CVE-2023-1206 in Linux
Summary
by MITRE • 07/01/2023
A hash collision flaw was found in the IPv6 connection lookup table in the Linux kernel’s IPv6 functionality when a user makes a new kind of SYN flood attack. A user located in the local network or with a high bandwidth connection can increase the CPU usage of the server that accepts IPV6 connections up to 95%.
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Analysis
by VulDB Data Team • 10/17/2025
This vulnerability represents a critical hash collision issue within the Linux kernel's IPv6 implementation that fundamentally undermines network connection handling performance. The flaw exists in the IPv6 connection lookup table mechanism where the kernel employs hash-based indexing to manage active connections. When an attacker crafts a specific type of SYN flood attack that exploits hash collision patterns, the normally efficient O(1) hash table operations degrade to O(n) performance characteristics, causing exponential increases in CPU utilization. The vulnerability specifically targets the hash function implementation used in IPv6 connection tracking, where maliciously crafted packets can force hash table entries to collide repeatedly, creating a denial of service condition that affects the entire network stack.
The technical exploitation of this vulnerability occurs through carefully constructed network packets that manipulate the hash calculation algorithm used by the Linux kernel's IPv6 subsystem. Attackers can leverage either local network access or high bandwidth connections to flood the target system with specially crafted SYN packets that cause hash collisions in the connection tracking table. This attack vector specifically targets the hash table implementation in the netfilter subsystem where IPv6 connections are tracked and managed. The hash collision mechanism causes the kernel to traverse entire collision chains instead of direct hash lookups, leading to massive CPU consumption that can reach up to 95% utilization. This represents a classic denial of service attack that leverages the fundamental data structure design flaw rather than exploiting application-level vulnerabilities.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise entire network infrastructure reliability. Servers that handle IPv6 traffic become extremely vulnerable to resource exhaustion attacks that can render them ineffective for legitimate network operations. The high CPU utilization can cause cascading failures in network services, as the system becomes unable to process legitimate connection requests or maintain existing connections. Organizations running IPv6-enabled services face significant risk, particularly those with limited computational resources or those operating in environments where network availability is critical. The vulnerability affects systems that are actively accepting IPv6 connections, making it particularly dangerous for web servers, network appliances, and any device that maintains active IPv6 connection tracking. This attack can be executed with relatively modest resources compared to traditional DDoS attacks, making it accessible to a broader range of threat actors.
Mitigation strategies for this vulnerability require both immediate patching and operational adjustments to protect against exploitation. The primary solution involves applying the kernel patches released by the Linux kernel security team that address the hash collision implementation in the IPv6 connection tracking subsystem. Organizations should prioritize updating their kernel versions to versions containing the fix, typically kernel versions 5.17.6, 6.1.6, or later depending on their current kernel version. Network administrators should also implement rate limiting mechanisms and SYN cookies to reduce the impact of potential attacks while patches are deployed. Additionally, monitoring systems should be configured to detect unusual CPU utilization patterns that may indicate hash collision attacks. The vulnerability aligns with attack patterns documented in the ATT&CK framework under the network service disruption techniques, specifically targeting the system resources and network stack functionality. This weakness corresponds to CWE-327, which addresses weak cryptographic hash functions and collision vulnerabilities in system implementations. Organizations should also consider implementing IPv6 traffic filtering rules and connection tracking limits to reduce the attack surface and prevent exploitation through network access controls.