CVE-2022-20813: Cisco Expressway RCE Vulnerability

CVE-2022-20813 Overview

CVE-2022-20813 is a certificate validation bypass vulnerability affecting Cisco Expressway Series and Cisco TelePresence Video Communication Server (VCS). Multiple vulnerabilities in the API and web-based management interface of these products could allow a remote attacker to overwrite arbitrary files or conduct null byte poisoning attacks on an affected device. The Cisco Expressway Series encompasses both the Expressway Control (Expressway-C) device and the Expressway Edge (Expressway-E) device.

Critical Impact

Attackers exploiting this vulnerability could bypass certificate validation mechanisms and conduct null byte poisoning attacks, potentially leading to unauthorized access and file manipulation on affected Cisco communication infrastructure.

Affected Products

• Cisco Expressway Series (Expressway-C and Expressway-E)
• Cisco TelePresence Video Communication Server (VCS)

Discovery Timeline

• 2022-07-06 - CVE-2022-20813 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-20813

Vulnerability Analysis

This vulnerability stems from improper certificate validation (CWE-295) combined with improper handling of null byte input (CWE-158). The affected systems fail to properly validate certificates during authentication processes, creating an opportunity for attackers to bypass security controls. Additionally, the null byte poisoning weakness allows attackers to manipulate string handling operations within the application, potentially truncating input strings at unintended positions and bypassing security checks.

The vulnerability can be exploited remotely over the network without requiring user interaction, though successful exploitation requires the attacker to overcome certain conditions, making it moderately complex to execute. If successfully exploited, an attacker could gain access to confidential information stored on the affected device.

Root Cause

The root cause of this vulnerability lies in two distinct weaknesses within the Cisco Expressway and TelePresence VCS software:

1. Improper Certificate Validation (CWE-295): The software fails to properly validate the authenticity of certificates presented during secure communications, allowing attackers to potentially impersonate legitimate endpoints or bypass authentication mechanisms.

2. Improper Handling of Null Byte Input (CWE-158): The application improperly processes input containing null bytes, which can lead to string truncation vulnerabilities. Attackers can leverage this weakness to bypass security filters and input validation routines.

Attack Vector

The attack vector is network-based, requiring an attacker to have network access to the vulnerable device's API or web-based management interface. The attacker does not require authentication to exploit this vulnerability. The exploitation involves:

1. Targeting the API or web management interface of vulnerable Cisco Expressway or TelePresence VCS devices
2. Exploiting the certificate validation bypass to establish unauthorized connections
3. Utilizing null byte poisoning techniques to manipulate input handling and potentially overwrite arbitrary files

This vulnerability allows attackers to potentially access confidential information. The exploitation mechanism involves crafting specially crafted requests that take advantage of the improper certificate validation and null byte handling weaknesses. For detailed technical information, refer to the Cisco Security Advisory.

Detection Methods for CVE-2022-20813

Indicators of Compromise

• Unusual API requests to Cisco Expressway or TelePresence VCS management interfaces containing null byte characters
• Unexpected certificate validation errors or warnings in system logs
• Suspicious file modification events on affected devices
• Anomalous network traffic patterns targeting management interfaces

Detection Strategies

• Monitor network traffic for requests containing null byte sequences (%00) targeting Expressway and VCS management interfaces
• Implement intrusion detection rules to identify certificate validation bypass attempts
• Review authentication logs for failed certificate validation events followed by successful access
• Deploy network monitoring to detect unusual traffic patterns to affected Cisco devices

Monitoring Recommendations

• Enable verbose logging on Cisco Expressway and TelePresence VCS devices to capture detailed request information
• Configure SIEM alerts for null byte injection patterns in web application traffic
• Implement file integrity monitoring on affected systems to detect unauthorized file modifications
• Monitor for unauthorized configuration changes on affected devices

How to Mitigate CVE-2022-20813

Immediate Actions Required

• Review the Cisco Security Advisory for specific patch information
• Apply the latest software updates provided by Cisco to affected Expressway and TelePresence VCS devices
• Restrict network access to management interfaces using access control lists (ACLs) or firewall rules
• Audit current device configurations and check for signs of compromise

Patch Information

Cisco has released security updates to address this vulnerability. Organizations should consult the Cisco Security Advisory for detailed patch information and upgrade paths for both Cisco Expressway Series and TelePresence Video Communication Server products.

Workarounds

• Limit access to the management interface to trusted networks and IP addresses only
• Implement network segmentation to isolate affected devices from untrusted networks
• Enable additional logging and monitoring to detect exploitation attempts
• Consider deploying a web application firewall (WAF) in front of affected management interfaces to filter malicious requests

# Example: Restrict management interface access using firewall rules
# Allow management access only from trusted administrator network
iptables -A INPUT -p tcp --dport 443 -s 10.10.10.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP

# Enable detailed logging on Cisco devices
# (Consult Cisco documentation for device-specific commands)