security practices to minimize risks.

Timeline

Published on: 03/01/2023 08:15:00 UTC
Last modified on: 03/10/2023 16:11:00 UTC