CVE-2026-20004: Cisco IOS XE TLS Memory DoS Vulnerability

CVE-2026-20004 Overview

A vulnerability in the TLS library of Cisco IOS XE Software could allow an unauthenticated, adjacent attacker to exhaust the available memory of an affected device. This vulnerability is classified as a Resource Exhaustion vulnerability (CWE-771: Missing Reference to Active Allocated Resource) that enables Denial of Service (DoS) attacks against network infrastructure.

The vulnerability stems from improper management of memory resources during TLS connection setup. An attacker could exploit this vulnerability by repeatedly triggering the conditions that cause memory consumption to increase. This could be accomplished through various methods, including repeatedly attempting Extensible Authentication Protocol (EAP) authentication when local EAP is enabled on an affected device, or by using a machine-in-the-middle attack to reset TLS connections between the affected device and other devices.

Critical Impact

Successful exploitation allows an adjacent network attacker to exhaust available memory on affected Cisco IOS XE devices, resulting in an unexpected reload and denial of service condition that disrupts network operations.

Affected Products

• Cisco IOS XE Software with TLS library functionality
• Cisco devices with local EAP authentication enabled
• Network infrastructure utilizing TLS connections on affected Cisco platforms

Discovery Timeline

• 2026-03-25 - CVE-2026-20004 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-20004

Vulnerability Analysis

This vulnerability exists due to improper management of memory resources during TLS connection setup in the Cisco IOS XE Software TLS library. The root cause is classified under CWE-771 (Missing Reference to Active Allocated Resource), which indicates that the software fails to properly track and release allocated memory resources during TLS handshake operations.

When processing TLS connections, the affected TLS library allocates memory for connection state management but does not properly release these resources under certain conditions. This creates a memory leak that can be systematically exploited by an attacker with adjacent network access.

The attack requires no authentication or user interaction, making it particularly dangerous in enterprise environments where Cisco IOS XE devices serve critical network infrastructure roles. The scope is changed, meaning the impact extends beyond the vulnerable component itself, potentially affecting other services and devices dependent on the affected network infrastructure.

Root Cause

The vulnerability originates from CWE-771: Missing Reference to Active Allocated Resource. The TLS library in Cisco IOS XE Software fails to maintain proper references to memory allocated during TLS connection setup, preventing the system from properly deallocating these resources when connections are terminated or fail. Over time, repeated connection attempts cause cumulative memory consumption that eventually exhausts available system memory.

Attack Vector

The attack requires adjacent network access (not remotely exploitable over the internet without first gaining local network presence). An attacker positioned on the same network segment as the vulnerable device can exploit this vulnerability through two primary methods:

Method 1: EAP Authentication Abuse
When local EAP is enabled on an affected device, an attacker can repeatedly attempt EAP authentication. Each authentication attempt triggers TLS connection setup, and the improper memory management causes memory to accumulate with each attempt.

Method 2: Machine-in-the-Middle TLS Reset
An attacker positioned between the affected device and its communication peers can intercept and reset TLS connections. By continuously resetting connections during the handshake phase, the attacker forces repeated memory allocations without proper cleanup.

The attack complexity is low, requiring no special privileges or user interaction. The impact is limited to availability (no confidentiality or integrity impact), but the denial of service condition can be severe, causing device reloads and network disruption.

Detection Methods for CVE-2026-20004

Indicators of Compromise

• Abnormally high memory utilization on Cisco IOS XE devices, particularly in processes related to TLS or EAP handling
• Repeated TLS connection attempts or failures logged from the same source within short time intervals
• Unexpected device reloads preceded by memory exhaustion error messages
• Unusual volume of EAP authentication attempts from unauthorized endpoints

Detection Strategies

• Monitor system memory utilization on Cisco IOS XE devices and alert when thresholds exceed normal operating parameters
• Implement logging for TLS connection failures and analyze patterns indicating potential exploitation attempts
• Configure SNMP monitoring for memory-related OIDs to track resource consumption trends
• Review authentication logs for anomalous patterns of repeated EAP authentication failures

Monitoring Recommendations

• Enable detailed logging for the TLS subsystem on affected Cisco IOS XE devices
• Implement network traffic analysis to detect abnormal connection patterns targeting device management interfaces
• Configure automated alerting for memory utilization above established baselines
• Monitor for repeated connection resets in TLS traffic flows using network detection tools

How to Mitigate CVE-2026-20004

Immediate Actions Required

• Review the Cisco Security Advisory for affected versions and available patches
• Assess network architecture to identify devices with local EAP enabled or those handling TLS connections from potentially untrusted adjacent networks
• Implement network segmentation to limit adjacent network access to critical infrastructure devices
• Consider disabling local EAP functionality on affected devices if not operationally required until patching is complete

Patch Information

Cisco has published a security advisory addressing this vulnerability. Organizations should consult the Cisco Security Advisory for specific patch information, affected software versions, and detailed remediation guidance.

Administrators should prioritize patching based on device criticality and exposure to adjacent network threats. Devices in environments with untrusted adjacent network segments should be prioritized for immediate remediation.

Workarounds

• Implement strict network access control to limit which devices can communicate with affected Cisco IOS XE systems
• Disable local EAP authentication on affected devices when operationally feasible
• Configure rate limiting for authentication attempts where supported
• Deploy 802.1X port-based network access control to restrict adjacent network access to authorized endpoints only

# Example: Review memory utilization on Cisco IOS XE
show memory statistics
show processes memory sorted

# Example: Check EAP configuration status
show running-config | section eap

# Example: Monitor TLS-related processes
show processes cpu | include TLS|SSL