CVE-2021-40119: Cisco Policy Suite Auth Bypass Flaw

CVE-2021-40119 Overview

A vulnerability in the key-based SSH authentication mechanism of Cisco Policy Suite could allow an unauthenticated, remote attacker to log in to an affected system as the root user. This vulnerability is due to the re-use of static SSH keys across installations. An attacker could exploit this vulnerability by extracting a key from a system under their control, then using that same key to authenticate to other Cisco Policy Suite installations. A successful exploit could allow the attacker to log in to an affected system as the root user, gaining complete control over the compromised system.

Critical Impact

Unauthenticated remote attackers can gain root-level access to Cisco Policy Suite systems by exploiting hardcoded SSH keys that are reused across all installations.

Affected Products

• Cisco Policy Suite (all versions prior to the security patch)

Discovery Timeline

• 2021-11-04 - CVE-2021-40119 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-40119

Vulnerability Analysis

This vulnerability falls under two critical weakness categories: CWE-321 (Use of Hard-coded Cryptographic Key) and CWE-798 (Use of Hard-coded Credentials). The fundamental issue lies in Cisco Policy Suite's implementation of SSH authentication, where static SSH keys are embedded in the software and reused across all installations of the product.

When organizations deploy Cisco Policy Suite, the same SSH key pair is used for authentication on every instance. This means that an attacker who obtains access to any single Cisco Policy Suite installation—whether through legitimate access, a compromised system, or other means—can extract the SSH private key. Once extracted, this key can be used to authenticate to any other Cisco Policy Suite deployment worldwide, as all installations share the identical cryptographic credentials.

The vulnerability is particularly severe because successful exploitation grants root-level access, providing attackers with the highest privilege level on the target system. This enables complete system compromise including data exfiltration, malware installation, lateral movement within the network, and persistence establishment.

Root Cause

The root cause is the re-use of static SSH keys across all Cisco Policy Suite installations. Rather than generating unique SSH key pairs during installation or initial setup, the product ships with pre-configured keys that are identical across all deployments. This violates fundamental cryptographic security principles where cryptographic keys should be unique per installation to prevent credential reuse attacks.

Attack Vector

The attack vector is network-based and requires no prior authentication or user interaction. An attacker can exploit this vulnerability remotely by:

1. Obtaining access to any Cisco Policy Suite installation (potentially their own or a compromised system)
2. Extracting the static SSH private key from that installation
3. Using the extracted key to authenticate via SSH to other Cisco Policy Suite systems
4. Gaining root-level access to the target systems

The attack does not require specialized skills beyond basic SSH knowledge once the key is obtained. The static nature of the keys means that a single key extraction can be leveraged against any vulnerable Cisco Policy Suite deployment accessible over the network.

Detection Methods for CVE-2021-40119

Indicators of Compromise

• Unexpected SSH login sessions as root user from unfamiliar IP addresses
• SSH authentication logs showing successful logins without corresponding user activity
• Multiple systems showing SSH connections from the same external IP addresses
• Presence of unauthorized processes or scheduled tasks running as root

Detection Strategies

• Monitor SSH authentication logs (/var/log/auth.log or /var/log/secure) for root login events
• Implement network segmentation and monitor for SSH connections (port 22) from untrusted networks to Cisco Policy Suite systems
• Deploy SIEM rules to alert on successful root SSH authentications from external IP addresses
• Compare SSH host keys across Cisco Policy Suite installations to identify static key usage

Monitoring Recommendations

• Enable verbose SSH logging to capture detailed authentication attempts
• Implement baseline analysis for normal SSH connection patterns and alert on deviations
• Monitor for any data exfiltration or lateral movement following SSH sessions to Policy Suite systems
• Review system integrity and audit trails for unauthorized configuration changes

How to Mitigate CVE-2021-40119

Immediate Actions Required

• Apply the security patch provided by Cisco immediately
• Restrict network access to SSH services on Cisco Policy Suite systems using firewall rules
• Implement network segmentation to limit exposure of Cisco Policy Suite management interfaces
• Review SSH authentication logs for evidence of unauthorized access

Patch Information

Cisco has released a security update to address this vulnerability. Organizations should consult the Cisco Security Advisory for detailed patch information and remediation guidance. After applying the patch, verify that unique SSH keys are generated for each installation.

Workarounds

• Restrict SSH access to Cisco Policy Suite systems using network ACLs or firewall rules to allow only trusted management networks
• Implement jump hosts or bastion servers for SSH access, limiting direct connectivity to Policy Suite systems
• Consider disabling SSH access entirely if alternative management methods are available until patches can be applied
• Deploy intrusion detection/prevention systems to monitor and block suspicious SSH authentication attempts

# Configuration example - Restrict SSH access via iptables
# Allow SSH only from trusted management network
iptables -A INPUT -p tcp --dport 22 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j DROP

# Verify SSH key uniqueness after patching
ssh-keygen -lf /etc/ssh/ssh_host_rsa_key.pub