CVE-2023-20186: Cisco IOS Auth Bypass Vulnerability

CVE-2023-20186 Overview

A critical vulnerability exists in the Authentication, Authorization, and Accounting (AAA) feature of Cisco IOS Software and Cisco IOS XE Software that allows an authenticated, remote attacker to bypass command authorization and perform unauthorized file operations. By exploiting improper processing of Secure Copy Protocol (SCP) commands within AAA command authorization checks, an attacker with valid credentials and level 15 privileges can copy files to or from the file system of affected devices.

This authorization bypass vulnerability poses significant risks to enterprise network infrastructure, as Cisco IOS and IOS XE devices are foundational components in many corporate environments. Successful exploitation could allow attackers to exfiltrate sensitive configuration data, deploy malicious configurations, or retrieve credential information stored on the device.

Critical Impact

Authenticated attackers with level 15 privileges can bypass AAA command authorization to obtain or modify device configurations and transfer files, potentially leading to full network compromise.

Affected Products

• Cisco IOS Software (multiple versions from 12.2 through 15.9)
• Cisco IOS XE Software (versions 3.2.x through 17.11.x)

Discovery Timeline

• September 27, 2023 - CVE-2023-20186 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-20186

Vulnerability Analysis

This vulnerability stems from a fundamental flaw in how Cisco IOS and IOS XE Software process SCP commands during AAA command authorization checks. The AAA framework is designed to enforce strict access controls, ensuring that users can only execute commands they are explicitly authorized to perform. However, the SCP command processing logic contains a flaw that allows it to bypass these authorization checks entirely.

When an attacker establishes an SCP connection from an external machine to an affected device, the AAA subsystem fails to properly validate whether the user has authorization to perform file transfer operations. This allows an authenticated user with level 15 privileges—which typically grants administrative access—to transfer files without the expected authorization verification.

The impact of this vulnerability extends beyond simple file transfers. An attacker could retrieve the running configuration, which often contains sensitive information such as SNMP community strings, routing protocol authentication keys, VPN credentials, and other secrets. Additionally, the ability to write files to the device could enable configuration tampering, backdoor installation, or preparation for further attacks.

Root Cause

The root cause of CVE-2023-20186 is improper authorization validation (CWE-285: Improper Authorization) within the AAA command processing subsystem. Specifically, the vulnerability exists because SCP commands are not subjected to the same rigorous authorization checks that apply to other administrative commands. This inconsistency in the security model creates an exploitable gap that attackers can leverage to circumvent intended access controls.

The authorization check bypass occurs because the SCP service processes file transfer requests through a code path that does not invoke the standard AAA command authorization mechanisms. This architectural oversight means that even when AAA command authorization is properly configured and functioning for other commands, SCP operations can proceed without proper verification.

Attack Vector

The attack requires network access to the affected device and valid credentials with level 15 privileges. The attacker must be able to establish an SCP connection to the target device from an external machine. Once connected, the attacker can use standard SCP operations to transfer files to or from the device's file system.

The attack flow typically follows these steps:

1. The attacker authenticates to the target Cisco device using valid credentials with level 15 privileges
2. From an external machine, the attacker initiates an SCP session to the target device
3. The SCP service processes the file transfer request without properly checking AAA command authorization
4. The attacker can then download sensitive files (such as running-config or startup-config) or upload malicious files to the device

This vulnerability is particularly dangerous in environments where command authorization is relied upon as a compensating control to restrict what authenticated administrators can do. Organizations that have implemented role-based access controls through AAA may be unaware that SCP operations bypass these controls entirely.

Detection Methods for CVE-2023-20186

Indicators of Compromise

• Unexpected SCP connections to Cisco IOS/IOS XE devices from external IP addresses
• Configuration file access or modifications that were not authorized through change management processes
• Log entries showing SCP file transfers by users who should not have file transfer permissions
• Presence of unfamiliar files in the device file system

Detection Strategies

• Monitor AAA accounting logs for SCP session establishment and file transfer activities
• Implement network traffic analysis to detect SCP connections (TCP port 22) to infrastructure devices
• Configure SNMP traps or syslog alerts for file system access events on network devices
• Establish baseline SCP usage patterns and alert on anomalies

Monitoring Recommendations

• Enable comprehensive logging for all authentication and file access events on Cisco devices
• Deploy network detection capabilities to identify lateral movement and unauthorized file transfers
• Integrate Cisco device logs with SIEM platforms for correlation and alerting
• Regularly audit device configurations for unauthorized changes

How to Mitigate CVE-2023-20186

Immediate Actions Required

• Review all Cisco IOS and IOS XE devices for vulnerable software versions
• Audit current SCP access and disable SCP functionality where not required
• Implement network segmentation to restrict management plane access to authorized systems only
• Review AAA configurations and ensure all administrative access is monitored

Patch Information

Cisco has released security updates to address this vulnerability. Organizations should consult the Cisco Security Advisory for specific fixed software versions applicable to their deployment. The advisory provides detailed information on which software versions contain the fix and guidance for determining the appropriate upgrade path.

Administrators should verify their current software version using the show version command and compare it against the fixed releases listed in the Cisco security advisory. Given the critical severity of this vulnerability, prioritize patching for devices that are exposed to untrusted networks or that contain sensitive configurations.

Workarounds

• Disable SCP server functionality if not required by using the no ip scp server enable command
• Implement strict access control lists (ACLs) to limit which IP addresses can establish SCP connections
• Use TACACS+ or RADIUS to implement additional authorization controls and comprehensive logging
• Consider using out-of-band management networks to isolate device management traffic

# Disable SCP server functionality
configure terminal
no ip scp server enable
end
write memory

# Restrict management access via ACL
ip access-list extended MGMT-ACCESS
 permit tcp host 10.0.0.100 any eq 22
 deny tcp any any eq 22
 permit ip any any