CVE-2021-1435: Cisco IOS XE Web UI RCE Vulnerability

CVE-2021-1435 Overview

A command injection vulnerability exists in the web UI of Cisco IOS XE Software that could allow an authenticated, remote attacker to inject arbitrary commands that can be executed as the root user. This vulnerability is due to insufficient input validation in the web-based management interface. An attacker could exploit this vulnerability by sending a crafted request to the web UI of an affected device with arbitrary commands injected into a portion of the request. A successful exploit could allow the attacker to execute arbitrary commands with root privileges, leading to complete device compromise.

Critical Impact

Authenticated attackers can achieve root-level command execution on affected Cisco IOS XE devices through the web UI, potentially leading to complete network infrastructure compromise.

Affected Products

• Cisco IOS XE versions 16.9.1 through 16.9.6 (including maintenance releases)
• Cisco IOS XE versions 16.10.1 through 16.10.3 (including maintenance releases)
• Cisco IOS XE versions 16.11.1 through 16.11.2 (including maintenance releases)
• Cisco IOS XE version 17.2.1v

Discovery Timeline

• 2021-03-24 - CVE-2021-1435 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-1435

Vulnerability Analysis

This command injection vulnerability stems from a failure to properly sanitize user-controlled input within the web-based management interface of Cisco IOS XE Software. The web UI processes HTTP requests from authenticated administrators, and certain request parameters are passed to underlying system commands without adequate input validation. This allows attackers with valid administrative credentials to inject shell metacharacters and arbitrary commands that are subsequently executed with root privileges on the underlying operating system.

The vulnerability is particularly concerning because it affects network infrastructure devices that often serve as critical security boundaries. Cisco IOS XE runs on a wide range of enterprise routers, switches, and wireless controllers. Successful exploitation grants attackers the highest level of system access, enabling them to modify device configurations, intercept network traffic, establish persistent backdoors, or pivot to other network segments.

Root Cause

The root cause of CVE-2021-1435 is insufficient input validation (CWE-22) in the web UI request handling code. The application fails to properly sanitize special characters and shell metacharacters in user-supplied input before incorporating that input into system commands. This allows command injection sequences such as semicolons, pipes, or backticks to break out of the intended command context and execute attacker-controlled commands.

Attack Vector

The attack vector for this vulnerability is network-based, requiring the attacker to have authenticated access to the Cisco IOS XE web management interface. The attacker constructs a malicious HTTP request containing embedded shell commands within vulnerable request parameters. When the web UI processes this request, the injected commands are executed with root privileges.

Exploitation involves sending crafted HTTP requests to the web UI containing command injection payloads in specific request fields. The injected commands bypass input validation checks and are passed directly to shell execution contexts. Because the web UI runs with elevated privileges, the injected commands execute as the root user, providing complete control over the device.

For detailed technical information about the vulnerability mechanism, refer to the Cisco Security Advisory.

Detection Methods for CVE-2021-1435

Indicators of Compromise

• Unusual HTTP requests to the Cisco IOS XE web UI containing shell metacharacters (;, |, $(), backticks) in request parameters
• Unexpected processes spawned by the web server process on IOS XE devices
• Unauthorized configuration changes or new user accounts created on affected devices
• Abnormal outbound network connections originating from network infrastructure devices

Detection Strategies

• Monitor web server access logs on Cisco IOS XE devices for requests containing command injection patterns
• Implement network intrusion detection rules to identify HTTP requests with shell metacharacters targeting IOS XE web interfaces
• Enable command accounting and authorization logging to detect unauthorized command execution
• Deploy endpoint detection on network management systems to identify anomalous administrative activity

Monitoring Recommendations

• Configure syslog forwarding from all Cisco IOS XE devices to a centralized SIEM platform
• Enable AAA accounting for all administrative access and command execution
• Implement alerting for any configuration changes outside approved maintenance windows
• Monitor for unexpected authentication attempts to web management interfaces

How to Mitigate CVE-2021-1435

Immediate Actions Required

• Apply Cisco security patches immediately to all affected IOS XE devices
• Restrict access to the web management interface using access control lists (ACLs) to trusted management networks only
• Disable the HTTP/HTTPS server feature if web-based management is not required
• Review administrative accounts and remove any unauthorized or unnecessary privileged accounts

Patch Information

Cisco has released software updates that address this vulnerability. Administrators should consult the Cisco Security Advisory for specific fixed software versions and upgrade guidance. Organizations should prioritize patching internet-facing and critical infrastructure devices.

Workarounds

• Disable the HTTP server and HTTPS server features if web-based management is not essential for operations
• Implement strict IP access restrictions using ACLs to limit web UI access to dedicated management networks
• Enable strong authentication mechanisms including multi-factor authentication where supported
• Consider using alternative management methods such as SSH/CLI management until patches can be applied

# Disable web UI if not required
configure terminal
no ip http server
no ip http secure-server
end
write memory

# Or restrict access to management network only
configure terminal
ip http access-class 10
access-list 10 permit 10.0.0.0 0.0.0.255
access-list 10 deny any
end
write memory