CVE-2021-1594: Cisco ISE REST API RCE Vulnerability

CVE-2021-1594 Overview

A command injection vulnerability exists in the REST API of Cisco Identity Services Engine (ISE) that could allow an unauthenticated, remote attacker to execute arbitrary commands with root privileges on the underlying operating system. This vulnerability stems from insufficient input validation for specific API endpoints used in internode communications between ISE personas.

Critical Impact

Successful exploitation allows an attacker to achieve root-level command execution on affected Cisco ISE deployments, potentially compromising the entire identity and access management infrastructure.

Affected Products

• Cisco Identity Services Engine 2.4(0.902)
• Cisco Identity Services Engine 2.6.0 through 2.6.0 Patch 9
• Cisco Identity Services Engine 2.7.0 through 2.7.0 Patch 4
• Cisco Identity Services Engine 3.0.0 through 3.0.0 Patch 3
• Cisco Identity Services Engine 3.1(0.440)

Discovery Timeline

• 2021-10-06 - CVE-2021-1594 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-1594

Vulnerability Analysis

This vulnerability combines command injection (CWE-78) with improper privilege management (CWE-266) to create a severe attack vector against Cisco ISE deployments. The flaw resides in the REST API endpoints that handle internode communications between different ISE personas in a distributed deployment.

When ISE nodes communicate with each other, certain API endpoints fail to properly validate and sanitize input data. An attacker positioned to intercept this traffic can inject malicious commands into the communication stream. Because these API calls execute with elevated privileges, the injected commands run as root on the underlying operating system.

The attack requires the adversary to be in a man-in-the-middle position between two ISE personas located on separate nodes. Additionally, the attacker must be able to decrypt the HTTPS traffic between these nodes, which adds complexity to the exploitation but does not eliminate the risk in compromised network environments.

Root Cause

The vulnerability originates from insufficient input validation in specific REST API endpoints that process internode communications. The API fails to properly sanitize user-controllable input before passing it to system-level command execution functions. This design flaw allows specially crafted input to break out of the intended command context and execute arbitrary commands.

The improper privilege management aspect compounds the severity—the vulnerable API endpoints operate with root privileges, meaning any injected commands execute with full system access rather than with restricted application-level permissions.

Attack Vector

The attack exploits the network-accessible REST API through a man-in-the-middle position. The attacker must:

1. Position themselves in the network path between two ISE personas on separate nodes
2. Intercept HTTPS traffic between the ISE nodes
3. Decrypt the encrypted communications (requiring access to certificates or exploiting TLS weaknesses)
4. Modify specific internode API calls to inject malicious commands
5. Forward the modified requests to the target ISE node

The command injection payload is processed by the vulnerable API endpoint, which executes the injected commands with root privileges on the underlying operating system. This can lead to complete system compromise, data exfiltration, or lateral movement within the network.

Detection Methods for CVE-2021-1594

Indicators of Compromise

• Unexpected or anomalous API calls between ISE nodes with unusual parameters
• Unexpected processes spawned by ISE services running as root
• Unusual network traffic patterns indicating potential man-in-the-middle activity between ISE nodes
• Suspicious entries in ISE application logs showing malformed API requests

Detection Strategies

• Monitor ISE system logs for unexpected command execution or error messages indicating injection attempts
• Implement network monitoring to detect anomalous traffic patterns between ISE nodes
• Deploy intrusion detection systems (IDS) with signatures for command injection patterns in REST API traffic
• Review audit logs for privilege escalation events or unauthorized root access

Monitoring Recommendations

• Enable comprehensive logging on all ISE nodes to capture API request details
• Configure alerts for unusual internode communication patterns or certificate anomalies
• Monitor for unexpected outbound connections from ISE servers that could indicate post-exploitation activity
• Implement network segmentation monitoring to detect potential man-in-the-middle positioning

How to Mitigate CVE-2021-1594

Immediate Actions Required

• Apply the security patches provided by Cisco immediately for all affected ISE versions
• Verify the integrity of TLS certificates used for internode communications
• Implement strict network segmentation to limit potential man-in-the-middle opportunities
• Review ISE node configurations and ensure all internode communications are properly secured

Patch Information

Cisco has released security patches addressing this vulnerability. Administrators should consult the Cisco Security Advisory for specific patch versions and upgrade instructions for their ISE deployments. Organizations should prioritize upgrading to patched versions as the vulnerability allows unauthenticated remote attackers to achieve root-level access.

Workarounds

• Implement strong network segmentation to isolate ISE internode traffic from potentially compromised network segments
• Deploy network monitoring solutions to detect man-in-the-middle attempts between ISE nodes
• Ensure TLS certificate validation is strictly enforced for all internode communications
• Limit physical and logical access to network infrastructure connecting ISE nodes

# Network monitoring configuration example for ISE internode traffic
# Monitor traffic between ISE nodes for anomalies
# Adjust interface and IP addresses for your environment

# Example: tcpdump capture for ISE internode traffic analysis
tcpdump -i eth0 -nn host ISE_NODE_1_IP and host ISE_NODE_2_IP -w ise_internode_traffic.pcap

# Example: Review captured traffic for anomalous API patterns
tshark -r ise_internode_traffic.pcap -Y "http.request.method == POST" -T fields -e http.request.uri