CVE-2025-20253: Cisco IKEv2 DoS Vulnerability

CVE-2025-20253 Overview

A vulnerability in the IKEv2 feature of Cisco IOS Software, IOS XE Software, Secure Firewall ASA Software, and Secure FTD Software could allow an unauthenticated, remote attacker to cause the device to reload, resulting in a Denial of Service (DoS) condition. This vulnerability is due to the improper processing of IKEv2 packets, which can trigger an infinite loop that exhausts system resources.

Critical Impact

Unauthenticated remote attackers can cause affected Cisco network devices to reload by sending specially crafted IKEv2 packets, potentially disrupting critical network infrastructure and VPN connectivity.

Affected Products

• Cisco IOS Software (IKEv2 feature enabled)
• Cisco IOS XE Software (IKEv2 feature enabled)
• Cisco Secure Firewall ASA Software
• Cisco Secure Firewall Threat Defense (FTD) Software

Discovery Timeline

• 2025-08-14 - CVE-2025-20253 published to NVD
• 2025-08-15 - Last updated in NVD database

Technical Details for CVE-2025-20253

Vulnerability Analysis

This vulnerability represents an Infinite Loop condition (CWE-835) affecting the IKEv2 implementation in multiple Cisco networking products. IKEv2 (Internet Key Exchange version 2) is a fundamental protocol used for establishing Security Associations (SA) within IPsec VPN tunnels. The flaw resides in how affected devices process incoming IKEv2 packets.

When a maliciously crafted IKEv2 packet is received by a vulnerable device, it triggers improper processing that leads to an infinite loop condition. This loop continuously consumes system resources—primarily CPU cycles—until the device exhausts available resources and is forced to reload. The attack requires no authentication and can be executed remotely over the network, making it particularly dangerous for internet-facing VPN concentrators and firewall appliances.

Root Cause

The root cause is classified as CWE-835 (Loop with Unreachable Exit Condition), commonly known as an Infinite Loop vulnerability. The IKEv2 packet processing logic contains a code path where certain malformed packet structures cause the parsing routine to enter a loop that cannot be terminated normally. This represents an input validation failure where boundary conditions or packet field values are not properly verified before entering iterative processing routines.

Attack Vector

The attack vector is network-based, requiring no prior authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying a target device running a vulnerable version of Cisco IOS, IOS XE, ASA, or FTD software with IKEv2 enabled
2. Crafting malicious IKEv2 packets designed to trigger the infinite loop condition
3. Sending the crafted packets to UDP port 500 or 4500 (standard IKEv2 ports) on the target device
4. The vulnerable device enters an infinite loop, exhausting resources and causing a system reload

The vulnerability mechanism exploits improper packet validation in the IKEv2 protocol handler. Specific technical details regarding the malformed packet structure are available in the Cisco Security Advisory.

Detection Methods for CVE-2025-20253

Indicators of Compromise

• Unexpected device reloads or crashes on Cisco IOS, IOS XE, ASA, or FTD devices with IKEv2 enabled
• System logs showing abnormal resource exhaustion or high CPU utilization prior to reload
• Crash dump files indicating infinite loop conditions in IKEv2 processing routines
• Unusual IKEv2 traffic patterns from unknown or suspicious source IP addresses

Detection Strategies

• Monitor for repeated device reloads or crashinfo files on Cisco network devices running IKEv2
• Implement IDS/IPS signatures to detect anomalous IKEv2 packet structures
• Deploy network traffic analysis to identify unusual UDP 500/4500 traffic volumes or patterns
• Review syslog messages for IKEv2-related errors or resource exhaustion warnings

Monitoring Recommendations

• Enable enhanced logging on affected Cisco devices to capture IKEv2 processing events
• Configure SNMP traps for device reload events and high CPU utilization thresholds
• Implement netflow analysis to track IKEv2 traffic sources and volumes
• Establish baseline metrics for normal IKEv2 operations to identify anomalous activity

How to Mitigate CVE-2025-20253

Immediate Actions Required

• Review the Cisco Security Advisory to determine if your software version is affected
• Apply available security patches from Cisco as soon as possible
• If patching is not immediately possible, consider implementing recommended workarounds
• Restrict IKEv2 traffic to trusted IP addresses using access control lists where feasible

Patch Information

Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for specific fixed software versions applicable to their deployment. The advisory provides version-specific guidance for Cisco IOS, IOS XE, ASA, and FTD software.

Workarounds

• Implement access control lists (ACLs) to restrict IKEv2 traffic to known, trusted peer IP addresses only
• Consider disabling IKEv2 on interfaces where it is not required for business operations
• Deploy network segmentation to limit exposure of vulnerable devices to untrusted networks
• Use infrastructure protection techniques such as rate limiting on IKEv2 ports (UDP 500/4500)

# Example ACL to restrict IKEv2 access to trusted peers
# Cisco IOS/IOS XE configuration example
access-list 100 permit udp host <trusted-peer-ip> any eq 500
access-list 100 permit udp host <trusted-peer-ip> 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

interface GigabitEthernet0/0
 ip access-group 100 in