CVE-2025-20239: Cisco IKEv2 Memory Leak DoS Vulnerability

CVE-2025-20239 Overview

A vulnerability exists in the Internet Key Exchange Version 2 (IKEv2) feature of Cisco IOS Software, IOS XE Software, Secure Firewall Adaptive Security Appliance (ASA) Software, and Secure Firewall Threat Defense (FTD) Software that could allow an unauthenticated, remote attacker to trigger a memory leak, resulting in a denial of service (DoS) condition.

This vulnerability is classified as CWE-401 (Missing Release of Memory after Effective Lifetime) and stems from improper processing of IKEv2 packets. An attacker could exploit this vulnerability by sending specially crafted IKEv2 packets to an affected device, causing device reloads on IOS/IOS XE platforms or system memory exhaustion on ASA/FTD platforms.

Critical Impact

Unauthenticated remote attackers can cause network infrastructure device reloads (IOS/IOS XE) or VPN session establishment failures (ASA/FTD), requiring manual device reboots to recover from the condition.

Affected Products

• Cisco IOS Software (with IKEv2 enabled)
• Cisco IOS XE Software (with IKEv2 enabled)
• Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
• Cisco Secure Firewall Threat Defense (FTD) Software

Discovery Timeline

• 2025-08-14 - CVE-2025-20239 published to NVD
• 2025-08-15 - Last updated in NVD database

Technical Details for CVE-2025-20239

Vulnerability Analysis

This vulnerability affects the IKEv2 protocol implementation across multiple Cisco networking and security products. IKEv2 is a critical protocol used for establishing VPN tunnels and is fundamental to IPsec-based secure communications. The flaw allows remote attackers to consume system memory resources without authentication, making it particularly dangerous for internet-facing devices.

The impact differs based on the affected platform. For Cisco IOS and IOS XE Software, successful exploitation causes the device to reload unexpectedly, resulting in complete service disruption. For Cisco ASA and FTD Software, exploitation leads to partial system memory exhaustion, causing system instability and preventing the establishment of new IKEv2 VPN sessions. In both scenarios, manual device reboot is required to restore normal operations.

Root Cause

The vulnerability is rooted in CWE-401: Missing Release of Memory after Effective Lifetime. The IKEv2 packet processing routines fail to properly release allocated memory when handling certain malformed or crafted packets. This missing memory cleanup leads to a gradual accumulation of unreleased memory blocks, eventually exhausting available system resources.

The lack of proper memory management in the IKEv2 processing path means that each malicious packet processed contributes to the memory leak, allowing attackers to progressively degrade device performance until critical failure occurs.

Attack Vector

The attack can be executed remotely over the network without requiring authentication. An attacker needs to send crafted IKEv2 packets to the UDP port 500 or 4500 (NAT-T) on an affected device. Since IKEv2 negotiation occurs before authentication, the attacker does not need valid credentials to trigger the vulnerability.

The attack methodology involves sending specially crafted IKEv2 packets that trigger the memory leak condition. Each packet causes additional memory to be allocated but not freed, progressively consuming device resources until either a reload occurs (IOS/IOS XE) or memory exhaustion prevents new VPN sessions (ASA/FTD).

Detection Methods for CVE-2025-20239

Indicators of Compromise

• Unusual increase in memory utilization on affected Cisco devices
• Unexpected device reloads without administrative action
• IKEv2 VPN session establishment failures
• Increased IKEv2 traffic from single or multiple sources
• System log entries indicating memory allocation failures or IKEv2 processing errors

Detection Strategies

• Monitor device memory utilization trends and alert on gradual increases without corresponding legitimate workload changes
• Implement IDS/IPS signatures to detect anomalous IKEv2 packet patterns targeting affected devices
• Configure syslog monitoring for memory-related errors and unexpected reload events
• Deploy network traffic analysis to identify unusual volumes of IKEv2 traffic from external sources
• Use SNMP polling to track memory usage patterns on critical network infrastructure

Monitoring Recommendations

• Enable detailed logging for IKEv2 events on affected devices
• Configure threshold-based alerts for memory utilization exceeding normal operational baselines
• Implement NetFlow or similar traffic analysis to baseline and monitor IKEv2 traffic patterns
• Set up automated health checks to detect unresponsive or reloading devices

How to Mitigate CVE-2025-20239

Immediate Actions Required

• Review the Cisco Security Advisory for detailed affected version information
• Identify all Cisco IOS, IOS XE, ASA, and FTD devices in your environment with IKEv2 enabled
• Apply vendor-provided patches as they become available
• Consider implementing rate limiting on IKEv2 traffic at perimeter firewalls
• Ensure network monitoring is in place to detect exploitation attempts

Patch Information

Cisco has released security advisories addressing this vulnerability. Administrators should consult the Cisco Security Advisory for specific patch versions applicable to their deployment. Organizations should prioritize patching internet-facing devices with IKEv2 enabled.

Workarounds

• If IKEv2 is not required, disable the feature and use alternative VPN protocols where possible
• Implement access control lists (ACLs) to restrict IKEv2 traffic to known, trusted IP addresses
• Deploy rate limiting controls on IKEv2 traffic at network perimeter
• Consider placing affected devices behind additional network protection layers
• Enable Control Plane Policing (CoPP) to limit the rate of IKEv2 packets processed by the device

# Example ACL configuration to restrict IKEv2 access (IOS/IOS XE)
# Replace x.x.x.x with trusted peer IP addresses
access-list 100 permit udp host x.x.x.x any eq 500
access-list 100 permit udp host x.x.x.x any eq 4500
access-list 100 deny udp any any eq 500
access-list 100 deny udp any any eq 4500
access-list 100 permit ip any any