CVE-2022-20773: Cisco Umbrella Auth Bypass Vulnerability

CVE-2022-20773 Overview

A vulnerability in the key-based SSH authentication mechanism of Cisco Umbrella Virtual Appliance (VA) could allow an unauthenticated, remote attacker to impersonate a VA. This vulnerability is due to the presence of a static SSH host key. An attacker could exploit this vulnerability by performing a man-in-the-middle attack on an SSH connection to the Umbrella VA. A successful exploit could allow the attacker to learn the administrator credentials, change configurations, or reload the VA.

Critical Impact

Successful exploitation enables attackers to intercept administrator credentials, modify appliance configurations, or force a reload of the Cisco Umbrella Virtual Appliance through man-in-the-middle attacks on SSH connections.

Affected Products

• Cisco Umbrella Virtual Appliance (versions prior to 3.3.2)
• cisco umbrella

Discovery Timeline

• 2022-04-21 - CVE CVE-2022-20773 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-20773

Vulnerability Analysis

This vulnerability stems from a hardcoded credentials issue (CWE-798) combined with the use of a hard-coded cryptographic key (CWE-321). The Cisco Umbrella Virtual Appliance ships with a static SSH host key that is identical across all deployed instances of the appliance. This cryptographic design flaw fundamentally undermines the security model of SSH authentication, which relies on unique host keys to establish trust between clients and servers.

When an administrator connects to the Umbrella VA via SSH, the client verifies the server's identity using the host key. Because this key is static and known, an attacker positioned on the network path can present the same key to impersonate a legitimate Umbrella VA instance. The administrator's SSH client would accept this connection as authentic, enabling the attacker to capture credentials and sensitive configuration data.

It is important to note that SSH is not enabled by default on the Umbrella VA, which limits the attack surface to environments where administrators have explicitly enabled SSH access for management purposes.

Root Cause

The root cause of this vulnerability is the presence of a static SSH host key embedded within the Cisco Umbrella Virtual Appliance firmware. Rather than generating unique cryptographic keys during initial appliance deployment or configuration, the appliance uses a pre-defined key that is shared across all instances. This represents a violation of cryptographic best practices, as SSH host keys should be unique per installation to ensure that man-in-the-middle attacks cannot be conducted using known key material.

Attack Vector

The attack vector for CVE-2022-20773 requires network access and the ability to perform a man-in-the-middle attack on SSH connections destined for the Umbrella VA. An attacker would need to:

1. Position themselves on the network path between an administrator and the Umbrella VA (through ARP spoofing, DNS hijacking, or network compromise)
2. Intercept SSH connection attempts to the legitimate appliance
3. Present the static SSH host key to the administrator's SSH client
4. Capture administrator credentials as they are entered
5. Optionally relay communications to the real appliance to avoid detection

The vulnerability does not require authentication to exploit, as the attacker intercepts the authentication process itself. However, high attack complexity exists due to the requirement of establishing a man-in-the-middle position on the network.

Detection Methods for CVE-2022-20773

Indicators of Compromise

• Unexpected SSH host key changes or warnings in administrator SSH client logs
• Multiple SSH sessions originating from unusual network locations or IP addresses
• Configuration changes to the Umbrella VA that were not authorized by administrators
• Unexplained appliance reloads or service disruptions
• Network traffic anomalies suggesting ARP spoofing or DNS manipulation attacks

Detection Strategies

• Monitor for SSH connection anomalies including duplicate sessions or connections from unexpected source addresses
• Implement network-based detection for ARP spoofing and DNS hijacking attempts that could enable MITM positioning
• Deploy network traffic analysis to identify potential interception of SSH traffic destined for Umbrella VA management interfaces
• Review SSH client host key caches across administrator workstations for consistency

Monitoring Recommendations

• Enable verbose logging on administrator workstations for SSH client connections
• Implement network segmentation monitoring to detect unauthorized traffic flows toward management interfaces
• Configure SIEM rules to alert on configuration changes to Umbrella VA appliances
• Monitor for indicators of network-layer attacks (ARP cache poisoning, DHCP spoofing) that could facilitate exploitation

How to Mitigate CVE-2022-20773

Immediate Actions Required

• Upgrade Cisco Umbrella Virtual Appliance to version 3.3.2 or later immediately
• If SSH is enabled on vulnerable appliances, consider disabling it until patching is complete
• Implement network segmentation to limit exposure of VA management interfaces
• Review recent administrator sessions and configuration changes for signs of compromise
• Verify SSH host key fingerprints manually when connecting to Umbrella VA instances

Patch Information

Cisco has released a security patch addressing this vulnerability in Umbrella Virtual Appliance version 3.3.2. The patch regenerates unique SSH host keys for each appliance instance, eliminating the static key vulnerability. Administrators should consult the Cisco Security Advisory for detailed upgrade instructions and version information.

Workarounds

• Disable SSH access on the Umbrella VA if not required for management operations (SSH is disabled by default)
• Implement strict network access controls to limit which hosts can reach the VA management interface
• Use out-of-band management networks isolated from general network traffic
• Deploy network-based intrusion prevention systems to detect and block potential MITM attacks

# Verify current Umbrella VA version and SSH status
# Consult Cisco documentation for specific commands
# Disable SSH if not operationally required until upgrade is complete