Recently, a serious flaw identified as CVE-2023-1206 was discovered in the Linux kernel’s handling of IPv6 connections. This bug opens up a new way for attackers to overload Linux servers by simply sending a specialized type of SYN flood. Unlike traditional SYN flood attacks, this one uses a hash collision weakness in the IPv6 connection lookup table, causing the server’s CPU usage to spike to extremely high levels, often reaching up to 95%. This article breaks down how the flaw works, what makes it unique, and how attackers can use it.

What’s the Problem? – The Hash Collision Flaw

The Linux kernel — the core part of every Linux-based system — keeps track of active network connections. For each new connection attempt, like a TCP SYN packet, the kernel checks its "connection table" to decide what should happen.

To do this quickly, the kernel uses a hash table. Each new connection (based on client and server IP addresses and ports) is hashed. Ideally, connections distribute randomly in this table, making each lookup fast.

The Catch: Hash Collision

A hash collision happens when two or more connections end up at the same spot in the table. Normally, this isn’t a big deal, as collisions are rare. But the code affected by CVE-2023-1206 in the IPv6 handling part of the kernel lets an attacker deliberately cause lots of collisions.

When this happens, the lookup table gets too many entries in a single "bucket", meaning every time the kernel checks for a new connection, it has to wade through a long line instead of quickly finding the right spot. With enough piles in the bucket, CPU usage goes through the roof.

The attacker creates a large number of specially-crafted IPv6 TCP SYN packets.

2. They pick source addresses, destination ports, and other values that will *all* hash to the same value in the kernel’s lookup table.

The server’s connection table gets overloaded with entries in one place ("bucket").

4. Whenever the server tries to match incoming SYN packets to connections, it has to search through an extra-long list, forcing the CPU to do way more work.

Result: CPU climbs as high as 95% — even on robust, modern servers.

This is nastier than a normal SYN flood, where the system is limited by memory (with half-open connections). In this case, the limiting resource is CPU, which can completely cripple the entire server. Legitimate traffic will grind to a halt.

Pseudocode Showing the Flaw

Here’s a simplified representation comparing the *intended* behavior vs. what’s happening with this attack:

// Normally:
bucket = hash(src_ip, dst_ip, src_port, dst_port) % TABLE_SIZE;
for (conn in conn_table[bucket]) {
  if (conn matches incoming SYN) {
    // Process connection
  }
}

// With attack:
many SYNs sent to generate same hash
conn_table[target_bucket] = [conn1, conn2, ..., conn10000] // Huge list!
for (conn in conn_table[target_bucket]) {
  // Server checks every entry, wasting CPU
}

This special attack can be imagined as a “traffic jam” in a parking lot: All cars trying to enter through the *same* gate, instead of spreading out, causing lines to build up and slow everything down.

Exploit Details: Proof-of-Concept Attack

Here’s a very basic proof-of-concept (python3 with scapy) to send lots of SYN packets that hash to the same bucket (for illustration; real exploiters would optimize hashing):

from scapy.all import *
target_ip   = "2001:db8::2"
target_port = 22

for i in range(10000):
    spoofed_src = "2001:db8::" + str(i)
    pkt = IPv6(src=spoofed_src, dst=target_ip)/TCP(sport=12345, dport=target_port, flags='S')
    send(pkt, verbose=)

An advanced attacker would reverse-engineer the kernel’s hash function to generate colliding address/port sets, maximizing bucket overload.

Mitigation and Fixes

Linux kernel developers released patches for CVE-2023-1206 in April 2023. Servers should:

References (Read More)

- Red Hat CVE-2023-1206 Security Advisory
- Linux Kernel Patch Commit
- NIST NVD Entry for CVE-2023-1206

Summary

CVE-2023-1206 is a new kind of threat that can severely overload Linux servers using IPv6, not by filling up memory with bogus connections, but by causing a CPU meltdown via targeted hash collisions. Keeping your Linux systems updated is crucial, especially if you’re using IPv6. Watch out for abnormal CPU usage — it could be a sign that someone is hitting you with this very attack.

Timeline

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