CVE-2022-20676: Cisco IOS XE Privilege Escalation Flaw

CVE-2022-20676 Overview

A vulnerability in the Tool Command Language (Tcl) interpreter of Cisco IOS XE Software could allow an authenticated, local attacker to escalate from privilege level 15 to root-level privileges. This privilege escalation vulnerability stems from insufficient input validation of data that is passed into the Tcl interpreter. An attacker could exploit this vulnerability by loading malicious Tcl code on an affected device, potentially gaining complete control over the underlying operating system.

Critical Impact

Authenticated attackers with privilege level 15 access can escalate to root-level privileges, enabling arbitrary command execution as root and complete system compromise of affected Cisco IOS XE network devices.

Affected Products

• Cisco IOS XE versions 16.12.1z2
• Cisco IOS XE versions 17.2.x through 17.2.3
• Cisco IOS XE versions 17.3.x through 17.3.4c
• Cisco IOS XE versions 17.4.x through 17.4.2a
• Cisco IOS XE versions 17.5.1, 17.5.1a, and 17.5.1c

Discovery Timeline

• April 15, 2022 - CVE-2022-20676 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2022-20676

Vulnerability Analysis

This vulnerability exists within the Tcl interpreter component of Cisco IOS XE Software, which is used for scripting and automation tasks on network devices. The Tcl shell provides powerful capabilities for device management and configuration automation, making it an attractive target for privilege escalation attacks.

The vulnerability allows an authenticated attacker who has already obtained privilege level 15 access (the highest administrative privilege level in IOS) to escape the IOS execution environment and gain root-level access to the underlying Linux operating system. This represents a significant security boundary violation, as privilege level 15 users are intended to have full control over IOS configuration but not direct access to the underlying OS.

By default, Tcl shell access requires privilege level 15, meaning the attacker must first have administrative credentials to the device. However, once this access is obtained, the vulnerability enables a complete bypass of the security separation between the IOS management plane and the root operating system.

Root Cause

The root cause of this vulnerability is insufficient input validation (CWE-20) in the Tcl interpreter when processing user-supplied data. Additionally, the vulnerability is associated with execution with unnecessary privileges (CWE-250), where the Tcl interpreter has access to elevated system capabilities beyond what is strictly required for its intended functionality.

The combination of improper input validation and excessive privileges creates an opportunity for attackers to craft malicious Tcl code that breaks out of the intended execution sandbox and achieves root-level command execution.

Attack Vector

The attack requires local access to the device and authentication with privilege level 15 credentials. The attacker exploits the vulnerability by crafting and loading malicious Tcl code that takes advantage of the insufficient input validation. When executed by the Tcl interpreter, this code can escape the intended execution context and run arbitrary commands with root privileges on the underlying operating system.

The attack does not require user interaction beyond the initial authentication, and while the attacker must already have administrative access, successful exploitation significantly amplifies their capabilities by granting direct OS-level access.

Detection Methods for CVE-2022-20676

Indicators of Compromise

• Unusual Tcl script execution or loading activity on IOS XE devices
• Unexpected process creation or command execution originating from the Tcl interpreter
• Evidence of shell escapes or attempts to access underlying Linux OS from IOS CLI
• Unauthorized configuration changes or file system modifications
• Anomalous privilege level 15 session activity, particularly involving Tcl shell commands

Detection Strategies

• Monitor and audit all Tcl shell usage on IOS XE devices, including tclsh command invocations
• Implement logging for privilege level 15 sessions and correlate with unusual activity patterns
• Deploy file integrity monitoring on IOS XE devices to detect unauthorized system modifications
• Review AAA (Authentication, Authorization, Accounting) logs for suspicious administrative access patterns

Monitoring Recommendations

• Enable comprehensive logging on all Cisco IOS XE devices, including command accounting
• Configure centralized syslog collection for all network infrastructure devices
• Implement SIEM rules to alert on Tcl shell usage followed by unexpected system-level activity
• Establish baselines for normal administrative activity and alert on deviations

How to Mitigate CVE-2022-20676

Immediate Actions Required

• Apply Cisco security patches immediately to all affected IOS XE devices
• Restrict privilege level 15 access to only essential personnel with strict authentication controls
• Audit current privilege level 15 account usage and disable unnecessary administrative accounts
• Implement strong multi-factor authentication for administrative access to network devices
• Review and restrict Tcl shell access where not operationally required

Patch Information

Cisco has released security updates to address this vulnerability. Organizations should consult the Cisco Security Advisory for specific fixed software versions and upgrade guidance. Upgrading to a patched version of Cisco IOS XE is the definitive remediation for this vulnerability.

Workarounds

• Limit and strictly control which users have privilege level 15 access on affected devices
• Disable Tcl shell functionality if it is not required for operational purposes using the no scripting tcl init command
• Implement network segmentation to restrict management plane access to trusted networks only
• Deploy jump hosts or privileged access management solutions for administrative access to network infrastructure
• Monitor all administrative sessions for suspicious Tcl interpreter usage

# Disable Tcl shell if not required (configuration example)
configure terminal
no scripting tcl init
no scripting tcl encdir
end
write memory