CVE-2021-34795: Cisco Catalyst PON Switch ONT RCE Flaw

CVE-2021-34795 Overview

CVE-2021-34795 is a critical vulnerability affecting the web-based management interface of Cisco Catalyst Passive Optical Network (PON) Series Switches Optical Network Terminal (ONT). This vulnerability allows an unauthenticated, remote attacker to exploit multiple security weaknesses including default credential authentication bypass when Telnet is enabled, command injection, and unauthorized configuration modification.

Critical Impact

Unauthenticated remote attackers can gain complete control of affected network devices through default credentials, command injection, or configuration manipulation, potentially compromising critical network infrastructure.

Affected Products

• Cisco Catalyst PON Switch CGP-ONT-1P (Firmware)
• Cisco Catalyst PON Switch CGP-ONT-4P (Firmware)
• Cisco Catalyst PON Switch CGP-ONT-4PVC (Firmware)
• Cisco Catalyst PON Switch CGP-ONT-4TVCW (Firmware)
• Cisco Catalyst PON Switch CGP-ONT-4PV (Firmware)

Discovery Timeline

• 2021-11-04 - CVE-2021-34795 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-34795

Vulnerability Analysis

This vulnerability encompasses multiple weaknesses in the web-based management interface of Cisco Catalyst PON Series Switches ONT devices. The affected systems contain hardcoded credentials (CWE-1188) and improper access control mechanisms (CWE-284) that can be exploited by unauthenticated remote attackers.

The vulnerability allows three distinct attack scenarios: First, attackers can authenticate using default credentials when the Telnet protocol is enabled on the device. Second, the web interface is susceptible to command injection attacks that allow arbitrary command execution on the underlying operating system. Third, attackers can modify device configurations without proper authorization, potentially disrupting network operations or establishing persistent access.

Root Cause

The root cause stems from two fundamental security weaknesses: Insecure Default Configuration (CWE-1188) where the devices ship with hardcoded default credentials that are not required to be changed, and Improper Access Control (CWE-284) where the web-based management interface fails to properly validate user authentication and authorization before processing sensitive operations.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker with network access to the management interface can exploit these vulnerabilities remotely. When Telnet is enabled, attackers can leverage default credentials for direct device access. Additionally, the command injection vulnerability allows attackers to craft malicious requests to the web interface that execute arbitrary commands with elevated privileges on the device.

The web-based management interface processes user-supplied input without adequate sanitization, enabling command injection. Configuration modification attacks exploit missing authorization checks that fail to verify whether a user has permission to alter device settings.

Detection Methods for CVE-2021-34795

Indicators of Compromise

• Unexpected Telnet connections to Cisco Catalyst PON Series Switch management interfaces from unauthorized IP addresses
• Authentication logs showing successful logins using default or unknown credentials
• Web server logs containing unusual or malformed HTTP requests targeting the management interface
• Unexplained configuration changes on PON switch devices

Detection Strategies

• Monitor network traffic for Telnet (port 23) connections to PON switch management interfaces
• Implement log analysis for authentication events on affected Cisco Catalyst PON devices
• Deploy network intrusion detection signatures for known command injection patterns targeting Cisco management interfaces
• Establish baseline configurations and alert on unauthorized modifications

Monitoring Recommendations

• Enable comprehensive logging on all Cisco Catalyst PON Series Switch devices
• Configure SIEM rules to detect multiple failed login attempts followed by successful authentication
• Monitor for command injection patterns in HTTP request logs targeting device management URLs
• Implement real-time alerting for configuration changes outside maintenance windows

How to Mitigate CVE-2021-34795

Immediate Actions Required

• Apply the latest firmware updates from Cisco that address this vulnerability
• Disable Telnet protocol and use SSH for remote management where possible
• Change all default credentials on affected devices immediately
• Restrict management interface access to trusted networks using access control lists

Patch Information

Cisco has released security updates to address these vulnerabilities. Organizations should refer to the Cisco Security Advisory for specific firmware versions that resolve CVE-2021-34795. The advisory provides detailed information on affected versions and the corresponding fixed releases for each Catalyst PON Series Switch model.

Workarounds

• Disable Telnet protocol on all affected devices and use secure alternatives like SSH
• Implement network segmentation to isolate management interfaces from untrusted networks
• Configure access control lists (ACLs) to restrict management access to authorized administrator IP addresses only
• Deploy a jump server or bastion host for all management access to critical network infrastructure

# Configuration example - Disable Telnet and restrict management access
# Example ACL to restrict management access (apply to management interface)
access-list 10 permit 192.168.1.0 0.0.0.255
access-list 10 deny any log

# Disable Telnet service
no service telnet

# Enable SSH for secure remote access
ip ssh version 2
line vty 0 4
 transport input ssh
 access-class 10 in