NatJack Attack Class Hijacks TCP Sessions and Spoofs DNS by Manipulating NAT Tables

New Attack Class Discovered

Security researcher Malcolm Stagg has revealed a new attack class named NatJack. This method exploits vulnerabilities in Network Address Translation (NAT) implementations to manipulate connection states, enabling attackers to hijack active TCP sessions, spoof DNS responses, expose mapped ports, and exhaust NAT tables. The findings were presented at Black Hat USA 2026.

Affected Systems and Specific Vulnerabilities

NatJack affects various independently developed NAT implementations, including those in Windows and Linux. Two specific implementation flaws have been assigned CVEs: CVE-2026-56181 (CVSS score: 8.3) for Windows NAT used by Hyper-V, and CVE-2026-63913 (CVSS score: 8.2) for Linux Netfilter conntrack. The attack generally requires an attacker to have privileged access to a system behind the same NAT as the victim.

Attack Mechanisms

The research describes four main attack paths. Attackers can redirect traffic from an active TCP connection by replacing its NAT mapping or interfere with a victim's DNS request to send a forged response. Other techniques involve disclosing externally mapped ports or filling the NAT connection table with spoofed flows, preventing legitimate clients from establishing new connections.

Mitigation and Recommendations

There is no single patch for the broader NatJack attack class. Organizations should apply available Windows and Linux updates and encrypt traffic even within internal networks. The research also recommends implementing Internet Protocol (IP) Source Guard where applicable. Mitigation guidance emphasizes separating untrusted workloads from trusted systems that share NAT infrastructure.

Background and Current Status

The NatJack research, conducted independently by Stagg through SODIUM-24, targets an assumption in many NAT implementations that hosts behind the same NAT do not manipulate each other's connection state. While Synack confirmed Stagg tested the techniques against real-world products, there is no public evidence of NatJack techniques being exploited in the wild as of August 7, 2026.