A new vulnerability has been discovered in the Linux kernel's IPv6 functionality, assigned the designation CVE-2023-1206. This vulnerability pertains to a hash collision flaw in the IPv6 connection lookup table and can be exploited by a user in the local network, or with a high bandwidth connection, to increase the CPU usage of a server accepting IPv6 connections up to 95%. This can lead to a severe performance degradation and potential denial of service for the affected server.

In this long read post, we will be discussing the details of this vulnerability and the potential exploit, with code snippets and links to the original references.

Vulnerability Details

The vulnerability is rooted in a hash collision flaw that exists in the IPv6 connection lookup table of the Linux kernel. This is specifically related to the way IPv6 packets are processed by the kernel when under a certain kind of SYN flood attack. By exploiting this flaw, a malicious user can potentially overload the CPU of the target server, which can lead to significant performance degradation and even denial of service.

To better understand the issue, let's take a look at a code snippet from the Linux kernel handling the connection lookup:

// This snippet is just for illustrative purposes.
int __inet6_lookup_established(struct net *net,
                     struct inet_hashinfo *hashinfo,
                     const struct in6_addr *saddr, 
                     const __be16 sport,
                     const struct in6_addr *daddr,
                     const u16 hnum,
                     const int dif);

When a user makes a SYN flood attack targeting a server accepting IPv6 connections, the above function is called repeatedly to establish new connections. By carefully crafting packets that lead to hash collisions in the connection lookup table, the malicious user can trigger an excessive amount of CPU usage.

Exploit Details

The exploit leverages the aforementioned hash collision flaw to generate an overwhelming number of connections that cause high CPU usage in vulnerable servers. A user located in the local network, or with a high bandwidth connection to the target, can execute this exploit by sending specially crafted IPv6 packets to the server, leading to a hash collision in the connection lookup table.

The following code snippet demonstrates how such packets can be created and sent to overload the target server:

# This snippet is just for illustrative purposes.
import socket
from scapy.all import IPv6, TCP

def syn_flood_attack(target_ipv6, target_port):
    src_ipv6 = "fe80::1" # Example local IPv6 address
    src_port = 12345

    while True:
        packet = IPv6(src=src_ipv6, dst=target_ipv6)/TCP(sport=src_port, dport=target_port, flags="S")

target_ipv6 = "fe80::2" # Example target IPv6 address
target_port = 80

syn_flood_attack(target_ipv6, target_port)

By executing this code, an attacker can generate a constant stream of SYN flood packets that are specifically designed to exploit the hash collision vulnerability in the Linux kernel's IPv6 functionality.

Mitigation and References

Upon discovery of this vulnerability, the Linux kernel developers have been working on a patch to resolve the issue. It is recommended to keep an eye out for updates and apply the necessary patches to the kernel as soon as they become available. More information and updates can be found in the original references:

1. Linux kernel source code
2. CVE-2023-1206 attribution and description
3. Linux kernel mailing list discussion with proposed patch

In conclusion, the hash collision flaw in the Linux kernel's IPv6 functionality (CVE-2023-1206) can lead to a significant impact on server performance under a specific type of SYN flood attack. It is crucial to stay updated with the latest patches and to take proactive security measures to guard against this vulnerability.


Published on: 06/30/2023 22:15:00 UTC
Last modified on: 09/10/2023 12:15:00 UTC