CVE-2024-20308: Cisco IOS IKEv1 DoS Vulnerability

CVE-2024-20308 Overview

A vulnerability in the IKEv1 fragmentation code of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause a heap underflow, resulting in an affected device reloading. This vulnerability exists because crafted, fragmented IKEv1 packets are not properly reassembled. An attacker could exploit this vulnerability by sending crafted UDP packets to an affected system. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a denial of service (DoS) condition.

Note: Only traffic that is directed to the affected system can be used to exploit this vulnerability. This vulnerability can be triggered by both IPv4 and IPv6 traffic, expanding the potential attack surface for organizations running dual-stack network configurations.

Critical Impact

Unauthenticated remote attackers can cause network infrastructure devices to reload, leading to service disruption across enterprise and service provider networks relying on Cisco IOS and IOS XE devices for VPN connectivity.

Affected Products

• Cisco IOS Software (multiple versions from 12.4 through 15.9)
• Cisco IOS XE Software (versions 3.3.0sg through 17.12.1w)

Discovery Timeline

• 2024-03-27 - CVE-2024-20308 published to NVD
• 2025-06-30 - Last updated in NVD database

Technical Details for CVE-2024-20308

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), specifically manifesting as a heap underflow condition. The flaw resides in the IKEv1 (Internet Key Exchange version 1) fragmentation handling code, which is responsible for reassembling fragmented IKE packets during VPN tunnel establishment and maintenance.

When IKEv1 packets exceed the maximum transmission unit (MTU) of the network path, they are fragmented for transmission and must be reassembled by the receiving device. The vulnerability occurs during this reassembly process when the affected device processes specially crafted fragmented IKEv1 packets. The improper handling of these packets causes a heap underflow condition, which corrupts memory and leads to device instability.

The attack requires no authentication and can be executed remotely over the network, making it particularly dangerous for internet-facing devices. Since IKEv1 operates over UDP port 500 (or UDP port 4500 for NAT traversal), any Cisco device with an active IKEv1 configuration accepting packets on these ports is potentially vulnerable.

Root Cause

The root cause is improper validation and handling of fragmented IKEv1 packet reassembly in the Cisco IOS and IOS XE operating systems. Specifically, the code fails to properly validate fragment boundaries and size parameters before performing memory operations during packet reconstruction. This allows an attacker to craft malicious fragments that, when reassembled, cause the device to write data before the beginning of an allocated heap buffer (heap underflow).

The heap underflow can corrupt adjacent memory structures critical to device operation, ultimately triggering a crash and reload of the affected device.

Attack Vector

An attacker can exploit this vulnerability by sending specially crafted, fragmented IKEv1 UDP packets to an affected Cisco device. The attack can be launched from any network location that can route packets to the target device's IP address on UDP port 500 or 4500.

The attack flow involves:

1. Identifying a target Cisco device with IKEv1 enabled
2. Crafting malicious IKEv1 fragmented packets with parameters designed to trigger the heap underflow
3. Sending the crafted packets to the target device
4. The device attempts to reassemble the fragments, triggering the heap underflow
5. Memory corruption causes the device to crash and reload

The vulnerability affects both IPv4 and IPv6 traffic paths, meaning devices configured for dual-stack operation are vulnerable regardless of which protocol is used to deliver the malicious packets.

Detection Methods for CVE-2024-20308

Indicators of Compromise

• Unexpected device reloads with crash logs indicating memory corruption or heap-related errors
• Syslog messages showing IKEv1 processing failures immediately before device crashes
• Crash dump files referencing IKEv1 fragmentation code or packet reassembly functions
• Multiple consecutive device reboots without administrative intervention

Detection Strategies

• Monitor for anomalous IKEv1 traffic patterns, particularly fragmented packets with unusual size or fragment offset values
• Implement IDS/IPS signatures to detect malformed IKEv1 fragmentation headers
• Configure syslog monitoring to alert on device crash and reload events
• Enable NetFlow or similar traffic analysis to identify unusual UDP port 500/4500 traffic patterns

Monitoring Recommendations

• Deploy network monitoring to track device availability and uptime metrics for Cisco infrastructure
• Configure SNMP traps for device reload events and memory utilization anomalies
• Implement centralized log collection to correlate IKEv1-related events across multiple devices
• Establish baseline traffic patterns for IKEv1 connections to identify deviations

How to Mitigate CVE-2024-20308

Immediate Actions Required

• Review the Cisco Security Advisory and determine if your devices are running affected software versions
• Apply the appropriate software update from Cisco to remediate the vulnerability
• If immediate patching is not possible, implement access control lists (ACLs) to restrict IKEv1 traffic to trusted sources only
• Consider migrating from IKEv1 to IKEv2 where possible, as IKEv2 is not affected by this vulnerability

Patch Information

Cisco has released software updates that address this vulnerability. Organizations should consult the Cisco Security Advisory for specific fixed software versions for their deployed IOS or IOS XE releases. The advisory provides detailed information on fixed releases and software upgrade paths.

Workarounds

• Implement ACLs to limit IKEv1 traffic (UDP ports 500 and 4500) to known, trusted VPN peer IP addresses only
• Consider disabling IKEv1 entirely if IKEv2 can be used as an alternative for VPN connectivity
• Deploy control plane policing (CoPP) to rate-limit IKEv1 traffic and reduce the impact of potential exploitation attempts

# Example ACL to restrict IKEv1 to trusted peers
access-list 100 permit udp host 192.168.1.10 any eq 500
access-list 100 permit udp host 192.168.1.10 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