CVE-2024-20309: Cisco IOS XE AUX Port DoS Vulnerability

CVE-2024-20309 Overview

A vulnerability in auxiliary asynchronous port (AUX) functions of Cisco IOS XE Software could allow an authenticated, local attacker to cause an affected device to reload or stop responding. This vulnerability is due to the incorrect handling of specific ingress traffic when flow control hardware is enabled on the AUX port. An attacker could exploit this vulnerability by reverse telnetting to the AUX port and sending specific data after connecting. A successful exploit could allow the attacker to cause the device to reset or stop responding, resulting in a denial of service (DoS) condition.

Critical Impact

An authenticated attacker with local access can cause network device reloads, disrupting network operations and potentially impacting business continuity for organizations relying on affected Cisco infrastructure.

Affected Products

• Cisco IOS XE versions 3.2.x through 3.18.x (various builds including SE, SQ, S, SP variants)
• Cisco IOS XE versions 16.1.x through 16.12.x
• Cisco IOS XE versions 17.1.x through 17.12.x

Discovery Timeline

• March 27, 2024 - CVE-2024-20309 published to NVD
• July 30, 2025 - Last updated in NVD database

Technical Details for CVE-2024-20309

Vulnerability Analysis

This vulnerability resides in the auxiliary asynchronous port (AUX) functionality of Cisco IOS XE Software. The AUX port is traditionally used for out-of-band management access, modem connections, or secondary console access on Cisco network devices. The vulnerability is triggered when flow control hardware is enabled on the AUX port and specific malformed ingress traffic is processed by the device.

The fundamental issue lies in how the IOS XE software handles incoming data on the AUX port when hardware flow control mechanisms are active. When an authenticated attacker establishes a reverse telnet session to the AUX port and transmits carefully crafted data, the improper handling of this traffic can cause the device to enter an unstable state, ultimately leading to a device reload or complete unresponsiveness.

Root Cause

The root cause of this vulnerability is classified as CWE-828 (Signal Handler with Functionality that is not Asynchronous-Safe). The incorrect handling of specific ingress traffic when flow control hardware is enabled on the AUX port creates a condition where the signal handler processes data in a manner that is not safe for asynchronous execution. This results in resource handling errors that can destabilize the device's operating state, causing it to reload or become unresponsive.

Attack Vector

The attack requires local access and authentication to the affected device. An attacker must:

1. Have valid credentials to access the Cisco IOS XE device
2. Be able to initiate a reverse telnet connection to the AUX port
3. The target device must have flow control hardware enabled on the AUX port

Once these conditions are met, the attacker can send specifically crafted data through the reverse telnet connection to trigger the denial of service condition. While this requires authenticated local access, the impact on network availability makes it a significant concern for enterprise environments where device uptime is critical.

The exploitation method involves establishing a reverse telnet session to the AUX port and transmitting malicious data patterns designed to exploit the improper signal handling, causing the device to crash or become unresponsive.

Detection Methods for CVE-2024-20309

Indicators of Compromise

• Unexpected device reloads or system crashes without apparent cause
• Increased number of reverse telnet sessions to AUX ports logged in system authentication records
• Abnormal traffic patterns or data transmission on AUX port interfaces
• Syslog entries indicating signal handler errors or memory corruption events

Detection Strategies

• Monitor authentication logs for unusual AUX port access attempts, particularly reverse telnet sessions
• Implement log correlation to identify patterns of AUX port access followed by device instability
• Configure SNMP traps to alert on unexpected device reloads
• Review AAA accounting records for connections to auxiliary line interfaces

Monitoring Recommendations

• Enable detailed logging on all line AUX configurations using logging synchronous and logging level informational
• Configure centralized syslog collection to capture device reload events and correlate with access logs
• Implement device health monitoring to detect unplanned outages indicative of exploitation attempts
• Deploy network behavior analysis tools to identify abnormal management plane traffic patterns

How to Mitigate CVE-2024-20309

Immediate Actions Required

• Review all Cisco IOS XE devices in your environment to determine if AUX ports are in use and if flow control is enabled
• Disable hardware flow control on AUX ports if not required for operational purposes
• Restrict AUX port access using access control lists (ACLs) and authentication mechanisms
• Implement strict user authentication and authorization policies for management access
• Consider disabling the AUX port entirely if not needed for business operations

Patch Information

Cisco has released security patches addressing this vulnerability. Organizations should consult the Cisco Security Advisory for specific fixed software versions applicable to their deployment. It is recommended to upgrade to a patched version of Cisco IOS XE as soon as possible following proper change management procedures.

Workarounds

• Disable the AUX port if not required using the no exec configuration under the AUX line
• Remove flow control configuration from the AUX port using no flowcontrol hardware if flow control is not essential
• Implement strict access controls using access-class ACLs to limit which users and source addresses can connect to management interfaces
• Use transport input none on the AUX line to prevent any inbound connections if remote access is not needed

# Configuration example to disable AUX port access
configure terminal
line aux 0
  no exec
  transport input none
  transport output none
end
write memory